EP4612412A1

EP4612412A1 - Operating wind turbine at reduced power output

Info

Publication number: EP4612412A1
Application number: EP23801687.7A
Authority: EP
Inventors: Poul Brandt Christensen
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2022-11-02
Filing date: 2023-10-31
Publication date: 2025-09-10
Also published as: WO2024094263A1; CN120457277A

Abstract

A method of operating a wind turbine at reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising: receiving a de-rating request, the de-rating request indicating a reduced power output level of the wind turbine; in response to the de-rating request, reducing the power output level of the wind turbine; as the power output level reduces, monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox; and in response to the detection of a backlash risk condition, modifying the reduction of the power output level to reduce the risk of backlash from occurring in the gearbox.

Description

OPERATING WIND TURBINE AT REDUCED POWER OUTPUT

FIELD OF THE INVENTION

The present invention relates to a method of operating a wind turbine at reduced power output, a wind turbine comprising a control system configured to carry out the method, and a computer program product.

BACKGROUND OF THE INVENTION

WO2012/139584 discloses a method of reducing the power output of a wind turbine. A Minimum Power Setpoint (MPS) of the turbine is dependent on the mean oncoming wind speed. The MPS is the lowest power output (expressed as a percentage of the nominal power) that the turbine is allowed to generate.

At Vmax, a cut-out wind speed, the MPS is 40% of the nominal power for example. This means that if the grid operator wishes to de-rate the turbine when the wind speed is at Vmax, the power can only be reduced to 40% of the nominal power and it cannot be reduced further.

At a lowerwind speed Vx, the MPS is 25% ofthe nominal power, but at wind speeds lowerthan Vx the power cannot be reduced further because of the risk of damaging components in the drive train, for example though gear torque reversals.

In the method of WO2012/139584 a reduction of the power output to a low level (close to zero power) is prevented but may nevertheless be desirable - for example when the wind turbine is in an island operation state.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method of operating a wind turbine at reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising: receiving a de-rating request, the de-rating request indicating a reduced power output level of the wind turbine; in response to the de-rating request, reducing a power output level of the wind turbine; as the power output level reduces, monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox; and in response to the detection of a backlash risk condition, modifying the reduction of the power output level to reduce the risk of backlash from occurring in the gearbox. Optionally the reduction of the power output level is modified by reversing, stopping or slowing the reduction of the power output level.

Optionally the de-rating request indicates a reduced power output level of the wind turbine below a current operating level, such as below a rated power level, wherein the rated power level is a design maximum power output level of the wind turbine.

Optionally the power output level of the wind turbine is at the rated power level when the derating request is received.

Optionally the wind turbine is in a full load state when the de-rating request is received. In the full load state a speed of the rotor may be controlled by controlling a pitch of blades of the rotor.

Operation of the wind turbine at the reduced power output level of the wind turbine may be obtained by operating the turbine in a de-rated full load control operational mode by controlling a pitch of the blades of the rotor to maintain the power output level at the reduced power output level. In a situation where the turbine is operating in a partial load control operational mode when the de-rating request is received, the control mode may be switched to de-rated full load control operational mode.

Optionally the operating parameter comprises a torque.

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

Optionally the torque is a low speed side torque obtained by estimation based on a generated power, a generator rotation speed, a combined efficiency, a gear ratio of the gearbox and a drivetrain inertia on a high speed side of the gearbox.

Optionally the operating parameter comprises a power.

Optionally the backlash risk condition is detected by the operating parameter crossing a reference value.

Optionally the reference is set based on an input condition. In an embodiment, the reference value may be varied based on an input condition. The reference value may be varied in response to a change of a condition, such as a wind speed or a condition of the wind turbine. Optionally a condition monitoring system monitors a condition of the wind turbine (for instance wear of gear teeth, or a rate of change of torque) and varies the reference value accordingly. In such embodiment the condition monitoring system determines the reference value based on an input condition in the form of the monitored condition.

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

Optionally the method further comprises detecting a state of the wind turbine in which gear backlash cannot be tolerated with a condition monitoring system, and performing the method following the detection of the change of state.

Optionally after the operating parameter crosses the reference value, the reduction of the power output level is modified to return the operating parameter to the reference value.

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.

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

Optionally the method further comprises increasing the minimum power reference value.

Optionally the method further comprises increasing the minimum power reference value as the power output level of the wind turbine reduces in response to the de-rating request.

