Method for operating a wind turbine and wind turbine

Abstract

According to an embodiment, the method is for operating a wind turbine (100) having a rotor (10) with at least two rotor blades (1, 2, 3) and a pitch adjustment system (500) for adjusting the pitch angles (β_i) of the at least two rotor blades. The method comprises a step of determining first information (I1) which is representative of the appearance of a failure in the pitch adjustment system while the rotor is rotating. In a further step, second information (I2) is provided which is representative of the actual rotational speed (n_a) of the rotor. Then, third information (I3) is determined depending on the first and the second information, wherein the third information is representative of a pitch angle setpoint (SP_β_i) for at least one rotor blade. The pitch angle setpoint depends on the actual rotational speed and is chosen to reduce the rotational speed of the rotor. When the wind turbine is operated according to the third information, a pitch angle of at least one rotor blade is adjusted to the pitch angle setpoint.

Description

[0001] P2023,1440 WO N / PA 1253 WO May 15, 2025- 1 -Description Method for operating a wind turbine and wind turbine The present disclosure relates to a method for operating awind turbine. Furthermore, the disclosure relates to acomputer program, a computer-readable data carrier, a control device and a wind turbine. Wind turbines are widely known and are used to convert wind energy into electrical energy. In case of a failure in the wind turbine, e.g. a failure in the pitch adjustment system thereof, it might be necessary to stop the wind turbine. One object to be achieved is to provide a method which contributes to a safe and efficient operation of the wind turbine, particularly in the case of a failure in the pitch adjustment system. Further objects to be achieved are to provide a computer program, a computer-readable data carrier, a control device and a wind turbine for executing such a method. First, the method for operating a wind turbine is specified. According to an embodiment, the method is for operating a wind turbine having a rotor with at least two rotor bladesand a pitch adjustment system for adjusting the pitch anglesof the at least two rotor blades. The method comprises a stepof determining first information which is representative ofthe appearance of a failure in the pitch adjustment systemwhile the rotor is rotating. In a further step, second information is provided which is representative of the actual rotational speed of the rotor. Then, third information isP2023,1440 WO N / PA 1253 WO May 15, 2025- 2 -determined depending on the first and the second information, wherein the third information is representative of a pitchangle setpoint for at least one rotor blade. The pitch anglesetpoint depends on the actual rotational speed and is chosento reduce the rotational speed of the rotor. When the wind turbine is operated according to the third information, a pitch angle of at least one rotor blade is adjusted to the pitch angle setpoint. When a failure appears in the pitch adjustment system of awind turbine, the pitch angle of one or more rotor blades maynot be appropriate for the wind conditions anymore, which maylead to an overloading and thus to a damage of the windturbine components. In order to avoid such damage, the windturbine must be slowed down or even stopped in a fast manner,e.g. via an emergency stopping procedure. This can be done,for example, by pitching the rotor blades towards or intotheir feathering positions. The present invention is based,inter alia, on the recognition that increasing the pitchangles of the rotor blades towards the feathering positionsin case of appearance of a pitch adjustment system failure,e.g. during an emergency stopping procedure, can induce ahigh aerodynamic imbalance in the rotor during rotation. Particularly, this happens if a pitch angle differencebetween two rotor blades becomes large due to one rotor bladebeing stuck or only movable slowly and only the remainingrotor blade(s) can be set towards the feathering position(s).Such a high aerodynamic imbalance causes high loads on the components and support structures arranged in the nacelle. In particular, a high aerodynamic imbalance causes high loads inthe yaw system which could lead to slippage and damage of theyaw drives or the yaw gears. The aerodynamic imbalance canP2023,1440 WO N / PA 1253 WO May 15, 2025- 3 -become particularly high when the rotor is still rotating athigh speeds. The present disclosure provides, inter alia, a method, in which slowing down of the rotor is achieved by taking into account the aerodynamic imbalance, for example, by exploitingthe maximum allowable imbalance. Indeed, by determining thepitch angle setpoint depending on the actual rotational speedof the rotor, the aerodynamic imbalance is considered andcontrollable when slowing down the rotation of the rotor.The method specified herein is, in particular, a computer- implemented method, i.e. is performed with the help of acomputer or a processor. For example, the method isexecutable by a control device of the wind turbine, such as a main controller of the wind turbine, also called “turbine controller”. Herein, when information is representative of a certain quantity or certain quantities, this means that the quantity or quantities can be extracted from the information, either directly, or the quantity / quantities can at least be derived from the information. In other words, the quantity / quantities is / are stored in the information, or at least data are stored in the information, from which the quantity / quantities can be derived or determined or calculated, respectively. Furthermore, here and in the following, information is, in particular, electronic information, like electronic data. A setpoint herein defines a certain target to be achievedwhen operating the wind turbine. For example, the pitch anglesetpoint is the target value of the pitch angle of a rotorblade. The realization of the pitch angle to become the pitchP2023,1440 WO N / PA 1253 WO May 15, 2025- 4 -angle setpoint can be done with help of a controller, e.g. ina closed feedback loop. The wind turbine comprises a pitch adjustment system. The pitch adjustment system is configured to change or set oradjust the pitch angles of the at least two rotor blades. Forexample, the pitch adjustment system comprises, for eachrotor blade of the rotor, a pitch bearing for movablysupporting the rotor blade and a pitch drive for driving the movement of the rotor blade around the pitch bearing axis.The pitch drives may be electromechanical or hydraulicdrives. For example, the pitch drives comprise electricmotors. For example, in addition to the pitch bearings andthe pitch drives, the pitch adjustment system comprises a control device, e.g. a so-called “pitch controller”, which controls the pitch drives so that the pitch angles of the at least two rotor blades are brought to the pitch angle setpoint(s). In another example, the pitch adjustment system comprises a pitch bearing, a pitch drive and a control device, e.g. a