Operating wind turbine with reduced power output

Abstract

A method of operating a wind turbine with a reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising: receiving a derating request indicative of a reduced power output level of the wind turbine; reducing the power output level of the wind turbine in response to the derating request; as the power output level decreases, monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox; and in response to detecting a backlash risk condition, modifying a reduction in the power output level to reduce the risk of backlash occurring in the gearbox.

Description

Technical Field

[0001] The present invention relates to a method of operating a wind turbine at a reduced power output, a wind turbine comprising a control system configured to perform the method, and a computer program product. Background Art

[0002] WO2012 / 139584 discloses a method for reducing the power output of a wind turbine. The turbine's minimum power setpoint (MPS) depends on the average oncoming wind speed. The MPS is the lowest power output (expressed as a percentage of nominal power) that the turbine is allowed to generate.

[0003] At Vmax (cut-out wind speed), the MPS is, for example, 40% of the nominal power. This means that if the grid operator wants to derate the turbine when the wind speed is at Vmax, the power can only be reduced to 40% of the nominal power and not further.

[0004] At lower wind speeds Vx, the MPS is 25% of the nominal power, but at wind speeds below Vx, the power cannot be reduced further due to the risk of damaging components in the drive train, for example through gear torque reversal.

[0005] In the method of WO 2012 / 139584, the power output is prevented from dropping to low levels (near zero power), but may still be desirable, for example when the wind turbine is in islanded operation. Summary of the Invention

[0006] A first aspect of the invention provides a method of operating a wind turbine at a reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising:

[0007] receiving a derate request indicating a reduced power output level of the wind turbine;

[0008] reducing a power output level of the wind turbine in response to the derate request;

[0009] As the power output level decreases, monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox; and

[0010] In response to detecting a backlash risk condition, the reduction in the power output level is modified to reduce the risk of backlash occurring in the gearbox.

[0011] Optionally, the reduction in the power output level is modified by reversing, stopping or slowing the reduction in the power output level.

[0012] Optionally, the derate request indicates a reduced power output level of the wind turbine below a current operating level, such as below a rated power level, where the rated power level is the designed maximum power output level of the wind turbine.

[0013] Optionally, when the derate request is received, the power output level of the wind turbine is at a rated power level.

[0014] Optionally, when the derate request is received, the wind turbine is in a full load state. In the full load state, the speed of the rotor can be controlled by controlling the pitch of the rotor blades.

[0015] Operation of the wind turbine at a reduced power output level of the wind turbine may be achieved by operating the turbine in a derated full load control operating mode by controlling the pitch of the rotor blades to maintain the power output level at the reduced power output level. In the event that the turbine is operating in the part load control operating mode when a derate request is received, the control mode may be switched to the derated full load control operating mode.

[0016] Optionally, the operating parameter comprises torque.

[0017] The gearbox has a low speed side connected to the rotor and a high speed side connected to the generator; and if the operating parameter includes torque, the torque is optionally on the low speed side of the gearbox or the high speed side of the gearbox.

[0018] Alternatively, the torque is a low-speed side torque obtained by estimation based on the generated power, the generator rotation speed, the combined efficiency, the gear ratio of the gearbox, and the drive train inertia on the high-speed side of the gearbox.

[0019] Optionally, the operating parameter includes power.

[0020] Optionally, a backlash risk condition is detected by an operating parameter crossing a reference value.

[0021] Alternatively, the reference is set based on input conditions. In an embodiment, the reference value may vary based on the input conditions. The reference value may vary in response to changes in conditions such as wind speed or the condition of the wind turbine.

[0022] Optionally, the condition monitoring system monitors conditions of the wind turbine (eg wear of gear teeth or rate of change of torque) and varies the reference value accordingly. In such embodiments, the condition monitoring system determines the reference value based on input conditions in the form of monitored conditions.

[0023] In an embodiment, the input condition may also be based on a determined or estimated energy level of a torque reversal.In such an embodiment, the operating parameter may be torque, and the input condition may be based on a slope of the torque signal.