Optionally the reduction of the power output level is modified to prevent a zero-crossing of the operating parameter.

Optionally the backlash risk condition of the gearbox is detected by comparing the operating parameter with a reference.

Optionally the operating parameter comprises a running minimum, and the method further comprises obtaining the operating parameter by a running minimum calculation algorithm. Optionally the running minimum calculation algorithm comprises an envelope calculation algorithm which determines a lower envelope of a signal.

Optionally the reduction of the power is modified based on a difference between the operating parameter and a setpoint.

Optionally the power output level of the wind turbine is reduced by a factor of more than 10 of the rated power. The factor may be set based on operating conditions of the wind turbine in accordance with the embodiments of the present invention, and may even be a factor of 20 or 100. It is an advantage of the present invention that the reduced power level of the wind turbine need not be fixed, but can be set in accordance with the experienced operating conditions.

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

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

A further aspect of the invention provides a wind turbine comprising a control system configured to carry out the method of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a wind turbine;

Figure 2A shows a control system of the wind turbine;

Figure 2B shows a general control scheme;

Figure 3 shows elements of the control system configured to reduce the risk of backlash by monitoring torque;

Figure 4 shows variation of various torque and power parameters over time;

Figure 5 shows elements of the control system configured to reduce the risk of backlash by monitoring power.

DESCRIPTION OF EMBODIMENT(S)

Figure 1 shows a wind turbine 1 comprising a nacelle 3 mounted on a tower 2. A rotor 4, 5 is rotationally 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.

The wind turbine 1 may be included among a collection of other wind turbines belonging to a wind power plant, also referred to as a wind farm or wind park, that serve as a power generating plant connected by transmission lines with a power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities.

Figure 2A schematically illustrates an embodiment of a control system 20 together with elements of the wind turbine 1. The rotor 4, 5 is mechanically connected to an electrical generator 7 via a gearbox 9. The electrical power generated by the generator 7 is injected into a power grid 24 via an electrical converter 25. The electrical 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.

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.

The control system 20 comprises a number of elements, including at least one main controller 10 with a processor and a memory, so that the processor is capable of executing computing tasks based on instructions stored in the memory. In general, the main controller 10 ensures that in operation the wind turbine generates a requested power output level. This is obtained by adjusting the pitch angle of the blades and/or the power extraction of the converter 25. To this end, the control system 20 comprises a pitch system including a pitch controller 27 using a pitch reference signal 28, and a 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 comprises an individual pitch system which is capable of individual pitching of the rotor blades 5, and may comprise a common pitch system which adjusts all pitch angles on all rotor blades at the same time. The control system 20, or elements of the control system 20, may be placed in a power plant controller (not shown) so that the turbine may be operated based on externally provided instructions.

Figure 2B gives a general example of how the control system 20 may control a power output level 50, a rotor speed 51 and a blade pitch angle 52 as the wind speed varies between a cutin wind speed 53 and a cut-out wind speed 54.

When the wind is below a rated wind speed 55 the wind turbine operates in a partial load state in which the rotor speed 51 is controlled by varying the power 50 as shown, and above the rated wind speed 55 the wind turbine operates in a full load state in which the rotor speed 51 is controlled by controlling the blade pitch angle 52 as shown. In the partial load state the power output level 50 is below a rated power level, and in the full load state the power output level 50 is at a rated power level, which is a maximum amount of power that the wind turbine is designed to generate.

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

In the full load state, the wind turbine may operate in a constant speed and constant power state 59 in which the rotor speed 51 is constant and the power output level 50 is constant (at the rated power level).

Therefore in normal operation of the wind turbine, when the wind speed is sufficiently high and the wind turbine is in the full load state, the requested power output level may be the rated power level, so the wind turbine injects power into the power grid 24 at the rated power level. However, in certain circumstances it may be necessary to operate the wind turbine at reduced power output, below the power level available from the operating conditions, e.g. wind speed. In these circumstances the requested power output level may comprise a de-rating request 22, the de-rating request 22 indicating a reduced power output level of the wind turbine below the rated power level.

The circumstances which prompt the de-rating request 22 may be, for example over-production from power sources coupled to the power grid 24, or an island operation in which the wind turbine is operating in isolation from the power grid. Another example is a situation where the wind turbines are instructed to operate with a spinning reserve for grid stability support.

The de-rating request 22 may be received from a source external to the wind turbine, such as an operator of the grid 24.

The de-rating request 22 may indicate the reduced power output level as an absolute power level, as a percentage of the rated power, or in any other way.