so-called “pitch controller”, for each rotorblade of the rotor. The pitch adjustment system may beconfigured to adjust the pitch angles of all rotor bladesindividually, so-called individual pitch control, IPC forshort, or may only be able to adjust the pitch angles of all rotor blades collectively, so-called collective pitch control, CPC for short. The first information is representative of the appearance ofa failure in the pitch adjustment system. For example, thefirst information is representative of whether a failure in the pitch adjustment system appears or if everything is ok. The first information may be determined depending on measurements. The measurements may be taken with the help ofP2023,1440 WO N / PA 1253 WO May 15, 2025- 5 -at least one sensor, e.g. incremental encoders assigned tothe rotor blades. The second information is representative of the actualrotational speed of the rotor. The second information may bedetermined depending on measurements. The measurements may betaken with the help of at least one sensor. The sensor may bean incremental encoder assigned to the rotor, for example.The third information is determined depending on the firstand the second information and is representative of a pitchangle setpoint for at least one rotor blade. For example, if the first information is representative of the appearance ofa failure in the pitch adjustment system, the thirdinformation is determined such that the pitch angle setpointof the at least one rotor blade depends on the rotationalspeed and is selected to reduce the rotational speed. Forexample, the pitch angle setpoint is determined such that the pitch angle of the at least one rotor blade is increased towards the feathering position. Here and in the following, the expression “if” is herein meant to introduce a necessary condition. It may but does have to introduce a sufficient condition.The third information may be representative of two or morepitch angle setpoints for different rotor blades. Alternatively, the pitch angle setpoint is common for two or more rotor blades. For example, if one of the rotor blades isdamaged or hindered or impaired in its movement or is stuck,the pitch angle setpoint(s) of the third information is atleast for the one or more other (movable) rotor blades.P2023,1440 WO N / PA 1253 WO May 15, 2025- 6 -The pitch angle setpoint depends on the actual rotationalspeed, i.e. the rotational speed the rotor has at the moment of determining the third information. For example, the pitchangle setpoint increases with decreasing rotational speed.Moreover, the pitch angle setpoint is chosen to reduce therotational speed of the rotor. For example, the pitch anglesetpoint is chosen to increase in order to reduce therotational speed. Providing the second information and determining the third information is, for example, done repeatedly or continuously. Accordingly, the third information or the pitch anglesetpoint, respectively, may be repeatedly or continuouslyupdated according to the current actual rotational speed. Thepitch angle setpoint may thereby repeatedly or continuously increased as a function of time.When the wind turbine, particularly when the pitch adjustmentsystem, is operated according to the third information, the pitch angle of at least one rotor blade is adjusted to the pitch angle setpoint. In other words, the third information is an operating information for the wind turbine or the pitchadjustment system, respectively, and is configured to causean operation of the wind turbine or the pitch adjustment system such that the pitch angle of at least one rotor blade is brought to the pitch angle setpoint. For example, if the third information is determined by theturbine controller, it may then be sent to a control deviceof the pitch adjustment system, e.g. to the so-called “pitchcontroller”, which then operates the pitch adjustment systemaccording to the third information. Alternatively, the thirdP2023,1440 WO N / PA 1253 WO May 15, 2025- 7 -information could be directly determined by the pitch controller. According to a further embodiment, the failure in the pitch adjustment system is a fault event in which at least one ofthe rotor blades is hindered or impaired, respectively, inits mobility (in the following also called “impaired rotorblade”). At least one other rotor blade is, however, stillmovable as intended (in the following called “movable-as-intended rotor blade)”. In other words, the fault event is an event in which at least one but not all rotor blades arehindered in its mobility and at least one rotor blade but notall rotor blades is still working correctly. That a rotorblade is hindered in its mobility can mean that the rotorblade is stuck, i.e. cannot be moved around this pitchbearing axis anymore, or that it can only be moved much moreslowly than intended. Examples of fault events in which themobility of a rotor blade is hindered or impaired may be a damage or malfunction of one or more of a pitching bearing, a pitch drive and a pitch controller.According to a further embodiment, the pitch angle setpointis for the at least one movable-as-intended rotor blade andis chosen such that the difference between the pitch anglesetpoint and the pitch angle of the at least one impairedrotor blade is lower or equal to a maximum allowable pitchangle difference. The maximum allowable pitch angledifference depends on the actual rotational speed of therotor. For example, the maximum allowable pitch angle difference decreases with increasing actual rotational speedof the rotor. For example, the pitch angle setpoint is chosensuch that the difference between pitch angle setpoint and thepitch angle of the at least one impaired rotor blade followsP2023,1440 WO N / PA 1253 WO May 15, 2025- 8 -or equals the maximum allowable pitch angle difference as afunction of the actual rotational speed.Indeed, in the case of an impaired rotor blade, the movementof the at least one movable-as-intended rotor blade towardsthe feathering position causes a large difference between thepitch angles of the rotor blades. This, however, creates theabove-mentioned aerodynamic imbalance which might harm theyaw system. By defining the maximum allowable pitch angledifference depending on the rotational speed of the rotor,the imbalance can be kept in an acceptable range. For example, in order to determine the third information or the pitch angle setpoint for the at least one movable-as-intendedrotor blade, respectively, a lookup table is used. The lookuptable may have been generated offline by postprocessing ofsimulations with static wind speeds, rotor speeds and pitchangles, so-called rotor maps. In another example, thedetermination of the third information or the pitch angle setpoint for the at least one movable-as-intended rotorblade, respectively, is performed in real-time based on loadcalculations, wherein the load calculations consist incalculating in real-time the loads acting on the wind turbine components based on current sensor data. The third information may also be representative of a pitch angle setpoint of the at least one impaired rotor blade. For example, the pitch angle