[0024] Optionally, the method further comprises detecting, by a condition monitoring system, a state of the wind turbine in which gear backlash cannot be tolerated, and performing the method after detecting a change in state.

[0025] Optionally, after the operating parameter crosses the reference value, the reduction in the power output level is modified to return the operating parameter to the reference value.

[0026] Optionally, the method further comprises setting a minimum power reference value, and modifying the reduction of the power output level when the power output level reaches the minimum power reference value.

[0027] Optionally, the minimum power reference value is a minimum saturation limit of the saturation dynamics. Optionally, the method comprises inputting a pre-saturation power reference signal into the saturation dynamics, outputting a power reference signal from the saturation dynamics, and controlling a power output level of the wind turbine based on the power reference signal.

[0028] Optionally, the method further comprises increasing the minimum power reference value.

[0029] Optionally, the method further comprises, in response to the derate request, increasing the minimum power reference value as the power output level of the wind turbine decreases.

[0030] Optionally, the reduction in power output level is modified to prevent the operating parameter from crossing zero.

[0031] Optionally, a backlash risk condition of the gearbox is detected by comparing the operating parameters with a reference.

[0032] Optionally, the operating parameter comprises a running minimum, and the method further comprises obtaining the operating parameter by a running minimum calculation algorithm.

[0033] Optionally, running the minimum calculation algorithm comprises an envelope calculation algorithm that determines a lower envelope of the signal.

[0034] Optionally, the reduction in power is modified based on a difference between the operating parameter and a set point.

[0035] Optionally, the power output level of the wind turbine is reduced to a factor of 10 below the rated power. According to embodiments of the present invention, this factor can be set based on the operating conditions of the wind turbine and can even be a factor of 20 or 100. An advantage of the present invention is that the reduced power level of the wind turbine does not need to be fixed, but can be set according to the operating conditions experienced.

[0036] Optionally, the de-rating request is received from a source external to the wind turbine.

[0037] A further aspect of the invention provides a computer program product comprising software code adapted to operate a wind turbine at a reduced power output when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0038] Yet another aspect of the present invention provides a wind turbine comprising a control system configured to perform the method of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0040] Figure 1 A wind turbine is shown;

[0041] Figure 2A A control system for a wind turbine is shown;

[0042] Figure 2B The general control scheme is shown;

[0043] Figure 3 Elements of a control system configured to reduce the risk of backlash by monitoring torque are shown;

[0044] Figure 4 shows the variations of various torque and power parameters over time;

[0045] Figure 5 Elements of a control system configured to reduce the risk of backlash by monitoring power are shown. DETAILED DESCRIPTION

[0046] Figure 1 A wind turbine 1 is shown comprising a nacelle 3 mounted on a tower 2. Rotors 4, 5 are rotatably mounted to the nacelle 3. The rotor comprises a hub 4 and blades 5 extending from the hub. In this example, the rotor comprises three blades 5. The nacelle 3 can be rotated about a vertical yaw axis to change its yaw angle.

[0047] The wind turbine 1 may be included in a collection of other wind turbines belonging to a wind power plant (also known as a wind farm or wind park), which serves as an electricity generation plant connected to an electric power grid via a transmission line. The electric power grid typically consists of a network of power stations, transmission circuits, and substations connected by a network of transmission lines that transmit electricity to loads in the form of end users and other customers of electric utilities.

[0048] Figure 2AAn embodiment of a control system 20 is schematically shown together with elements of a wind turbine 1. The rotors 4, 5 are mechanically connected to a generator 7 via a gearbox 9. The electricity generated by the generator 7 is injected into an electricity grid 24 via an electrical converter 25. The generator 7 and the converter 25 may be based on a full-scale converter (FSC) architecture or a doubly fed induction generator (DFIG) architecture, but other types may be used.

[0049] The gearbox 9 has a low-speed side connected to the rotor via a low-speed-side shaft 9a and a high-speed side connected to the generator 7 via a high-speed-side shaft 9b.