The reduced power output level may be significantly lower than the rated power level. By way of example the rated power level may be 2MW and the reduced power output level may be 100kW (i.e. 5% of the rated power level). Backlashing can occur within the gearbox 9 when the driving torque (on the low or high speed side of the gear box) crosses zero torque. Reducing the power in response to a de-rating request brings a risk of such a backlash occurring.

The reducing of the power output level of the wind turbine in response to the de-rating request from the operating power level 60 to a reduced power output level Pred, 61 may be received by the wind turbine controller, here shown with examples receiving such request at three wind speeds 60. In each case, the reduced power level may be set as the output power level and the turbine operated by the full-load controller in de-rated full load operation. In an embodiment, in de-rated full load operation the full load controller is used and the power output level at the reduced power output level is maintained by controlling a pitch of the blades. In operation region 58 and 59 the speed of the generator will be set to the rated speed. In region 57, i.e. in the variable speed partial load region, different strategies may be implemented in order to set the generator speed. In one embodiment the speed may be set to follow the speed curve 51. As the power is reduced a higher pitch angle is typically needed to keep the power down compared to the pitch angle shown 52 for the normal operation situation.

Reference is made to operating the turbine operated by the full-load controller in de-rated full load operation. De-rated full load operation may also be referred to as curtailed full load operation.

Figure 3 shows various elements of the control system 20 which are configured to reduce the risk of backlash when responding to a de-rating request 22, according to a first embodiment of the invention.

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

The power reference maximum saturation limit 37 may be controlled by the main controller 10 via a control signal 40. The power reference minimum saturation limit 38 may be controlled by a feedback controller 33, e.g. in the form of a PI (proportional-integral) controller or a PID (proportional-integral- derivate) controller as explained below.

The control system of Figure 3 operates as follows. The main controller 10 receives a de-rating request 22, the de-rating request 22 indicating a reduced power output level of the wind turbine below the rated power. In response to the de-rating request, the main controller 10 reduces the power output level of the wind turbine by gradually reducing the power reference maximum saturation limit 37 via the control signal 40. This causes the power reference signal 26 to reduce when the reducing power reference maximum saturation limit 37 reaches the level of the presaturation power reference signal 36.

After the reducing power reference maximum saturation limit 37 reaches the level of the presaturation power reference signal 36, the wind turbine enters into a de-rated full load operation state, in which the wind speed is above the rated wind speed but the power output level is below the rated power.

As the power output level of the wind turbine reduces in the de-rated full load operation state, the control system 20 monitors an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox. The backlash risk condition may be a condition in which the gearbox 9 is not yet in a state of backlash, but is at increased risk of entering such a state. For example there may be a high risk of backlash when variations in the driving torque are larger than the counter torque from the generator, such that gear torque reversals can occur.

In the case of Figure 3, the operating parameter comprises a torque, as will be explained in more detail below.

In response to the detection of a backlash risk condition, the control system 20 modifies the reduction of the power output level (for example by reversing, stopping or slowing the reduction of the pre-satu ration power reference signal 36) to reduce the risk of backlash from occurring in the gearbox 9.

The control system 20 comprises a running minimum evaluation unit 30 which takes as its input a driving torque value, which may be an estimated torque M_{LSS est} (in kNm) on the low speed side of the gearbox or a measured torque M_LSSimeas (in kNm) on the low speed side of the gearbox (obtained by strain gauges or other sensors on the low speed side shaft 9a).

The estimated torque M_LSS,_est may be obtained by Equation 8 below. where:

• P-mech is ^a mechanical power

• P_genis an active power of the generator 7 (stemming from a generator reaction torque calculated from the power delivered to the grid 24 and power losses)

• Pmech,gen'^{s a} mechanical power stemming from the generation of electric power

• ^pmec , inertia is ^a mechanical power used for acceleration and deceleration of the rotor 4, 5

• J_HSS is a total drivetrain inertia on a high speed side of the gearbox 9

• M_gen is a generator rotation speed

• ^MHss,est is ^an estimated torque on a high speed side of the gearbox 9

• ^MLss,est is ^an estimated torque on a low speed side of the gearbox 9

• 77 is a combined efficiency

• acceleration

• Ngear is a gear ratio of the gearbox 9

Equation 8 enables an estimated torque on a low speed side of the gearbox 9 to be obtained by monitoring various operating dynamic parameters of the wind turbine (for example active power P_gen and generator rotation speed M_gen) and inputting them into Equation 8 along with various static parameters of the wind turbine (for example combined efficiency 77, total drivetrain inertia J_HSS on a high speed side of the gearbox, and gear ratio N_gear of the gearbox).