setpoint for the at least oneimpaired rotor blade is equal to the pitch angle setpoint forthe movable as intended rotor blade or the same pitch angle setpoint is used. This has the advantage that, if the failure disappears, e.g. the impaired rotor blade becomes loose, it is adjusted to the same pitch angle as the other rotor blades and the imbalance reduces.P2023,1440 WO N / PA 1253 WO May 15, 2025- 9 -According to a further embodiment, the wind turbine further comprises a generator coupled to the rotor in order to convert mechanical power into electrical power. Particularly, the generator is configured to convert the rotation of therotor into electrical power. The generator can either bedirectly coupled to the rotor or indirectly via, for example, a gearbox. According to a further embodiment, the method furthercomprises a step of determining fourth information dependingon the first and the second information. The fourthinformation is representative of a generator torque setpoint.The generator torque setpoint depends on the actualrotational speed of the rotor and is chosen to reduce therotational speed of the rotor. Alternatively, the generatortorque setpoint is chosen to reduce the rotational speed ofthe rotor while ensuring that a maximum allowable generatortorque is not exceeded, wherein the maximum allowablegenerator torque depends on the actual rotational speed. Inparticular, the fourth information is determined as specifiedif the first information is representative of a failure in the pitch adjustment system.The generator torque setpoint or the maximum allowablegenerator torque may increase with increasing rotationalspeed of the rotor. For example, the generator torquesetpoint or the maximum allowable generator torque is chosento be greater than a nominal generator torque of the windturbine. The generator torque setpoint or the maximumallowable generator torque may exceed the nominal generatortorque by at least 5% and / or at most 20%, for example. By wayof example, the generator torque setpoint or the maximumP2023,1440 WO N / PA 1253 WO May 15, 2025- 10 -allowable generator torque is chosen to be greater than thenominal generator torque for a limited time, e.g. of at least5 s. After that, the generator torque setpoint or the maximum allowable generator torque is chosen, for example, to beequal to or lower than the nominal generator torque.According to a further embodiment, the fourth information isdetermined such that, when the wind turbine is operatedaccording to the fourth information, a generator torque ofthe generator is adjusted to the generator torque setpoint.In other words, the fourth information is an operatinginformation which is configured to cause an operation of the wind turbine such that the generator torque is brought to thegenerator torque setpoint. For example, the fourthinformation is sent to or determined by a generator torqueadjustment system coupled to the generator in order to adjustthe generator torque. For example, the generator torqueadjustment system comprises a generator torque controller.For example, the generator torque adjustment system additionally comprises a generator side converter coupled to the generator, in which case the generator torque controller is (part of) a generator side converter controller.Increasing the generator torque setpoint is another measurewhich enables to slow down the rotor rotation. However, inorder to not damage the generator or the electrical systemsor a gearbox between the rotor and the generator or to notviolate safety regulation, the generator torque must be keptwithin certain limits or must not exceeds these limits bymore than a limited time. The torque limit may also changewith the rotational speed of the rotor. Thus, since thegenerator torque cannot be made arbitrarily high in order to brake the rotation of the rotor, it is particularlyP2023,1440 WO N / PA 1253 WO May 15, 2025- 11 -advantageous to additionally use the aerodynamic braking by adjusting the pitch angles(s).According to a further embodiment, the third and the fourthinformation are determined such that, when the wind turbineis operated according to the third and the fourthinformation, the adjustment of the pitch angle of the atleast one rotor blade to the pitch angle setpoint and theadjustment of the generator torque to the generator torquesetpoint are done simultaneously. This turned out to beparticularly efficient in slowing down the rotor rotationand, at the same time, to protect the wind turbine againstdamage. According to a further embodiment, the method furthercomprises a step of determining fifth information dependingon the first and the second information. The fifthinformation is representative of a pitch angle change rate,also called “pitch speed”, wherein the pitch angle changerate depends on the actual rotational speed of the rotor anddefines the maximum speed with which the pitch angle of at least one rotor blade is adjusted to the pitch angle setpointor defines the speed with which the pitch angle setpoint ischanged. For example, the fifth information is determined asspecified, if the first information is representative of a failure in the pitch adjustment system.The sensitivity of the wind turbine loading to changes inoperational parameters typically depends on the rotational speed of the rotor. For example, a rapid change of pitch angle at a high rotational speed of the rotor, e.g. at a rotational speed close or equal to the nominal rotationalspeed, may cause a higher aerodynamic imbalance on the rotorP2023,1440 WO N / PA 1253 WO May 15, 2025- 12 -than at a low rotational speed of the rotor. Therefore, itcan help to change the pitch angle change rate depending onthe actual rotational speed of the rotor. For example, thepitch angle change rate increases with decreasing rotationalspeed of the rotor. The pitch angle change rate as a functionof the actual rotational speed of the rotor may be astaircase function with two constant values of the pitchangle change rate.The fifth information is, in particular, an operatinginformation, e.g. for the pitch adjustment system, so that,when the wind turbine or the pitch adjustment system isoperated according to the third and the fifth information,the pitch angle of at least one rotor blade is adjusted tothe pitch angle setpoint, wherein the pitch angle setpointchanges with the pitch angle change rate or the pitch angleof at least one rotor blade is adjusted to the pitch anglesetpoint with a speed which is equal to or lower than thepitch angle change rate. As mentioned above, the third information may be determined repeatedly or continuously. Accordingly, the pitch anglesetpoint may be changed repeatedly or continuously due to achange in the actual rotational speed of the rotor. The adjustment of the pitch angle of the at least one rotor blade to the pitch angle setpoint usually happens much faster thanthe change in the pitch angle setpoint caused by the changeof the rotational speed of the rotor. According to a further embodiment, the rotor comprises at least three rotor blades. In this case, the above-mentionedfault event is, in particular, an event in which two or