Controlling a wind turbine in accordance with a maximum allowable rotor torque

Abstract

The invention involves controlling a wind turbine in accordance with a maximum allowable rotor torque. The invention involves determining maximum allowable rotor torque based on an obtained rotor speed target. The invention involves determining respective first and second operational parameter references of the wind turbine based on the rotor speed target and on the maximum allowable torque, where the second operational parameter is different from the first operational parameter. When wind speed in the vicinity of the wind turbine is less than a defined wind speed threshold, the invention involves controlling the wind turbine to operate in accordance with the first operational parameter reference. When the obtained wind speed is greater than the defined wind speed threshold, the invention involves controlling the wind turbine to operate in accordance with the second operational parameter reference.

Description

[0001] CONTROLLING A WIND TURBINE IN ACCORDANCE WITH A MAXIMUM

[0002] ALLOWABLE ROTOR TORQUE

[0003] TECHNICAL FIELD

[0004] The invention relates to controlling a wind turbine in accordance with a maximum allowable torque of a rotor of the wind turbine. In particular, the invention relates to controlling the wind turbine in accordance with a respective reference value of different operational parameters of the wind turbine in dependence on wind speed in order to ensure that the maximum allowable rotor torque is not exceeded.

[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, and monitoring and controlling operational parameters of a wind turbine in view of such conditions is important for maximising power production and / or minimising wind turbine component loading.

[0007] A wind turbine typically operates with a certain rotational speed. This may generally involve operating a wind turbine at a relatively low rotational speed at low wind speeds, which increases for increasing wind speed until a maximum rotational speed that is utilised for high wind speeds.

[0008] Wind turbine components, such as a wind turbine gearbox, are built to withstand certain levels of torque in the wind turbine rotor. A wind turbine operating at nominal values of rotational speed and output power may give rise to acceptable levels of rotor torque. However, a decrease in rotational speed while rotor torque remains at a relatively high level may cause issues such as component wear. For instance, in a wind turbine gearbox, such operational conditions may cause bearings to slide instead of roll, which can cause undue wear of the bearings. Wind turbine rotational speed may be reduced for different reasons. One example is when a noise mode of the wind turbine is changed to reduce noise generated by the wind turbine. It is desired to be able to perform appropriate control of operation of a wind turbine to ensure that wind turbine rotor torque is at an acceptable level, e.g. that does not lead to undue component wear, for different wind turbine rotational speeds, e.g. when the rotational speed is reduced from a relatively high value.

[0009] It is against this background to which the present invention is set.

[0010] SUMMARY OF THE INVENTION

[0011] According to an aspect of the present invention there is provided a method of controlling a wind turbine in accordance with a maximum allowable torque of a rotor of the wind turbine. The method comprises obtaining a rotor speed target for the wind turbine rotor speed. The method comprises determining maximum allowable torque of the rotor based on the obtained rotor speed target. The method comprises determining a first operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque. The method comprises determining a second operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque. The second operational parameter is different from the first operational parameter. The method comprises obtaining a wind speed signal indicative of wind speed in a vicinity of the wind turbine.

[0012] When the obtained wind speed is less than a defined wind speed threshold, the method comprises controlling the wind turbine to operate in accordance with the determined first operational parameter reference. When the obtained wind speed is greater than the defined wind speed threshold, the method comprises controlling the wind turbine to operate in accordance with the determined second operational parameter reference.

[0013] The second operational parameter reference may be a maximum output power of the wind turbine.

[0014] Controlling the wind turbine to operate in accordance with the determined second operational parameter reference may comprise controlling pitch angle of one or more rotor blades of the wind turbine.

[0015] The first operational parameter reference may be a minimum pitch angle of one or more rotor blades of the wind turbine. The method may comprise determining a maximum output power of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque. The method may comprise determining a maximum rotor power of the wind turbine based on the determined maximum output power. The minimum pitch angle may be determined based on the determined maximum rotor power.

[0016] Determining the minimum pitch angle, emin, may comprise solving for emin. Here, Pmaxis the determined maximum rotor power, r is a length of each rotor blade of the wind turbine, p is air density, v is wind speed, A is tip speed ratio, and Cpis a power coefficient.

[0017] Controlling the wind turbine to operate in accordance with the determined first operational parameter reference may comprise controlling a generator speed of the wind turbine.