[0050] The control system 20 includes a plurality of elements, including at least one main controller 10 having a processor and a memory, so that the processor can perform computing tasks based on instructions stored in the memory. Typically, the main controller 10 ensures that the wind turbine generates the requested power output level during operation. This is achieved by adjusting the pitch angle of the blades and / or the power extraction of the converter 25. To this end, the control system 20 includes a pitch system and a power system, the pitch system including a pitch controller 27 using a pitch reference signal 28, and the power system including a power controller 29 using a power reference signal 26. The rotor blades 5 can be pitched by a pitch mechanism. The rotor includes individual pitch systems that can individually pitch the rotor blades 5, and can include a common pitch system that simultaneously adjusts all pitch angles on all rotor blades. The control system 20 or elements of the control system 20 can be placed in a power plant controller (not shown) so that the turbine can be operated based on instructions provided externally.

[0051] Figure 2B A general example is given of how control system 20 may control power output level 50 , rotor speed 51 , and blade pitch angle 52 as wind speed varies between a cut-in wind speed 53 and a cut-out wind speed 54 .

[0052] When the wind is below the rated wind speed 55, the wind turbine operates in a part-load condition, where the rotor speed 51 is controlled by varying the power 50 as shown, and when the wind is above the rated wind speed 55, the wind turbine operates in a full-load condition, where the rotor speed 51 is controlled by controlling the blade pitch angle 52 as shown. In the part-load condition, the power output level 50 is below the rated power level, and in the full-load condition, the power output level 50 is at the rated power level, which is the maximum amount of power the wind turbine is designed to generate.

[0053] In a part load state, the wind turbine may operate in a first constant speed state 56 when the rotor speed 51 is constant, in a variable speed state 57 when the rotor speed 51 varies with wind speed, and in a second constant speed state 58 when the rotor speed 51 is constant.

[0054] In a fully loaded state, the wind turbine may be operated in a constant speed and constant power state 59 , where the rotor speed 51 is constant and the power output level 50 is constant (at a rated power level).

[0055] Thus, in normal operation of the wind turbine, when the wind speed is high enough and the wind turbine is at full load, the requested power output level may be the rated power level, so the wind turbine injects power into the electric power grid 24 at the rated power level. However, in certain circumstances, it may be necessary to operate the wind turbine at a reduced power output that is lower than the power level that can be obtained from the operating conditions (e.g., wind speed). In these circumstances, the requested power output level may include a derate request 22 that indicates a reduced power output level of the wind turbine that is lower than the rated power level.

[0056] Circumstances prompting a derate request 22 may be, for example, overproduction from a power source coupled to the power grid 24, or island operation where a wind turbine operates isolated from the power grid. Another example is a situation where a wind turbine is instructed to operate in spinning reserve for grid stability support.

[0057] The derate request 22 may be received from a source external to the wind turbine, such as an operator of the grid 24 .

[0058] The derate request 22 may indicate the reduced power output level as an absolute power level, a percentage of rated power, or in any other manner.

[0059] The reduced power output level may be significantly lower than the rated power level. For example, the rated power level may be 2 MW and the reduced power output level may be 100 kW (ie 5% of the rated power level).

[0060] When the drive torque (on the low speed side or high speed side of the gearbox) crosses zero torque, backlash may occur within the gearbox 9. Reducing power in response to a derating request carries the risk of such backlash occurring.

[0061] In response to the power level 60 being reduced to the power output level P red, 61, a reduction in the power output level of the wind turbine may be received by the wind turbine controller, with examples of receiving such requests at three wind speeds 60 being shown here. In each case, the reduced power level may be set to the output power level and the turbine is operated by the full load controller in derated full load operation. In one embodiment, in derated full load operation, the full load controller is used and the power output level at the reduced power output level is maintained by controlling the pitch of the blades. In operating regions 58 and 59, the speed of the generator will be set to the rated speed. In region 57, i.e., in the variable speed part load region, different strategies may be implemented to set the generator speed. In one embodiment, the speed may be set to follow (follow) the speed curve 51. As the power is reduced, a higher pitch angle is typically required to maintain the power reduction compared to the pitch angle 52 in normal operation.