morerotor blades are movable as intended and one or more rotorP2023,1440 WO N / PA 1253 WO May 15, 2025- 13 -blades are hindered in their mobility, e.g. are stuck. Thethird information is then, in particular, representative ofthe pitch angle setpoint(s) for the at least two movable-as-intended rotor blades. Likewise, the fifth information maythen be representative of the pitch angle change rate(s) ofthe at least two movable-as-intended rotor blades.According to a further embodiment, the third information isdetermined such that, when the wind turbine is operatedaccording to the third information, the pitch angles of atleast two movable-as-intended rotor blades are synchronized.That is, the pitch angle setpoints for the two movable-as- intended rotor blades are equal or the same pitch anglesetpoint is used for both rotor blades. Particularly,collective pitch control (CPC) is applied for the at leasttwo moveable-as-intended rotor blades. According to a further embodiment, the wind turbine is operable in a first operating mode and a second operating mode. The first operating mode is a mode in which the pitch angles of the rotor blades are controlled individually (individual pitch control, IPC) and the second operating mode is a mode in which the pitch angles of the rotor blades are controlled collectively (collective pitch control, CPC). “Controlled individually” means, in particular, that thepitch angle setpoints are determined individually for thedifferent rotor blades and can assume different values.Controlled collectively means that the pitch angle setpoints are determined to be equal or the same pitch angle setpoint is used for all rotor blades. According to a further embodiment, if the wind turbine isoperated in the first operating mode when the firstP2023,1440 WO N / PA 1253 WO May 15, 2025- 14 -information is determined to be representative of a failurein the pitch adjustment system, the method further comprisesa step of generating a switch command which is configured tocause a transition from the first operating mode into the second operating mode. In other words, it is switched fromIPC to CPC. Indeed, by switching into the second operatingmode in the case of a failure in the pitch adjustment system,the aerodynamic imbalance caused by the individual pitch control can be avoided. According to a further embodiment, determining the first information comprises determining whether the pitch angle ofone rotor blade differs from the pitch angle of at least oneother rotor blade by more than a defined threshold.According to a further embodiment, the generator torquesetpoint or the maximum allowable generator torque increaseswith increasing actual rotational speed. In other words, thegenerator torque setpoint or the maximum allowable generatortorque used in the case of a failure in the pitch adjustmentsystem is an increasing function of the actual rotationalspeed. For example, the generator torque setpoint or themaximum allowable generator torque increases monotonically orstrictly monotonically with increasing rotational speed ofthe rotor. The generator torque setpoint or the maximumallowable generator torque as a function of the actual rotational speed may be determined from nominal values of thedifferent power modes of the wind turbine, for example.According to a further embodiment, the maximum allowablepitch angle difference decreases with increasing actualrotational speed of the rotor. In other words, the maximumallowable pitch angle difference used in the case of aP2023,1440 WO N / PA 1253 WO May 15, 2025- 15 -failure in the pitch adjustment system is a decreasingfunction of the actual rotational speed. For example, itdecreases in a monotonical or strictly monotonical manner.According to the further embodiment, the pitch angle setpoint is chosen to reduce the rotational speed of the rotor up to a stop of the rotation of the rotor. Additionally oralternatively, the generator torque setpoint or the maximumallowable generator torque may be chosen to reduce therotational speed of the rotor up to a stop of the rotation of the rotor. Next, the computer program, the computer-readable datacarrier and the control device are specified.According to an embodiment, the computer program comprises instructions which, when the program is executed by a controldevice, cause the control device to carry out the method foroperating a wind turbine according to any one of the embodiments described herein. According to an embodiment, the computer-readable data carrier has the computer program stored thereon.According to an embodiment, the control device is configuredto execute the method for operating a wind turbine according to any one of the embodiments described herein. Particularly, the method is carried out when the above-mentioned computerprogram is executed by the control device.The control device may comprise at least one processor and / orat least one programmable logic controller, plc for short.The control device may be part of the wind turbine. ForP2023,1440 WO N / PA 1253 WO May 15, 2025- 16 -example, the control device comprises or consists of theturbine controller. The control device may further comprisethe pitch controller and / or the generator torque controller.Next, the wind turbine is specified. The wind turbine is, inparticular, configured to execute the method according to anyof the embodiments described herein or to be operatedaccording to the method according to any of the embodimentsdescribed herein. Thus, all features disclosed in connectionwith the method are also disclosed for the wind turbine and vice versa. According to an embodiment, the wind turbine comprises a rotor with at least two rotor blades, the control deviceaccording to any of the embodiments described herein and apitch adjustment system for adjusting the pitch angles of therotor blades. The control device is signally connected orconnectable to the pitch adjustment system. The wind turbineis configured such that, upon provision of the third information, the pitch adjustment system adjusts the pitch angle of at least one rotor blade to the pitch angle setpoint. According to a further embodiment, the wind turbine further comprises a generator and a generator torque adjustmentsystem for adjusting a generator torque of the generator. Thecontrol device is signally connected or connectable to the generator torque adjustment system. The wind turbine is configured such that, upon provision of the fourthinformation, the generator torque adjustment system adjuststhe generator torque to the generator torque setpoint. Forexample, the generator torque adjustment system comprises a generator torque controller. For example, the generatorP2023,1440 WO N / PA 1253 WO May 15, 2025- 17 -torque adjustment system additionally comprises a generator side converter coupled to the generator, in which case the generator torque controller is (part of) a generator side converter controller. Hereinafter, the method for operating a wind turbine, thecontrol device and the wind turbine will be explained in moredetail with reference to the drawings on the basis ofexemplary embodiments. The accompanying figures are