[0018] The maximum allowable torque may be determined using a defined speed-torque curve describing maximum allowable torque as a function of rotor speed.

[0019] According to the defined speed-torque curve, maximum allowable torque may be less at a first rotor speed than at a second rotor speed greater than the first rotor speed. Optionally, the defined speed-torque curve may describe maximum allowable torque as a nondecreasing function of the rotor speed.

[0020] The method may comprise gain-scheduling a first control output signal with a first gain to control the wind turbine to operate in accordance with the determined first operational parameter reference. The first gain may be a minimum gain value when the obtained wind speed is greater than the defined wind speed threshold. The first gain may be greater than the minimum gain value when the obtained wind speed is less than the defined wind speed threshold.

[0021] The method may comprise gain-scheduling a second control output signal with a second gain to control the wind turbine to operate in accordance with the determined second operational parameter reference. The second gain may be the minimum gain value when the obtained wind speed is less than the defined wind speed threshold. The second gain may be greater than the minimum gain value when the obtained wind speed is greater than the defined wind speed threshold.

[0022] The first gain may ramp down from a maximum gain value at a first wind speed, less than the threshold wind speed, to the minimum gain value at the threshold wind speed.

[0023] The second gain may ramp up from the minimum gain value at the threshold wind speed to the maximum gain value at a second wind speed, greater than the threshold wind speed.

[0024] The step of determining maximum allowable torque of the rotor may be performed in response to the rotor speed, indicated in the obtained rotor speed target, being derated from a rated rotor speed.

[0025] The method may comprise performing the step of obtaining the rotor speed target in response to receiving an indication that a noise mode of operation of the wind turbine has been activated. Optionally, activation of the noise mode may comprise switching from a first noise mode to a second noise mode.

[0026] 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, causes the one or more computer processors to perform the method defined above.

[0027] According to another aspect of the invention there is provided a controller for controlling a wind turbine. The controller is configured to obtain a rotor speed target for wind turbine rotor speed, and to determine maximum allowable torque of the rotor based on the obtained rotor speed target. The controller is configured to determine a first operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque, and to determine a second operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque. The second operational parameter is different from the first operational parameter. The controller is configured to obtain a wind speed signal indicative of wind speed in a vicinity of the wind turbine. When the obtained wind speed is less than a defined wind speed threshold, the controller is configured to control the wind turbine to operate in accordance with the determined first operational parameter reference. When the obtained wind speed is greater than the defined wind speed threshold, the controller is configured to control the wind turbine to operate in accordance with the determined second operational parameter reference.

[0028] According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0031] Figure 1 schematically illustrates a wind turbine in accordance with an aspect of the invention;

[0032] Figure 2 is a schematic plot of maximum allowable rotor torque against rotor speed according to which the wind turbine of Figure 1 is operated;

[0033] Figure 3 shows the steps of a method of operating the wind turbine of Figure 1 in accordance with an aspect of the invention;

[0034] Figure 4 is a schematic plot of a gain scheduling strategy used as part of implementing the method of Figure 3; and

[0035] Figure 5 illustrates a plot of power generated by the wind turbine as a function of wind speed obtained from a simulation of the method of Figure 3 implemented using the gain scheduling strategy of Figure 4.

[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 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 is operated at a certain rotational speed, i.e. in accordance with a certain speed target, based on a control strategy for the wind turbine 10, e.g. maximise output power. Typically, turbine or rotor speed varies with wind speed in the vicinity of the wind turbine 10. In particular, turbine or rotor speed may increase (e.g. linearly) with increasing wind speed up to a certain (rated) wind speed, i.e. ‘partial load control’, and turbine or rotor speed may remain constant at a maximum / rated value for wind speeds greater than the rated wind speed, i.e. ‘full load control’.

[0039] Components of the wind turbine 10 such as the gearbox or other drivetrain components are designed to withstand certain maximum levels of rotor torque. Indeed, these maximum levels may be relatively tightly constrained, e.g. to minimise costs associated with drivetrain components. Operating the wind turbine 10 with a rotor torque higher than a defined maximum level may cause undue wear to the drivetrain components, which can reduce component life.