[0062] Reference is made to operating a turbine operated by a full load controller in derated full load operation. Derated full load operation may also be referred to as curtailed full load operation.

[0063] Figure 3 Various elements of a control system 20 according to a first embodiment of the present invention are shown that are configured to reduce backlash risk in response to a derating request 22 .

[0064] The main controller 10 generates a pre-saturation power reference signal 36, which is input into the saturation dynamics 35, and the power reference signal 26 is output from the saturation dynamics 35. The saturation dynamics 35 prevents the power reference signal 26 from rising above a power reference maximum saturation limit 37 or falling below a power reference minimum saturation limit 38. The power reference signal 26 is either the same as the pre-saturation power reference signal 36 (when the pre-saturation power reference signal 36 is between the maximum and minimum power reference values) or is limited to the maximum and minimum power reference values (when the pre-saturation power reference signal 36 is above the power reference maximum saturation limit 37 or below the power reference minimum saturation limit 38).

[0065] The power reference maximum saturation limit 37 may be controlled by the main controller 10 via a control signal 40 .

[0066] The power reference minimum saturation limit 38 may be controlled by a feedback controller 33, for example in the form of a PI (Proportional Integral) controller or a PID (Proportional Integral Derivative) controller, as described below.

[0067] Figure 3The control system operates as follows. The master controller 10 receives a derate request 22 indicating a reduced power output level of the wind turbine below the rated power. In response to the derate request, the master controller 10 reduces the power output level of the wind turbine by gradually reducing the power reference maximum saturation limit 37 via a control signal 40. When the reduced power reference maximum saturation limit 37 reaches the level of the pre-saturation power reference signal 36, this causes the power reference signal 26 to decrease.

[0068] After the reduced power reference maximum saturation limit 37 reaches the level of the pre-saturation power reference signal 36 , the wind turbine enters a derated full load operation state, where the wind speed is higher than the rated wind speed but the power output level is lower than the rated power.

[0069] As the wind turbine's power output level decreases in a derated, full-load operating state, control system 20 monitors wind turbine operating parameters to detect a gearbox backlash risk condition. A backlash risk condition may be one in which the gearbox 9 is not yet in a backlash state, but has an increased risk of entering such a state. For example, a high backlash risk may exist when the change in drive torque is greater than the reactive torque from the generator, such that gear torque reversal may occur.

[0070] exist Figure 3 In this case, the operating parameters include torque, which will be explained in more detail below.

[0071] In response to detecting a backlash risk condition, the control system 20 modifies the reduction in power output level (eg, by reversing, stopping, or slowing the reduction of the pre-saturation power reference signal 36 ) to reduce the risk of backlash occurring in the gearbox 9 .

[0072] The control system 20 comprises an operating minimum evaluation unit 30 which takes as its input a driving torque value which may be an estimated torque M on the low speed side of the gearbox. LSS,est (in kNm) or the measured torque M on the low-speed side of the gearbox LSS,meas (in kNm) (obtained from a strain gauge or other sensor on the low-speed side shaft 9a).

[0073] Estimated torque M LSS,est It can be obtained by the following equation 8.