included to provide a further understanding. In the figures, elements of the same structure and / or functionality may be referenced by the same reference signs. It is to be understood that the embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. Insofar as elements or components correspond to one another in terms of their function in different figures, the description thereof is not repeated for each of the following figures. For the sake of clarity, elements might not appear with corresponding reference symbols in all figures. Figure 1 shows an exemplary embodiment of the wind turbine,Figures 2 and 3 show flow charts of exemplary embodiments ofthe method for operating a wind turbine,Figure 4 shows an exemplary embodiment of a control system ofa wind turbine,Figures 5 to 7 show different graphs associated with theoperation of the wind turbine. Figure 1 shows a wind turbine 100 which comprises a tower 20. The tower 20 is fixed to the ground by means of a foundationP2023,1440 WO N / PA 1253 WO May 15, 2025- 18 -104. A nacelle 4 is rotatably mounted at one end of the tower20, opposite to the ground. The nacelle 4 comprises agenerator 40 which is coupled to a rotor 10, e.g. via agearbox (not shown). The rotor 10 comprises three (wind turbine) rotor blades 1, 2, 3, which are arranged on a rotor hub 112, said rotor hub 112 being connected to a rotor shaft (not shown). During operation, the rotor 10 is set in rotation by an air flow, for example wind. This rotational movement istransmitted to the generator 40 via the drive traincomprising, inter alia, the rotor shaft and optionally the gearbox. The generator 40 converts the mechanical energy of the rotor 10 into electrical energy. For optimizing the energy output of the wind turbine 100, the nacelle 4 has to be rotated into the wind. The pitch angles of the rotor blades 1, 2, 3 have to be set according to thewind speed. This is done with the help of drives which rotatethe rotor blades 1, 2, 3 and the nacelle 4 to a respectivetarget position. The drives for the rotor blades 1, 2, 3 arepart of a pitch adjustment system which is described in more detail in connection with Figure 4.In order to control and operate the drives, the wind turbinecomprises a control device 30 which determines pitch anglesetpoints for the blades 1, 2, 3 and a yaw angle setpoint ofthe nacelle 4. The control device 30 is located in thenacelle 4. The control device 30 is herein also called“turbine controller 30”.The control device 30 also determines a generator torquesetpoint with which the generator 40 is controlled. Not onlyP2023,1440 WO N / PA 1253 WO May 15, 2025- 19 -the pitch angles of the rotor blades but also the generatortorque influence the rotational speed of the rotor 10. In the case of a failure in the pitch adjustment system, it can become necessary to slow down or stop the rotation of therotor 10. In the situation that one of the rotor blades, e.g.rotor blade 1, is stuck, impaired or hindered in itsmobility, reducing the rotational speed of the rotor byincreasing the pitch angles of the remaining, movable rotorblades 2, 3 may lead to an aerodynamic imbalance which causesheavy loads on the yaw system of the wind turbine 100. On theother hand, increasing the generator torque in order to slow down the rotor rotation can only be done within certain limits so that using the generator torque alone to decelerate or stop the rotation of the rotor is not sufficient. Computer program comprising instructions which, when the program is executed by a control system, cause the controlsystem to carry out the method of any one of claims 1 to 11.13. Computer-readable data carrier having the computerprogram of claim 12 stored thereon.Figures 2 and 3 show flow charts of two different exemplaryembodiments of a method for operating a wind turbine with which a deceleration or stop of the rotor rotation isachievable in an efficient and gentle manner, since itconsiders the possible aerodynamic imbalance of the rotor. Furthermore, the described method for operating a windturbine can be a computer-implemented method in form of acomputer program or can be a part thereof. Consequently, thecomputer program can comprise or be the method for operatingP2023,1440 WO N / PA 1253 WO May 15, 2025- 20 -a wind turbine as indicated in Figures 2 and 3. In otherwords, the method can be implemented in a computer program,so that the computer program product comprises instructions which, when the computer program is executed by a computer or computer system, cause the computer or computer system to carry out the method for operating a wind turbine. Furthermore, the computer program can be stored on a computer-readable data carrier. Accordingly, Figures 2 and 3likewise show the computer program and the computer-readabledata carrier. Features and embodiments referring to the method also apply to the computer program and to the computer-readable data carrier. In the exemplary embodiment of Figure 2, first information I1 is determined which is representative of a failure in the pitch adjustment system. The failure may be a fault event inwhich one of the rotor blades is hindered or impaired in itsmobility. The remaining two rotor blades remain, for example,movable as intended. Moreover, second information I2 isdetermined which is representative of the actual rotationalspeed n_a of the rotor 10. Third information I3 is determineddepending on the first I1 and the second I2 information. Thethird information I3 is representative of a pitch anglesetpoint SP_β_1, SP_β_2, SP_β_3 (SP_β_i for short) for atleast one of the rotor blades 1, 2, 3. When the wind turbineis operated according to the third information I3, a pitchangle β_1 ,β_2 ,β_3 (β_i for short) of at least one of therotor blades 1, 2, 3 is adjusted to the pitch angle setpointSP_β_i. The pitch angle setpoint SP_β_i of the thirdinformation I3 depends on the actual rotational speed n_a andis chosen to reduce the rotational speed of the rotor 10.P2023,1440 WO N / PA 1253 WO May 15, 2025- 21 -For the sake of further illustration, the method is nowapplied to the wind turbine 100 of Figure 1 and it is assumedthat a failure in the pitch adjustment system appears inwhich the rotor blade 1 is impaired in its mobility or isstuck whereas the remaining rotor blades 2, 3 are stillmovable as intended. The third information I3 may then berepresentative of the pitch angle setpoints SP_β_2, SP_β_3for the two movable-as-intended rotor blades 2, 3. Themovable-as-intended rotor blades 2, 3 may be adjusted to thepitch angle setpoints SP_β_2, SP_β_3 in a collective manner,i.e. the pitch angle setpoints SP_β_2, SP_β_3 are equal. If,before the detection of the failure in the pitch adjustmentsystem, the wind turbine 100 has been operated in a firstoperating mode of individual pitch control (IPC), a switchcommand may have been generated after determining the firstinformation I1 which causes a switch from the first to asecond operating mode of collective pitch control (CPC).Figure 6 shows a graph indicating an example of how the pitchangle setpoint(s) SP_β_i for the movable-as-intended rotorblade(s) is / are determined. Coming back to