[0040] A maximum allowable rotor torque, e.g. defined to guard against undue wind turbine component wear, may typically be different for different rotational speeds of the wind turbine rotor 104. When the wind turbine 10 operates at rated I nominal rotor speed and rated I nominal output power, then relatively higher levels of rotor torque may be acceptable. However, lower rotor speeds combined with higher rotor torque levels may lead to issues in relation to drivetrain components, e.g. gearbox bearings may start to slide instead of roll, thereby risking undue wear I damage to the gearbox.

[0041] In order to guard against wind turbine operation that causes such component wear, the wind turbine 10 may be operated in accordance with a so-called speed-torque curve. Figure 2 illustrates an example of a speed-torque curve 20 for the gearbox or drivetrain of the wind turbine 10. In particular, the speed-torque curve 20 shows a defined maximum allowable rotor torque as a function of rotor speed. It is therefore desirable to control operation of the wind turbine 10 such that, for a given rotor speed, the associated I corresponding maximum allowable rotor torque is not exceeded. Expressed differently, the wind turbine 10 may be operated such that the wind turbine operating point (i.e. actual rotor torque and rotor speed) is under the speed-torque curve 20. However, in order to maximise power production within the defined constraints, it may be desirable to operate the wind turbine such that actual rotor torque is as close to the maximum allowable rotor torque - for a given rotor speed - as is possible.

[0042] In the example illustrated in Figure 2, the maximum allowable torque is a constant, first value 201 for values of rotor speed below a first rotor speed value 211. The maximum allowable torque then increases linearly from the first maximum torque value 201 at the first rotor speed value 211 to a second maximum torque value 202 at a second rotor speed value 212 greater than the first value 211. The maximum allowable torque then increases linearly at a greater rate from the second maximum torque value 202 at the second rotor speed value 212 to a third maximum torque value 203 at a third rotor speed value 213 greater than the second value 212. For values of the rotor speed greater than the third rotor speed value 213, the maximum allowable torque remains constant at the third maximum torque value 203. It will be understood that Figure 2 merely shows one illustrative example of a speed-torque curve and that any suitably defined speed-torque curve may be used instead, e.g. more or fewer different rates of change of maximum torque, nonlinear changes in maximum torque as a function of rotor speed, etc.

[0043] The present invention is advantageous in that it provides an improved approach for controlling a wind turbine in accordance with a defined speed-torque curve, i.e. in accordance with a maximum allowable rotor torque. The described approach is particularly useful for performing such rotor torque limitation when wind turbine rotor speed is reduced, e.g. from a rated rotor speed. The improvements of the present approach reside partly in the inventors’ realisation that different methods for controlling wind turbine operation to limit I reduce rotor torque are appropriate at different wind speeds in the vicinity of a wind turbine. This is discussed in greater detail below. Further advantages of the present invention will become apparent from the following description.

[0044] Returning to the example speed-torque curve 20 of Figure 2, rated rotor speed is typically lower than the third rotor speed value 213. The third maximum torque value 203 may correspond to nominal I rated rotor torque of the wind turbine 10 such that values of rotor torque below this value 203 are therefore below the nominal I rated rotor torque. When rotor speed is reduced, an appropriate way to reduce the rotor torque to remain below the speed-torque curve 20 is needed.

[0045] When controlling the wind turbine 10 in such a manner the aim is to balance input power / torque from the rotor 104 with output power / torque from the generator. Here it is noted that power is directly proportional to torque, and rotor speed is inversely proportional to torque, and so a reduction in power for a given I constant rotor speed results in a decrease in torque.

[0046] As such, one way to reduce the rotor torque is to reduce the output power being produced by the wind turbine 10. Derating the output power effectively instructs the generator to take out less power. A typical way to perform this is to pitch out (increase the pitch) of the rotor blades 106 of the wind turbine 10 in full load. In particular, a generator or output power reference of the wind turbine 10 is reduced such that the torque limitation is fulfilled for any current rotational speed in accordance with the speed-torque curve 20. This can be regarded as forcing the wind turbine 10 into a state in which the rotor-generator balance mentioned above is governed by rotor blade pitching as the power is dictated via the power reference.