[0074] Equation 1: P mec =P mech,gen +P mech,inertia

[0075] Equation 2: M HSS,est =J HSS ·ω gen

[0076] Equation 3: P mech =M HSS,est ·ω gen

[0077] Equation 4:

[0078] Equation 5:

[0079] Equation 6:

[0080] Equation 7:

[0081] Equation 8: M LSS,est =M HSS,est ·N gear

[0082] in:

[0083] ·P mech is the mechanical power

[0084] ·P gen is the active power of the generator 7 (derived from the generator reaction torque calculated from the power delivered to the grid 24 and the power losses)

[0085] ·P mech,gen is the mechanical power generated from electricity

[0086] ·P mech,inertia is the mechanical power used to accelerate and decelerate the rotor 4,5

[0087] ·J HSS is the total drive train inertia on the high speed side of the gearbox 9

[0088] ·ω gen is the generator rotation speed

[0089] ·M HSS,est is the estimated torque on the high-speed side of the gearbox 9

[0090] ·M LSS,est is the estimated torque on the low speed side of the gearbox 9

[0091] η is the combined efficiency

[0092] · is the acceleration

[0093] ·N gear is the transmission ratio of gearbox 9

[0094] Equation 8 enables the wind turbine to be operated by monitoring various operational dynamic parameters of the wind turbine (e.g., active power P genand the generator rotation speed ω gen ) and calculate them together with various static parameters of the wind turbine (e.g. combined efficiency η, total drive train inertia J on the high speed side of the gearbox HSS and the gearbox transmission ratio N gear ) are input together into Equation 8 to obtain the estimated torque on the low speed side of the gearbox 9.

[0095] In this example, the driving torque value input into the operating minimum evaluation unit 30 indicates torque on the low speed side of the gearbox, but in other embodiments, the driving torque value input into the operating minimum evaluation unit 30 may indicate torque on the high speed side of the gearbox.

[0096] Figure 4 is an indication of the driving torque value M when the power output level of the wind turbine is reduced in response to the derating request LSS,Est Schematic diagram of changes over time.

[0097] As shown in the figure, the operating minimum torque value M output by the operating minimum value evaluation unit 30 is LSS,min Decreases until it crosses the minimum torque reference value M at time t1 ref,min .

[0098] The operating minimum value evaluation unit 30 may use an envelope calculation algorithm to determine the lower envelope of the driving torque signal to obtain the operating minimum torque value M LSS,min This operating minimum torque value can be determined in various ways known to those skilled in the art. As an example, the envelope calculation algorithm can take a series of inputs and maintain each value as its output while the input is less than the previous input, and when the input begins to increase, the output of the envelope calculation algorithm begins to increase slowly (e.g., using a preset slope or using a low-pass filter) until the input begins to decrease again. The figure provides a schematic illustration of this situation.

[0099] The subtraction unit 31 is used to calculate the minimum torque reference value M ref,min Subtract the minimum operating torque value M LSS,min , the subtraction unit 31 generates an error signal 32 which is input to the feedback controller 33 .

[0100] The output of the feedback controller 33 is a power reference minimum saturation limit 38 of the saturation dynamics 35 .

[0101] Minimum torque reference value M ref,min is used as a set point by the feedback controller 33. If the minimum torque value M LSS,min Less than the set point (M ref,min), the power reference minimum saturation limit 38 is increased. When the minimum saturation limit becomes greater than the maximum saturation limit, the minimum saturation limit can be set to overrule (take precedence over) the maximum saturation limit. In this case, depending on the configuration, a warning can be generated or the turbine can be shut down.

[0102] Therefore, the power reference minimum saturation limit 38 is determined by the feedback controller 33 based on the reference value (M ref,min ) and the minimum operating torque value (M LSS,min ) is set by the difference between them.

[0103] When running the minimum torque value M LSS,min At time t1, it drops to the minimum torque reference value M ref,min When , the error signal 32 changes sign from negative to positive.

[0104] Figure 4 Included are traces indicating changes over time of the power reference minimum saturation limit 38 and the power reference signal 26 (when limited from above by the derate control signal 40 ) after receiving the derate request 22 .

[0105] After time t1, the feedback controller 33 increases the power reference minimum saturation limit 38. At time t2, the decreasing power reference signal 26 reaches the increasing power reference minimum saturation limit 38 of the saturation dynamics 35. This causes the power reference signal 26 to stop decreasing and begin increasing in line with the power reference minimum saturation limit 38. The minimum torque reference value M is reached again at time t4. ref,min .