In this example the driving torque value input into the running minimum evaluation unit 30 is indicative of torque on the low speed side of the gearbox, but in other embodiments the driving torque value input into the running minimum evaluation unit 30 may be indicative of torque on the high speed side of the gearbox. Figure 4 is a schematic sketch indicating variation of the driving torque value M_LSS,_Est over time as the power output level of the wind turbine reduces in response to the de-rating request.

A running minimum torque value Mi_ss,min output by the running minimum evaluation unit 30 reduces as shown, until it crosses a minimum torque reference value Mref,min at time t1 .

The running minimum evaluation unit 30 may use an envelope calculation algorithm to determine the lower envelope of the driving torque signal, to obtain the running minimum torque value Mi_ss,min. Such running minimum torque value may be determined in different ways as is known by the skilled person. As an example, the envelope calculation algorithm may take a series of inputs and keep each value as its output when the input is less than the previous input and when the inputs start increasing, the output of the envelope calculation algorithm starts increasing slowly (using a preset ramp or using a low-pass filter, for example) until the inputs start decreasing again. The figure provides a schematic illustration of this.

The running minimum torque value Mi_ss,min is subtracted from the minimum torque reference value Mref,min by a subtraction unit 31 which generates an error signal 32 which is input to the feedback controller 33.

The output of the feedback controller 33 is the power reference minimum saturation limit 38 of the saturation dynamic 35.

The minimum torque reference value Mref,min is used by the feedback controller 33 as a setpoint. If the running minimum torque value Mi_ss,min is less than the setpoint (/Wretmm) then the power reference minimum saturation limit 38 is increased. When the minimum saturation limit becomes larger than the maximum saturation limit, the minimum saturation limit may be set to overrule the maximum saturation limit. In such a situation a warning may be generated or the turbine may be shutdown, depending on configuration.

Thus the power reference minimum saturation limit 38 is set by the feedback controller 33 based on the difference between a reference value (Mimin') and a running minimum torque value (MLSS,min) .

When the running minimum torque value Mi_ss,min drops below the minimum torque reference value Mref,min at time t1 , the error signal 32 changes sign from negative to positive.

Figure 4 includes traces indicating variation of the power reference minimum saturation limit 38 and the power reference signal 26 (when limited from above by the reducing control signal 40) over time following receipt of a de-rating request 22. After time t1 , the power reference minimum saturation limit 38 is increased by the feedback controller 33. At time t2, the decreasing power reference signal 26 reaches the increasing power reference minimum saturation limit 38 of the saturation dynamic 35. This causes the power reference signal 26 to stop decreasing, and start increasing in line with the power reference minimum saturation limit 38. The minimum torque reference value Mref,min is reached again at time t4.

At time t4, the error signal 32 returns to zero so the power reference signal 26 stops increasing.

Thus the reduction of the power reference signal 26 (and the consequential reduction of the power output level of the wind turbine) is modified to reduce the risk of backlash from occurring in the gearbox 9. Before time t2, the power reference signal 26 reduces, but this reduction of power output level is reversed at time t2 until the error signal 32 reaches zero at time t4.

The reduction of 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 (as set by the saturated power reference signal 26) increases in line with the increasing power reference minimum saturation limit 38.

In the example of Figure 4, the backlash risk condition is detected by the operating parameter (running minimum torque value Mi_ss,min) crossing a reference value (/Wretmm). This backlash risk condition is a condition in which the risk of backlash occurring is relatively high.

The reference value (/Wretmm) may be constant, or it may be varied in response to a change of a condition, such as a wind speed or a condition of the wind turbine.

Optionally a condition monitoring system monitors a condition of the wind turbine (for instance wear of gear teeth, or a rate of change of torque) and varies the reference value accordingly.

For example the reference value (Mimin)' may be increased by the condition monitoring system if the condition monitoring system detects that a rate of change of the gear torque is high (then the consequence of a zero-crossing is higher and thus a lower risk is desirable). Alternatively the reference value (M_re/;mm) may be increased if the condition monitoring system detects evolving damage to gear teeth, bearings or other elements of the wind turbine. Alternatively the reference value (M_ref,mm) may change over time based on a learning process, for example as a function of wind direction.