[0047] However, while controlling the wind turbine 10 in the above-described manner may be a relatively simple approach, it has been understood that it may be undesirable for low wind speeds. As such, an alternative approach for limiting rotor torque when rotor speed is reduced is desired at least in such conditions. One option is to use partial load control of the wind turbine 10. In particular, a minimum pitch angle of the rotor blades 106 may be increased in accordance with the speed-torque curve 20, i.e. rotor power input is fixed as needed and then this fixed rotor power is achieved by pitching directly. That is, increasing the minimum pitch reference forces the wind turbine 10 into a state in which the rotorgenerator balance is governed by the generator power as the pitch is dictated via the pitch reference. While this approach may be useful at lower wind speeds, it can be more difficult to control the turbine at higher wind speeds and so controlling generator speed at high wind speeds may be undesirable.

[0048] It is noted that the two approaches described above for limiting I reducing torque may be regarded as competing methods and, as such, cannot be used together at the same time. One of the two approaches may therefore be implemented at any given time with reference to a wind speed threshold. This means that, beneficially, one approach that is appropriate for limiting rotor torque at higher wind speeds can be used when wind speed is above the wind speed threshold, and another, different approach that is appropriate for limiting rotor torque at lower wind speeds can be used when wind speed is below the wind speed threshold. Figure 3 shows steps of a method 30 performed by a controller of the wind turbine 10 in accordance with examples of the invention. In particular, the method 20 is for controlling the wind turbine 10 in accordance with a maximum allowable torque of the wind turbine rotor 104. At step 301 , the method 30 involves obtaining a rotor speed target for the wind turbine rotor speed. The rotor speed target is desired rotor speed of the wind turbine 10, and the wind turbine 10 operates in accordance with the target. A decrease in the rotor speed target indicates that the wind turbine 10 is to be operated at a lower rotational speed. Indeed, in one example the following steps of the method 30 are performed I triggered when there has been a change in the rotor speed target, e.g. a reduction in rotor speed, such that torque limitation is needed or appropriate.

[0049] At step 302, the method 30 involves determining maximum allowable torque of the rotor 104 based on the obtained rotor speed target. In some examples, the maximum allowable torque is determined using a defined speed-torque curve, e.g. the curve 20 in Figure 2, describing maximum allowable torque as a function of rotor speed. The wind turbine 10 may include a look-up table including values (or ranges / bins of values) of rotor speed and corresponding values of maximum allowable torque, and determining the maximum allowable torque may involve querying the look-up table based on the obtained rotor speed target.

[0050] At step 303, the method 30 involves determining a first operational parameter reference of the wind turbine 10 based on the obtained rotor speed target and on the determined maximum allowable torque. This first operational parameter reference may be a minimum pitch angle of the rotor blades 106 of the wind turbine 10.

[0051] The minimum pitch angle may be determined in any suitable manner. In one example, it is considered what the maximum possible output (generator) power would be given the rotor speed target and maximum allowable rotor torque, determined according to standard relationships between these parameters. The maximum possible rotor power may then be obtained from the maximum output (generator) power using known power loss values. The minimum rotor blade pitch may then be increased until the maximum possible rotor power is obtained. For instance, this may be performed computationally (in code).

[0052] In a specific implementation of this approach for determining the minimum pitch angle, the method 30 may involve solving the following equation for the minimum pitch angle, emin:

[0053] Here, Pmaxis the determined maximum rotor power, r is a length of each rotor blade 106 of the wind turbine 10, p is (known) air density, v is wind speed (e.g. measured or estimated), A is tip speed ratio, and Cpis a power coefficient. The power coefficient may be defined as a known function of tip speed ratio and pitch angle, e.g. via a look-up table comprising combinations of tip speed ratio and pitch angle values along with corresponding power coefficient values. The rotor speed is known at each time sample, as well as wind speed, rotor blade pitch angle and the other parameters. It is then possible to calculate the minimum pitch angle that guards against the power being breached.

[0054] At step 304, the method 30 involves determining a second operational parameter reference of the wind turbine 10 based on the obtained rotor speed target and on the determined maximum allowable torque. The first and second operational parameters are different. The first operational parameter reference may be a ‘rotor side’ reference, whereas the second operational parameter reference may be a ‘generator side’ reference. The second operational parameter reference may be a maximum output (generator) power of the wind turbine rotor blades 106. In particular, the power may be reduced until an allowable rotor torque is reached for the given rotor speed target, e.g. in line with a defined speed-torque curve, and then the power reference may be back calculated. A power loss may be needed to go from rotor power to generator or output power.

