Operating a wind turbine

Abstract

A method of operating a wind turbine (100) is provided. The wind turbine (100) comprises a wind turbine rotor (120) having at least three rotor blades (121, 122, 123), wherein a sensor (21, 22, 23) is arranged on at least one of the rotor blades (121, 122, 123). The method comprises: obtaining, from the sensor (21, 22, 23), measurements of a blade load of the rotor blade (121, 122, 123); determining a parameter (P) indicative of a change of thrust applied to the wind turbine rotor (120) based on the obtained blade load measurements; employing the parameter (P) to determine if a change of thrust applied to the wind turbine rotor (120) exceeds a predefined limit (31); and if the parameter (P) indicates that the change of thrust applied to the wind turbine rotor (120) exceeds the predefined limit (31), taking a mitigation action that comprises controlling the wind turbine (100) to counteract the change of thrust.

Description

[0001] 2024PF00525 PCT subsequent filing

[0002] 1

[0003] Description

[0004] Operating a wind turbine

[0005] FIELD OF THE INVENTION

[0006] The present invention relates to a method of operating a wind turbine . It further relates to a respective control system and to a computer program for controlling the operation of a wind turbine by a control system .

[0007] BACKGROUND

[0008] To make energy generation by wind turbines more ef ficient , larger wind turbines having longer blades are employed . The rotor blades are exposed to the wind, which applies respective forces to the blades . The force applied by the wind to the wind turbine rotor is generally referred to as thrust .

[0009] In particular during a stop procedure of the wind turbine , a sudden reduction of thrust applied to ( acting on) the wind turbine rotor may lead to signi ficant tower loading and vibrations , risking tower integrity . It is thus desirable to mitigate the reduction of thrust during a stop procedure of the wind turbine . The nacelle of the wind turbine may thus be equipped with respective accelerometers to detect the accelerations of the wind turbine nacelle . Based on the detected accelerations , a mitigation action may be taken to reduce tower loading and vibrations .

[0010] The document US 2021 / 0239092 Al describes a method of reducing a rotor thrust comprising determining a tower head speed or nacelle oscillation speed from an acceleration sensor arranged in the tower or the nacelle of the wind turbine . 2024PF00525 PCT subsequent filing

[0011] 2

[0012] However, such methods may interfere with other control methods that for example mitigate tower loads induced by a wind gust ( i . e . sudden change in wind amplitude and / or direction) . In particular, a bad coordination between these methods ( i . e . method for mitigating the reduction of thrust and method for mitigating the increase of thrust ) may lead to increased tower loads risking the integrity of the tower . For improving the coordination, for example in a case where a wind gust leading to an increased thrust is combined with a grid drop leading to a reduced thrust , some kind of scheduling between the control methods is necessary which increases the complexity of the controller .

[0013] WO 2024 / 132076 Al discloses a method of controlling a wind turbine , wherein the method comprises receiving a plurality of blade flap load signals from a blade flap load sensor of a respective blade indicative of measured flap loading on respective rotor blades . The rotor blade pitch angle is controlled such that the thrust experienced by a wind turbine is kept below a maximum thrust level .

[0014] EP 3 741 990 Al discloses a control device for controlling a wind turbine comprising a rotor having a plurality of blades . The control device comprises a detecting device for detecting an amount of a bending moment of the blade . A change element outputs a response to a di f ferential of the detected amount of the bending moment of the blade and controls the wind turbine based on the response .

[0015] It is desirable to provide a method of operating a wind turbine which reduces such risk and is less complex to implement on the wind turbine , and which in particular allows a more accurate mitigation of loading caused by a change of thrust acting on the wind turbine .

[0016] SUMMARY 2024PF00525 PCT subsequent filing

[0017] 3

[0018] Accordingly, there is a need to mitigate at least some of the drawbacks mentioned above and to provide an improved counteracting of a change of thrust acting on the wind turbine rotor .

[0019] This need is met by the features of the independent claims . The dependent claims describe embodiments of the invention .

[0020] According to an aspect of the invention, a method of operating a wind turbine is provided . The wind turbine comprises a wind turbine rotor having at least three rotor blades , wherein a ( load) sensor is arranged on at least one of the rotor blades . The method comprises obtaining, from the sensor, measurements of a blade load of the rotor blade , determining a parameter ( e . g . thrust change parameter ) indicative of a change of thrust applied to ( acting on) the wind turbine rotor based on the obtained blade load measurements , employing the parameter to determine i f a change of thrust applied to the wind turbine rotor exceeds a predefined limit , and i f the parameter indicates that the change of thrust applied to the wind turbine rotor exceeds the predefined limit , taking a mitigation action that comprises controlling the wind turbine to counteract the change of thrust .