the previousexample, in which rotor blade 1 is impaired or stuck, thechange of the pitch angles β_2, β_3 of the movable-as-intended rotor blades 2, 3 to the pitch angle setpoints SP_β_2, SP_β_3 results in a difference Δβ between the pitchangles β_2, β_3 and the pitch angle β_1 of the stuck / impairedrotor blade 1. In order to keep the resulting aerodynamicimbalance acceptable, it should be avoided that thisdifference Δβ exceeds a maximum allowable pitch angledifference Δβ_max which is indicated by the graph in figure6. As can be seen, this maximum allowable pitch angledifference Δβ_max decreases with increasing rotational speedn_a of the rotor 10. The pitch angle setpoints SP_ β_2,P2023,1440 WO N / PA 1253 WO May 15, 2025- 22 -SP_β_3 of the two movable-as-intended rotor blades 2, 3 arenow chosen such that the difference Δβ is equal to or lowerthan this maximum allowable pitch angle difference Δβ_max. In this way, an efficient deceleration of the rotor rotation can be achieved without inducing too high aerodynamic imbalances.The maximum allowable pitch angle difference Δβ_max as afunction of the rotational speed n_a of the rotor may be determined depending on simulations.Figure 3 shows another exemplary embodiment of the method foroperating a wind turbine. As described in connection withFigure 2, first information I1 being representative of afailure in the pitch adjustment system and second informationI2 being representative of the actual rotation speed n_a ofthe rotor 10 are determined. Then, three steps are executed.In one step, third information I3 is determined as explainedin connection with Figure 2. In addition to what is shown inFigure 2, two further information I4 and I5 are determined.The fourth information I4 is determined depending on thefirst information I1 and the second information I2. Thefourth information I4 is representative of a generator torquesetpoint SP_T. The generator torque setpoint SP_T therebydepends on the actual rotational speed n_a of the rotor 10and is chosen to reduce the rotational speed of the rotor 10.Alternatively, the generator torque setpoint SP_T is chosensuch that it reduces the rotational speed n_a of the rotor 10while ensuring that a maximum allowable generator torqueSP_T_max is not exceeded, wherein the maximum allowablegenerator torque SP_T_max depends on the actual rotationalspeed n_a. The fourth information I4 is determined such that,when the wind turbine 100 is operated according to the fourthP2023,1440 WO N / PA 1253 WO May 15, 2025- 23 -information I4, a generator torque of the generator 40 isadjusted to the generator torque setpoint SP_T.Figure 5 shows an example of how the generator torquesetpoint SP_T or the maximum allowable generator torqueSP_Tmax may be determined as a function of the actualrotational speed of the rotor 10. The y-axis shows thegenerator torque T_G and the x-axis shows the actualrotational speed n_a of the rotor 10. The solid lineindicates the nominal generator torque as a function of theactual rotational speed n_a of the rotor. The dash-dottedline is determined as a 15% excess over the solid line. Thegenerator torque setpoint SP_T or the maximum allowablegenerator torque SP_Tmax is determined according to the dash-dotted line for a limited time, for instance 10 s. Afterthat, the generator torque setpoint SP_T or the maximum allowable generator torque SP_Tmax is determined according to the solid line.Coming back to Figure 3, the fifth information I5 is alsodetermined depending on the first I1 and the second I2information and is representative of a pitch angle changerate d(β_i) / dt. The pitch angle change rate d(β_i) / dt therebydepends on the actual rotational speed n_a of the rotor 10.The pitch angle change rate d(β_i) / dt may either define themaximum speed with which the pitch angle of at least one rotor blade 1, 2, 3 is adjusted or defines the speed withwhich the pitch angle setpoint SP_β_i is changed.Figure 7 shows an example of how the pitch angle change rated(β_i) / dt may depend on the actual rotational speed n_a. Itis a function with a higher pitch angle change rate d(β_i) / dtP2023,1440 WO N / PA 1253 WO May 15, 2025- 24 -at lower rotational speeds n_a and a lower pitch angle changerate d(β_i) / dt at higher actual rotational speeds n_a.The combination of adjusting the pitch angles β_i dependingon the actual rotational speed n_a of the rotor and, at thesame time, adjusting the generator torque T_G depending onthe actual rotational speed n_a leads to a particularly safeand efficient reduction of the rotor rotation. The efficiencycan be further increased by also changing the pitch anglechange rate depending on the actual rotational speed n_a.Figure 4 shows an exemplary embodiment of a system for a wind turbine. This system may be used in the wind turbine 100 of Figure 1. The system comprises the control device 30, thegenerator 40, a drive train 41 coupled to the generator 40, agenerator torque adjustment system 400 and the pitchadjustment system 500.The generator torque adjustment system 400 comprises agenerator torque controller 42 and a generator side converter43 coupled to the generator 40. The generator torquecontroller 42 is (part of) a generator side convertercontroller and is configured to adjust the generator torqueof the generator 40. The generator controller 42 receives thefourth information I4 being representative of the generatortorque setpoint from the control device 30. A sensor, e.g. anincremental encoder, is coupled to the drive train 41 andprovides measurements M(n_a) to the control device 30. Themeasurements M(n_a) are representative of the actualrotational speed n_a of the rotor.The pitch adjustment system 500 comprises a pitch drive 50for each rotor blade (only one of the drives 50 is shown inP2023,1440 WO N / PA 1253 WO May 15, 2025- 25 -Figure 4). The pitch drive 50 is coupled to a motor side converter 51. The motor side converter 51 is coupled to a grid side converter 53 via a DC link intermediate circuit 52.Power from an electric grid is transmitted to the pitch drive50 via the grid side converter 53, the DC link intermediatecircuit 52 and the motor side converter 51. The pitchadjustment system 500 further comprises a pitch controller 54which receives the third I3 and fifth I5 information from thecontrol device 30 and controls the pitch drives 50 accordingto the corresponding pitch angle setpoint and pitch anglechange rate. A sensor, e.g. an incremental encoder, isassigned to each of the pitch drives 50. The measurements of the sensor can be used to determine the actual pitch angles β_i of the rotor blades. The measurements M(β_i) are sent to the control device 30. The control device 30 may determine the first information I1 depending on the measurements M(β_i). The invention described herein is not limited by the description in conjunction with the exemplary embodiments. Rather, the invention comprises any new feature as well as any combination of features, particularly including any combination of features in the patent claims, even if said feature or said combination per se is not explicitly stated in the patent claims or exemplary embodiments.