[0055] Returning to Figure 3, at step 305 the method 30 involves obtaining a wind speed signal indicative of wind speed in a vicinity of the wind turbine 10. The wind speed may be obtained in any suitable manner. The wind speed may be obtained via a measured wind speed signal, obtained using one or more suitable sensors, e.g. an anemometer, at or on the wind turbine 10, e.g. on the rotor 104 or nacelle 103 behind the rotor blades 106. The wind speed may alternatively be obtained via an estimated wind speed signal, e.g. using a Kalman filter approach (based on aerodynamic torque).

[0056] The wind speed obtained at step 305 is compared against a defined wind speed threshold value. If the obtained wind speed is less than the defined wind speed threshold, then at step 306a the method 30 involves controlling the wind turbine 10 to operate in accordance with the determined first operational parameter reference. In an example in which the first operational parameter is minimum rotor blade pitch angle, then this controlling step 306a may involve controlling a generator speed of the wind turbine 10. In particular, generator speed may be decreased to decrease the power and so limit rotor torque to an acceptable level.

[0057] On the other hand, if the obtained wind speed is greater than the defined wind speed threshold, then at step 306b the method 30 involves controlling the wind turbine 10 to operate in accordance with the determined second operational parameter reference. In an example in which the second operational parameter is maximum output power of the wind turbine 10, then this controlling step 306a may involve controlling pitch angle of the wind turbine rotor blades 106, specifically by controlling pitch actuators of the wind turbine 10. In particular, increasing the rotor blade pitch angle may be used to decrease the power so as to limit / reduce rotor torque to an acceptable level when rotor speed is decreased.

[0058] To ensure a smooth transition between the two competing approaches for limiting rotor torque, i.e. step 306a or step 306b, a gain scheduling approach may be implemented in the vicinity of the defined wind speed threshold. Figure 4 schematically illustrates how steps 306a, 306b of the method 30 may be implemented via the use of applied gains dependent on wind speed. In particular, a first gain 41 may be applied to a control signal generated for controlling the wind turbine 10 in accordance with the first operational parameter reference, i.e. step 306a, and a second gain 42 may be applied to a control signal generated for controlling the wind turbine 10 in accordance with the second operational parameter reference, i.e. step 306b. In some examples, the control signals may be signals output by full and load partial load controllers I control modules of a control system I controller of the wind turbine 10. Specifically, the first gain 41 may be applied to the output signal from a partial load controller of the wind turbine 10 and the second gain 42 may be applied to the output signal from a full load controller of the wind turbine 10.

[0059] In the example illustrated in Figure 4, the first gain 41 is a constant, maximum gain value for wind speeds up to a defined first wind speed 43 less than the wind speed threshold 44. The first gain 41 then ramps down, i.e. decreases, at a constant rate from the maximum gain value at the first wind speed 43 to a minimum gain value at the wind speed threshold 44. The first gain 41 remains at the minimum gain value for wind speeds greater than the wind speed threshold 44.

[0060] On the other hand, the second gain 42 is a constant, minimum gain value for wind speeds up to the wind speed threshold 44. The second gain 42 then ramps up, i.e. increases, at a constant rate from the minimum gain value at the wind speed threshold 44 to a maximum gain value at a defined second wind speed 45 greater than the wind speed threshold. The second gain 42 remains at the maximum gain value for wind speeds greater than the second wind speed 45.

[0061] The minimum and maximum gain values may be any suitable values, and the maximum gain value may be any suitable amount greater than the minimum gain value. In a purely illustrative example, the minimum gain value may be one and the maximum gain value may be a constant that is several multiples of the minimum gain value, e.g. five.

[0062] By adopting a gain-scheduling approach such as the one illustrated in Figure 4, the two approaches for limiting rotor torque, i.e. steps 306a and 306b, will not compete with one another and, in particular, a smoother transition between how rotor torque is limited around the wind speed threshold 44 is achieved. Indeed, the gain-scheduling approach means that repeated switching between different control routines I controllers (full load and partial load) around the wind speed threshold is avoided. It is generally desirable to avoid repeated switching between different controllers as each time a switch is performed then the respective controllers need to be reset. The method of the present invention implemented using the described gain-scheduling approach may therefore beneficially be regarded as a hybrid approach incorporating two different control routines I controllers. It will be understood that the gain-scheduling approach illustrated in Figure 4 is just one example of how this may be implemented.