[0021] By such method, a change of thrust may be mitigated by the mitigation action such that tower loading and vibrations may be reduced . In particular, the method may be used for operating a wind turbine during a stop or shut down procedure which may be initiated, e . g . , by a grid drop, a disconnection from the grid, an emergency stop or the like . Such stop or shut down procedure may lead to a sudden reduction / loss of thrust applied to the wind turbine rotor which can lead to signi ficant tower loading and vibrations , risking tower integrity . The mitigation action may prevent tower bottom bending in the upwind direction caused by the reduction of thrust after a stop procedure is initiated while the tower starts swaying upwind . 2024PF00525 PCT subsequent filing

[0022] 4

[0023] Thrust generally refers to the axial force applied by the wind on the rotor of a wind turbine .

[0024] Existing sensors on the rotor blades may be used by the method . In particular, existing sensors used to mitigate increased tower loads / thrust induced by a gust can also be used for initiating the mitigation action during a stop procedure ( i . e . for mitigating a reduction of thrust ) . This reduces costs and simpli fies installation . Usually, accelerometers on the nacelle are used for mitigation action during the stop procedure , which detect movements or accelerations from which moments are calculated . As these accelerometers are no longer necessary and existing sensors on the rotor blades are used for the mitigation action during a stop procedure , bad coordination between rotor blade sensors and accelerometers on the nacelle , which may lead to increased tower loads risking the integrity of the tower can be avoided . This may also result in a simpler implementation of the wind turbine controller . In particular, no scheduling between functionalities that reduce loading due to gusts and that reduce loading due to a stop or shut down process of the wind turbine may be necessary .

[0025] Alternatively, the sensors on the rotor blades used for initiating the mitigation action may act as a fallback in case the accelerometers at the nacelle are not available or operational and thus ensure the integrity of the turbine .

[0026] In an optional example , determining the parameter comprises determining a moment measure for the rotor blade from the obtained blade load measurements , wherein the parameter is determined from the moment measure . Preferably, determining the moment measure comprises determining, based on the blade load measurements , one or more of the following : a ( e . g . mean) flap-wise moment , and a ( e . g . mean) edge-wise moment . The flap-wise moment and the edge-wise moment may be defined with respect to a zero pitch reference line of the blade . The 2024PF00525 PCT subsequent filing

[0027] 5 method may be implemented with any or with any combination of the respective moment measures .

[0028] The parameter can also be determined from the blade load measurements without determining moments such as the moment measure . For example , out-of-plane blade loads may be determined for the rotor blades from the blade load measurements and the parameter indicative of a change of thrust may be determined from the out-of-plane blade loads .

[0029] Preferably, at least one sensor is arranged on each of at least two rotor blades , preferably on each of the rotor blades , and wherein the method comprises obtaining from each of the sensors , measurements of a blade load of the respective rotor blade , and determining a moment measure for each of the rotor blades from the respective obtained blade load . The parameter is preferably determined based on ( from) the moment measures determined for each rotor blade ( e . g . moment measures determined for two , three or more rotor blades ) .

[0030] In an optional example , determining the parameter comprises determining for the rotor blade based on the determined moment measure and a respective blade pitch angle of the rotor blade an out-of-plane moment ( i . e . a moment perpendicular to the wind turbine rotor plane ) , wherein the parameter is determined from the out-of-plane moment . Preferably, an out-of-plane moment is determined for each rotor blade ( in particular for each rotor blade comprising a sensor ) .

[0031] Preferably, at least one sensor is arranged on each of at least two , preferably on each, of the rotor blades , wherein the moment measure and the out-of-plane moment are determined for each rotor blade , and wherein determining the parameter comprises processing the out-of-of plane moments by averaging the out of plane moments determined for each rotor blade . The occurrence of outliers and incorrect detection of load 2024PF00525 PCT subsequent filing

[0032] 6 measurements from which the out-of-plane moments are determined may be reduced by averaging over the out-of-plane moments of each rotor blade . The averaged out-of-plane moment of the rotor blades may also be a good approximation of the total out-of-plane moment on the rotor .

[0033] Alternatively, determining the parameter comprises processing the out-of-of plane moments by selecting the maximum out-of- plane moment from the out-of-plane moments of each rotor blade . By selecting the maximum out-of-plane moment , it may be possible to prevent a local overload on one of the rotor blades from being disregarded .

[0034] Preferably, determining the parameter comprises filtering the out-of-plane moment , in particular the averaged or maximum out-of-plane moment , with one or more filters such as bandstop filter and / or low-pass filter, wherein the parameter is determined from the filtered out-of-plane moment .

[0035] By filtering the out-of-plane moment , in particular the averaged or maximum out-of-plane moment , undesired frequency content such as high frequency content like noise and / or harmonics originated from rotation of the wind turbine rotor and the rotor blades may be removed . Thus , these undesired frequencies may not af fect the determination of the parameter, making it possible to determine based on the determined parameter more accurately and reliably whether a mitigation action needs to be carried out or not . Thus , unnecessary usage of components and / or actuators for carrying out the mitigation action can be avoided . This reduces wear and maintenance costs of such components and / or actuators .