[0002] P2023,1440 WO N / PA 1253 WO May 15, 2025- 26 -Reference sign list:1, 2, 3 rotor blades4 nacelle10 rotor20 tower30 control device40 generator41 drive train42 generator torque controller43 generator side converter50 pitch drive51 motor side converter52 DC link intermediate circuit53 grid side converter54 pitch controller100 wind turbine104 foundation112 rotor hub400 generator torque adjustment system500 pitch adjustment systemI1 first informationI2 second informationI3 third informationI4 fourth informationI5 fifth informationβ_i pitch anglen_a actual rotational speed of the rotorSP_β_i pitch angle setpointΔβ differenceΔβ_max maximum allowable pitch angle differenceT_G generator torqueSP_T generator torque setpointP2023,1440 WO N / PA 1253 WO May 15, 2025- 27 -SP_T_max maximum allowable generator torqued(β_i) / dt pitch angle change rateM(n_a) measurementsM(β_i) measurements

Claims

P2023,1440 WO N / PA 1253 WO May 15, 2025- 28 -Claims (We claim) 1. Method for operating a wind turbine (100) having a rotor(10) with at least two rotor blades (1, 2, 3) and a pitchadjustment system (500) for adjusting the pitch angles (β_i)of the at least two rotor blades (1, 2, 3), wherein themethod comprises the steps of- determining first information (I1) which is representativeof the appearance of a failure in the pitch adjustment system(500) while the rotor (10) is rotating;- providing second information (I2) which is representativeof the actual rotational speed (n_a) of the rotor (10);- determining third information (I3) depending on the firstinformation (I1) and the second information (I2), wherein thethird information (I3) is representative of a pitch anglesetpoint (SP_β_i) for at least one rotor blade (1, 2, 3),wherein the pitch angle setpoint (SP_β_i)- depends on the actual rotational speed (n_a) and- is chosen to reduce the rotational speed of the rotor(10),- so that, when the wind turbine (100) is operated accordingto the third information (I3), a pitch angle (β_i) of atleast one rotor blade (1, 2, 3) is adjusted to the pitchangle setpoint (SP_β_i).

2. Method according to claim 1, wherein- the failure in the pitch adjustment system (20) is a faultevent in which at least one of the rotor blades (1) isimpaired in its mobility and at least one other rotor blade(2, 3) is still movable as intended,- the pitch angle setpoint (SP_β_2, SP_β_3) is for the atleast one movable-as-intended rotor blade (2, 3) and ischosen such that the difference (Δβ) between the pitch angleP2023,1440 WO N / PA 1253 WO May 15, 2025- 29 -setpoint (SP_β_2, SP_β_3) and the pitch angle (β_1) of the atleast one impaired rotor blade (1) is lower or equal to amaximum allowable pitch angle difference (Δβ_max), wherein the maximum allowable pitch angle difference (Δβ_max) depends on the actual rotational speed (n_a).