[0063] Figure 5 illustrates a plot of power generated by the wind turbine 10 as a function of wind speed obtained from a simulation of each of the two torque limitation approaches of the method 30 of Figure 3, and gain-scheduled according to the approach illustrated in Figure 4. In particular, the data points in Figure 4 illustrate ten-minute statistics for the two rotor torque limitation approaches of the method 30. Namely, data points 51 (i.e. the filled points) obtained when the first gain 41 is applied to an output signal for controlling the wind turbine 10 in accordance with the first operational parameter reference, i.e. adjusting generator speed in accordance with a minimum pitch angle reference (‘rotor power’ constraint). Also, data points 52 (i.e. the unfilled points) are obtained when the second gain 42 is applied to an output signal for controlling the wind turbine 10 in accordance with the second operational parameter reference, i.e. pitching the rotor blades 106 in accordance with a maximum output power reference (‘generator power’ constraint). It may be seen that the rotor power constraint - governed by the minimum pitch reference - is used for lower wind speeds, and the generator power constraint - governed by the maximum output power reference - is used for higher wind speeds. In particular, the rotor power constraint is ramped down as wind speed increases towards the wind speed threshold 44, whereas the generator power constraint is ramped up as the wind speed increases beyond the wind speed threshold 44.

[0064] It is mentioned above that the described method may be particularly useful in cases in which rotational speed of the wind turbine 10 is decreased, e.g. decreased from rated speed to a lower speed. One case in which this typically occurs is when noise (sound level) reduction associated with the wind turbine 10 is being performed. There is a link between noise emitted by a wind turbine and its rotational speed. Indeed, sound output of a wind turbine 10 is dictated to a relatively large extent by rotor speed, and decreasing wind turbine rotational speed can reduce noise output levels of the wind turbine.

[0065] It is known to control noise emissions of a wind turbine with the use of noise modes in the wind turbine. Each noise mode is associated with a specific noise-dB (decibel) curve, and for each noise mode the rotational speed and pitch angle are tuned for each one of a plurality of defined wind speed bins. Each noise mode has a fixed dB level, e.g. 100 dB, that the wind turbine must not exceed, and the fixed dB level is different for different modes. Tuning even a single noise mode can take a relatively large amount of time, and it is expected that the number of required noise modes will increase. A high number of required noise modes comes from the fact that each noise mode is associated with loss of AEP (Annual Energy Production), and thus, it is best to select a noise mode that is as close to, but below, the required noise level as possible. Each wind turbine 10 in a wind farm may be configured differently with different noise modes, and a different noise mode may be selected based on time of day, wind speed, geographical location (country), etc., in line with differing noise emission requirements and to ensure that these requirements are being satisfied.

[0066] In order to avoid the need for time consuming tuning of noise modes, a different approach that needs no tuning may simply define a rotational speed (target) that fulfils the noise level requirement. This may be performed by deriving the rotor rotational speed target based on a defined relationship between noise level and rotational speed. However, this approach may not take rotor / driving torque limitations into account, and so when the rotor speed target is reduced in line with noise emission requirements, the methods described herein in accordance with examples of the present invention may be used to control the wind turbine to ensure that rotor torque is less than a maximum allowable torque, e.g. in line with a defined speed-torque curve 20. In some examples, therefore, the method of the present invention to limit rotor torque may be implemented I triggered upon receiving an indication that a noise mode has been activated or a control action has been implemented for controlling (e.g. reducing) noise emitted by the wind turbine 10, where such a control action may typically involve wind turbine rotational speed being decreased.

[0067] It will be understood that noise reduction control is just one context in which the method of the present invention may be used; however, it will be understood that the described method may be used with other wind turbine control features in which rotational speed may vary. It will also be understood that the method of the present invention can be used to adjust / increase rotor torque when rotor speed increases (in addition to examples in which rotor torque is reduced as described above).

[0068] A controller 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.

[0069] Many modifications may be made to the described examples without departing from the scope of the appended claims.