[0106] At time t4, error signal 32 returns to zero, and thus power reference signal 26 stops increasing.

[0107] Therefore, the reduction in the power reference signal 26 (and the corresponding reduction in the power output level of the wind turbine) is modified to reduce the risk of backlash occurring in the gearbox 9. Prior to time t2, the power reference signal 26 is decreasing, but this reduction in the power output level is reversed at time t2 until the error signal 32 reaches zero at time t4.

[0108] The reduction in the power output level is modified at time t2 when the power output level reaches the power reference minimum saturation limit 38. After time t2, the power reference minimum saturation limit 38 continues to increase until time t4, and the power output level of the wind turbine (set by the saturated power reference signal 26) increases in unison with the increased power reference minimum saturation limit 38.

[0109] exist Figure 4 In the example, by operating parameters (minimum torque value M LSS,min ) exceeds the reference value (M ref,min) to detect a backlash risk condition. This backlash risk condition is a condition in which the risk of backlash occurring is relatively high.

[0110] Reference value (M ref,min ) may be constant, or it may vary in response to changes in conditions, such as wind speed or the condition of the wind turbine.

[0111] Optionally, a condition monitoring system monitors the condition of the wind turbine (eg wear of gear teeth or rate of change of torque) and changes the reference value accordingly.

[0112] For example, if the condition monitoring system detects that the rate of change of the gear torque is high (the result of crossing zero is high, so a lower risk is expected), the reference value (M) can be increased by the condition monitoring system. ref,min Alternatively, if the condition monitoring system detects developing damage to the gear teeth, bearings or other elements of the wind turbine, the reference value ((M ref,min ). Alternatively, the reference value (M ref,min ) can change over time (e.g. as a function of wind direction) based on a learning process.

[0113] Reference value based on input conditions (M ref,min ) is shown by box 62.

[0114] In the operating parameters (M LSS,min ) exceeds the reference value (M ref,min ), the reduction in the power output level is modified to return the operating parameter to the reference value (M ref,min ). In this example, the reduction in power output level is reversed at time t2, thereby modifying the reduction in power output level by increasing the power output level. In other embodiments, the reduction in power output level may be modified in different ways, such as by reducing the rate of reduction in power output level to a lower level or to zero. In either case, the reduction in power output level may be modified to prevent the operating parameter (in this case, the resulting torque) from crossing zero.

[0115] Figure 5 Shows something like Figure 3 An alternative control system to the control system of the . Figure 5 Most of the components in Figure 3 These elements are given the same reference numerals and will not be described again.

[0116] exist Figure 5 In this case, the operating parameter that is monitored to reduce the risk of backlash includes power rather than torque.

[0117] The mechanical power reserve value P is obtained by equation 9 mec,reserve :

[0118] Equation 9: P mech,reserve =P gen -P mech,inertia

[0119] exist Figure 5 The input of the operating minimum evaluation unit 30 is the power reserve value P mech,reserve , its output operating minimum power reserve value P mech,reserve,min , the minimum power reserve value P mech,reserve,min The subtraction unit 31 is used to calculate the minimum power reference value P ref,min is subtracted from the error signal to generate the error signal 32.

[0120] If P mech,reserve,min Less than the set point (P ref,min ), the power reference minimum saturation limit 38 increases, and if it is larger, the power reference minimum saturation limit 38 decreases.

[0121] exist Figure 5 In the example, the operating parameters (minimum power reserve value P mech,reserve,min ) exceeds the reference value (P ref,min ) to detect backlash risk conditions.

[0122] Reference value P ref,min can be constant, or it can be related to a reference value M as described above. ref,min change.

[0123] As in Figure 3 In the embodiment of the present invention, the reduction of the power output level is modified to prevent the operating parameter (in this case the operating minimum power reserve value P) from mech,reserve,min ) crosses zero.