3. Method according to claim 1 or 2, wherein- the wind turbine (100) comprises a generator (40) coupledto the rotor (10) in order to convert mechanical power into electrical power, the method further comprising- determining fourth information (I4) depending on the first(I1) and the second information (I2), wherein the fourth information (I4) is representative of a generator torque setpoint (SP_T), wherein -the generator torque setpoint (SP_T) depends on theactual rotational speed (n_a) of the rotor (10) and ischosen to reduce the rotational speed of the rotor (10), or -the generator torque setpoint (SP_T) is chosen toreduce the rotational speed of the rotor (10) while ensuring that a maximum allowable generator torque (SP_T_max) is not exceeded, wherein the maximumallowable generator torque (SP_T_max) depends on theactual rotational speed (n_a), wherein- the fourth information (I4) is determined such that, whenthe wind turbine (100) is operated according to the fourthinformation (I4), a generator torque of the generator (40) isadjusted to the generator torque setpoint (SP_T).

4. Method according to claim 3, wherein- the third information (I3) and fourth information (I4) aredetermined such that, when the wind turbine (100) is operatedaccording to the third information (I3) and fourthP2023,1440 WO N / PA 1253 WO May 15, 2025- 30 -information (I4), the adjustment of the pitch angle (β_i) of the at least one rotor blade (1, 2, 3) to the pitch anglesetpoint (SP_β_i) and the adjustment of the generator torqueto the generator torque setpoint (SP_T) are done simultaneously.

5. Method according to any one of the preceding claims, further comprising- determining fifth information (I5) depending on the firstinformation (I1) and the second information (I2) which is representative of a pitch angle change rate (d(β_i) / dt), wherein -the pitch angle change rate (d(β_i) / dt) depends on theactual rotational speed (n_a),- the pitch angle change rate (d(β_i) / dt) defines themaximum speed with which the pitch angle (β_i) of at least one rotor blade (1, 2, 3) is adjusted to the pitch angle setpoint (SP_β_i) or it defines the speed withwhich the pitch angle setpoint (SP_β_i) is changed.

6. Method according to claim 2 or any one of claims 3 to 5 in its dependency of claim 2, wherein- the rotor (10) comprises at least three rotor blades (1, 2,3),- the third information (I3) is determined such that, whenthe wind turbine is operated according to the thirdinformation (I3), the pitch angles (β_1, β_2) of at least twomovable-as-intended rotor blades (2, 3) are synchronized.

7. Method according to any one of the preceding claims,wherein- the wind turbine (100) is operable in a first and a secondoperating mode,P2023,1440 WO N / PA 1253 WO May 15, 2025- 31 -- the first operating mode is a mode in which the pitchangles (β_i) of the rotor blades (1, 2, 3) are controlled individually,- the second operating mode is a mode in which the pitchangles (β_i) of the rotor blades (1, 2, 3) are controlled collectively, andif the wind turbine (100) is operated in the first operatingmode when the first information (I1) is determined to be representative of a failure in the pitch adjustment system (500), the method further comprises the step of- generating a switch command which is configured to cause atransition from the first operating mode into the second operating mode.

8. Method according to any one of the preceding claims, wherein- determining the first information (I1) comprisesdetermining whether the pitch angle (β_1) of one rotor blade (1) differs from the pitch angle (β_2, β_3) of the at least one other rotor blade (2, 3) by more than a defined threshold.

9. Method according to claim 3 or any one of claims 4 to 8 inits dependency of claim 3, wherein- the generator torque setpoint (SP_T) or the maximumallowable generator torque (SP_T_max) increases withincreasing actual rotational speed (n_a).

10. Method according to claim 2 or any one of claims 6 to 9in its dependency of claim 2, wherein- the maximum allowable pitch angle difference (Δβ_max)decreases with increasing actual rotational speed (n_a).P2023,1440 WO N / PA 1253 WO May 15, 2025- 32 -11. Method according to any one of the preceding claims,wherein- the pitch angle setpoint (SP_β_i) is chosen to reduce therotational speed of the rotor (10) up to a stop of the rotation of the rotor (10).

12. Computer program comprising instructions which, when the program is executed by a control system, cause the controlsystem to carry out the method of any one of claims 1 to 11.

13. Computer-readable data carrier having the computerprogram of claim 12 stored thereon.

14. Control device (30) which is configured to execute themethod according to any one of claims 1 to 11.

15. Wind turbine (100) comprising- a rotor (10) with at least two rotor blades (1, 2, 3),- a generator (40) coupled to the rotor (10) in order toconvert a mechanical power of the rotor (10) into electrical power,- the control device (30) according to claim 14,- a generator torque adjustment system (400) for adjusting agenerator torque of the generator (40),- a pitch adjustment system (500) for adjusting pitch angles(β_i) of the rotor blades (1, 2, 3), wherein- the control device (30) is signally connected orconnectable to both the generator torque adjustment system(400) and the pitch adjustment system (500),- the wind turbine (100) is configured such that,- upon provision of the third information (I3), thepitch adjustment system (500) adjusts the pitch angleP2023,1440 WO N / PA 1253 WO May 15, 2025- 33 -(β_i) of at least one rotor blade (1, 2, 3) to the pitchangle setpoint (SP_β_i), and- upon provision of the fourth information (I4), thegenerator torque adjustment system (400) adjusts thegenerator torque to the generator torque setpoint (SP_T).

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