The optional setting of the reference value (Mref.mm) based on an input condition is shown by box 62. After the operating parameter (Mtss.mm) crosses the reference value (Mref.min) , the reduction of the power output level is modified to return the operating parameter to the reference value (Mref.min) . In this example the reduction of the power output level is reversed at time t2, so the reduction of the power output level is modified by increasing the power output level. In other embodiments the reduction of the power output level may be modified in a different way - for instance by reducing a rate of reduction of the power output level to a lower level or to zero. In either case the reduction of the power output level may be modified to prevent a zero-crossing of the operating parameter (in this case the resulting torque).

Figure 5 shows an alternative control system which is similar to the control system of Figure 3. Most elements of Figure 5 have equivalents in Figure 3. These elements are given the same reference number and will not be described again.

In the case of Figure 5, the operating parameter which is monitored to reduce backlash risk comprises a power rather than a torque.

A mechanical power reserve value P_meCh, reserve is obtained by equation 9:

Equation 9. Pmech, reserve ~ Pgen Pmech, inertia

In Figure 5 the input to the running minimum evaluation unit 30 is the power reserve value Pmech, reserve which outputs a running minimum power reserve value P me ch, reserve,min which is subtracted from a minimum power reference value Pref, min by the subtraction unit 31 to generate the error signal 32.

If Pmech, reserve, min is less than the setpoint (Pref, min) then the power reference minimum saturation limit 38 is increased, and if it is larger then the power reference minimum saturation limit 38 is decreased.

In the example of Figure 5, the backlash risk condition is detected by the operating parameter (running minimum power reserve value Pmech, reserve, min crossing a reference value (Pref, min) .

The reference value Pref, min may be constant, or it may be varied as described above in relation to the reference value Mref,min.

As in the embodiment of Figure 3, the reduction of the power output level is modified to prevent a zero-crossing of the operating parameter (in this case the running minimum power reserve Value Pmech .reserve. min) . The methods described above enable the power output level of the wind turbine to be reduced from a rated power level (for example 2MW) to a much lower power output level, with low risk of backlash. For example, if operating conditions allow it, the power output level may be reduced by a factor of 10 (i.e. to 10% of the rated power level) or by a factor of more than 10 (for instance to 5% of the rated power level).

The feedback controller 33 may operate at all times, or in other embodiments the feedback controller 33 may only operate when the wind turbine is in a state in which gear backlash cannot be tolerated - for instance as indicated by the condition monitoring system of the wind turbine.

In this case the method may comprise detecting a state of the wind turbine in which gear backlash cannot be tolerated with the condition monitoring system, and performing the method following the detection of the change of state.

Situations in which 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 thus low power, and the energy in the gear torque reversals is low (for example the slope of the torque when zero crossing is not high); b) in higher wind the slope of the zero crossing can also be low and thus tolerated; c) if there is a critical grid need for low power operation, for example to keep island operation stable.

Optionally the condition monitoring system is set up to detect a bearing at risk of failure, or that gear teeth of the gearbox are 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.

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

The computer program product may be provided on a computer readable storage medium or be downloadable from a communication network. The computer program product may comprise instructions to cause a data processing system, e.g. in the form of a controller, to carry out the instructions when loaded onto the data processing system. 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 reduced power output, the wind turbine comprising a rotor coupled to a gearbox, the method comprising: receiving a de-rating request, the de-rating request indicating a reduced power output level of the wind turbine; in response to the de-rating request, reducing a power output level of the wind turbine; as the power output level reduces, monitoring an operating parameter of the wind turbine to detect a backlash risk condition of the gearbox; and in response to the detection of a backlash risk condition, modifying the reduction of the power output level to reduce the risk of backlash from occurring in the gearbox.

2. A method according to claim 1 , wherein the operating parameter comprises a torque.

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

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

5. A method according to claim 1 , wherein the operating parameter comprises a power.

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

7. A method according to claim 6, wherein after the operating parameter crosses the reference value, the reduction of 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. A method according to claim 8, further comprising increasing the minimum power reference value.

10. A method according to any preceding claim, wherein the reduction of the power output level is modified to prevent a zero-crossing of the operating parameter.

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

12. A method according to claim 11 , wherein the reference is set based on an input condition.

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

14. A method according to any preceding claim, wherein the reduction of the power is modified based on a difference between the operating parameter and a setpoint.

15. A wind turbine comprising a control system configured to carry out the method of any preceding claim.

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