[0124] The above method enables the power output level of a wind turbine to be reduced from a rated power level (e.g., 2 MW) to a much lower power output level with low risk of backlash. For example, if operating conditions permit, the power output level can be reduced by a factor of 10 (i.e., to 10% of the rated power level) or more than 10 (e.g., to 5% of the rated power level).

[0125] Feedback controller 33 may operate at all times, or in other embodiments, feedback controller 33 may operate only when the wind turbine is in a condition where gear backlash cannot be tolerated (eg, as indicated by a condition monitoring system of the wind turbine).

[0126] In this case, the method may comprise detecting, by the condition monitoring system, a state of the wind turbine in which gear backlash cannot be tolerated, and performing the method after a change in state has been detected.

[0127] Situations where gear backlash is tolerated (and the feedback controller 33 is disabled by the condition monitoring system) may include: a) when the wind turbine is operating in low wind and therefore at low power, and the energy in the gear torque reversal is low (e.g., the slope of the torque crossing zero is not high); b) in higher winds, the slope crossing zero may also be low and therefore tolerated; c) if there is a critical grid need for low power operation, for example to keep island operation stable.

[0128] Optionally, a condition monitoring system is provided to detect a bearing at risk of failure, or a gear tooth of a gearbox that is in a worn state. In this case, the condition monitoring system may give an indication to the feedback controller 33 that it needs to start operating in order to reduce the risk of backlash.

[0129] Figure 3 or Figure 5 The various elements of the control system shown in are configured to operate the wind turbine at a reduced power output by the method described above. More specifically, the control system 20 includes a computer program product including software code adapted to perform the method when executed on a data processing system. The computer program product is adapted to perform the method described above.

[0130] The computer program product may be provided on a computer readable storage medium or may be downloaded from a communication network.The computer program product may include instructions so that when loaded onto a data processing system, the data processing system (eg in the form of a controller) executes the instructions.

[0131] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A method of operating a wind turbine at a reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising: receiving a derate request indicating a reduced power output level of the wind turbine; reducing a power output level of the wind turbine in response to the derate request; monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox as the power output level decreases; as well as In response to detecting a backlash risk condition, the reduction in the power output level is modified to reduce the risk of backlash occurring in the gearbox.

2. The method according to claim 1, wherein The operating parameters include torque.

3. The method according to claim 2, wherein: The gearbox has a low speed side connected to the rotor and a high speed side connected to the generator; and the operating parameter includes torque on the low speed side of the gearbox or the high speed side of the gearbox.

4. The method according to claim 3, wherein: The torque is a low-speed side torque obtained by estimation based on the generated power, the generator rotation speed, the combined efficiency, the gear ratio of the gearbox, and the drive train inertia on the high-speed side of the gearbox.

5. The method according to claim 1, wherein The operating parameters include power.

6. A method according to any preceding claim, wherein: A backlash risk condition is detected by the operating parameter crossing a reference value.

7. The method according to claim 6, wherein: After the operating parameter crosses the reference value, the reduction in the power output level is modified to return the operating parameter to the reference value.

8. A method according to any preceding claim, further comprising setting a minimum power reference value and modifying the reduction of the power output level when the power output level reaches the minimum power reference value.

9. The method of claim 8, further comprising increasing the minimum power reference value.

10. A method according to any preceding claim, wherein The reduction in the power output level is modified to prevent the operating parameter from crossing zero.

11. A method according to any preceding claim, wherein: A backlash risk condition of the gearbox is detected by comparing the operating parameter with a reference.

12. The method according to claim 11, wherein The reference is set based on input conditions.

13. A method according to any preceding claim, wherein: The operating parameter includes a running minimum, and the method further includes obtaining the operating parameter by a running minimum calculation algorithm.

14. A method according to any preceding claim, wherein: The reduction in power is modified based on a difference between the operating parameter and a set point.

15. A wind turbine comprising a control system configured to perform the method according to any preceding claim.

16. A computer program product comprising software code adapted to operate a wind turbine at a reduced power output when executed on a data processing system, the computer program product being adapted to perform the method according to any one of claims 1 to 14.

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