Claims

CLAIMS1. A method of controlling a wind turbine in accordance with a maximum allowable torque of a rotor of the wind turbine, the method comprising: obtaining a rotor speed target for the wind turbine rotor speed; determining maximum allowable torque of the rotor based on the obtained rotor speed target; determining a first operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque; determining a second operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque, wherein the second operational parameter is different from the first operational parameter; obtaining a wind speed signal indicative of wind speed in a vicinity of the wind turbine; when the obtained wind speed is less than a defined wind speed threshold, controlling the wind turbine to operate in accordance with the determined first operational parameter reference; and when the obtained wind speed is greater than the defined wind speed threshold, controlling the wind turbine to operate in accordance with the determined second operational parameter reference.

2. A method according to Claim 1 , wherein the second operational parameter reference is a maximum output power of the wind turbine.

3. A method according to Claim 1 or Claim 2, wherein controlling the wind turbine to operate in accordance with the determined second operational parameter reference comprises controlling pitch angle of one or more rotor blades of the wind turbine.

4. A method according to any previous claim, wherein the first operational parameter reference is a minimum pitch angle of one or more rotor blades of the wind turbine.

5. A method according to Claim 4, the method comprising: determining a maximum output power of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque; and determining a maximum rotor power of the wind turbine based on the determined maximum output power,wherein the minimum pitch angle is determined based on the determined maximum rotor power.

6. A method according to Claim 5, wherein determining the minimum pitch angle, emin, comprises solvingfor emin, where Pmaxis the determined maximum rotor power, r is a length of each rotor blade of the wind turbine, p is air density, v is wind speed, A is tip speed ratio, and Cpis a power coefficient.

7. A method according to any previous claim, wherein controlling the wind turbine to operate in accordance with the determined first operational parameter reference comprises controlling a generator speed of the wind turbine.

8. A method according to any previous claim, wherein the maximum allowable torque is determined using a defined speed-torque curve describing maximum allowable torque as a function of rotor speed.

9. A method according to Claim 8, wherein, according to the defined speed-torque curve, maximum allowable torque is less at a first rotor speed than at a second rotor speed greater than the first rotor speed; optionally, wherein the defined speed-torque curve describes maximum allowable torque as a non-decreasing function of the rotor speed.

10. A method according to any previous claim, the method comprising: gain-scheduling a first control output signal with a first gain to control the wind turbine to operate in accordance with the determined first operational parameter reference, wherein the first gain is a minimum gain value when the obtained wind speed is greater than the defined wind speed threshold, and wherein the first gain is greater than the minimum gain value when the obtained wind speed is less than the defined wind speed threshold; and gain-scheduling a second control output signal with a second gain to control the wind turbine to operate in accordance with the determined second operational parameter reference, wherein the second gain is the minimum gain value when the obtained wind speed is less than the defined wind speed threshold, and wherein the second gain isgreater than the minimum gain value when the obtained wind speed is greater than the defined wind speed threshold.

11. A method according to Claim 10, wherein: the first gain ramps down from a maximum gain value at a first wind speed, less than the threshold wind speed, to the minimum gain value at the threshold wind speed; and the second gain ramps up from the minimum gain value at the threshold wind speed to the maximum gain value at a second wind speed, greater than the threshold wind speed.

12. A method according to any previous claim, wherein the step of determining maximum allowable torque of the rotor is performed in response to the rotor speed, indicated in the obtained rotor speed target, being derated from a rated rotor speed.

13. A method according to any previous claim, the method comprising performing the step of obtaining the rotor speed target in response to receiving an indication that a noise mode of operation of the wind turbine has been activated; optionally, wherein activation of the noise mode comprises switching from a first noise mode to a second noise mode.

14. A controller for controlling a wind turbine, the controller being configured to: obtain a rotor speed target for wind turbine rotor speed; determine maximum allowable torque of the rotor based on the obtained rotor speed target; determine a first operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque; determine a second operational parameter reference of the wind turbine based on the obtained rotor speed target and on the determined maximum allowable torque, wherein the second operational parameter is different from the first operational parameter; obtain a wind speed signal indicative of wind speed in a vicinity of the wind turbine; when the obtained wind speed is less than a defined wind speed threshold, control the wind turbine to operate in accordance with the determined first operational parameter reference; and when the obtained wind speed is greater than the defined wind speed threshold, control the wind turbine to operate in accordance with the determined second operational parameter reference.

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

Images