[0036] For example , the out-of-plane moment , in particular the averaged or maximum out-of-plane moment , is filtered by a plurality of filters such as for example at least two , three or four filters . The out-of-plane moment , in particular the averaged or maximum out-of-plane moment , may be filtered by a first band-stop filter and / or a second band-stop filter 2024PF00525 PCT subsequent filing

[0037] 7 and / or a low-pass filter . The band-stop filter may be configured to remove a band of unwanted frequencies so that other frequencies can pass with minimal loss . Thus , harmonics originated from rotation of the wind turbine rotor and the rotor blades may be removed .

[0038] Preferably, determining the parameter comprises determining a time derivative of the out-of-plane moment ( e . g . of the averaged or maximum and / or filtered out of plane moment ) . For example , the parameter may be the time derivative of the filtered out-of-plane moment .

[0039] A negative derivative may indicate a reduction of thrust acting on the wind turbine rotor . The reduction of thrust , which may be caused by a grid drop, can lead to substantial tower loading in the upwind direction . A positive derivative may indicate an increase of thrust acting on the wind turbine rotor . The increase of thrust , which may be caused by a wind gust , may lead to substantial tower loading in the downwind direction .

[0040] During a stop or shut down procedure of the wind turbine , a reduction of thrust applied to the wind turbine rotor may occur which may result in a negative derivative . This reduction of thrust may be caused by a loss of electromagnetic torque at a generator coupled to the rotor of the wind turbine .

[0041] Exceeding the predefined limit may comprise comparing the parameter having a negative value with the predefined limit being a threshold with a negative value or comparing the parameter having a positive value with the predefined limit being a threshold having a positive value . The parameter having a positive value may be obtained by multiplying the (negative ) time derivative with - 1 . The parameter preferably contains information about whether the derivative is positive or negative . 2024PF00525 PCT subsequent filing

[0042] 8

[0043] In particular, when the time derivative is negative , the parameter may be identical to the time derivative and may have a negative value . In this case , the predefined limit ( to which the parameter is compared) may be a threshold having a negative value . Exceeding the predefined limit may mean that the value of the parameter may drop below the predefined (negative ) threshold, i . e . may become even more negative than the predefined threshold .

[0044] Alternatively, the parameter can be obtained by multiplying the negative time derivative with - 1 and the parameter therefore has a positive value . In this case the predefined limit ( to which the parameter is compared) may be a threshold having a positive value . Exceeding the predefined limit may then mean that the value of the parameter becomes larger than the predefined threshold .

[0045] Preferably, the method further comprises determining from the parameter an activation level parameter indicating how much the parameter exceeds the predefined limit ( or predefined threshold) , and taking the mitigation action dependent on the determined activation level parameter . Preferably, an intensity or type of the mitigation action depends on the extent to which the parameter exceeds the predefined limit . Further activation levels may be computed based on other operational parameters or variables of the turbine such as collective pitch and / or an estimated wind speed . An average activation level can be calculated from the various activation levels , for example , or the maximum activation level can be selected .

[0046] Preferably, the predefined limit is a first threshold corresponding to a first activation level of 0% and a second threshold corresponds to a second activation level of 100% indicating a maximum mitigation action, wherein the activation level parameter has a value from 0 to 100 % , and wherein an intensity or type of the mitigation action depends on the value of the activation level parameter . 2024PF00525 PCT subsequent filing

[0047] 9

[0048] The intensity of the mitigation action may indicate an amount or magnitude of a pitch of fset to a collective pitch reference and / or an of fset to a rotor speed reference . For example , a higher intensity of the mitigation action may mean that a negative pitch of fset to a collective pitch assumes an even more negative value and / or a positive of fset to a rotor speed reference assumes a higher value compared to a lower intensity of the mitigation action .

[0049] Type of mitigation action depends on the value of the activation level parameter may mean that depending on the value of the activation level parameter, di f ferent mitigation actions are executed or that at a lower value of the activation level parameter a first mitigation action is carried out and at a higher value of the activation level parameter the first and one or more further mitigation actions are carried out .

[0050] The mitigation action may only be executed when the first predefined limit is exceeded . An activation level parameter value of 0% may mean that no mitigation action is taken .

[0051] For example , the dependency between the time derivative and the activation level parameter may be linear or exponential .

[0052] When the parameter is identical to the time derivative and has a negative value , the first and second thresholds ( to which the parameter is compared) are also negative .

[0053] Alternatively, the parameter can be obtained by multiplying the time derivative with - 1 and therefore has a positive value . In this case the first and second thresholds ( to which the parameter is compared) are also positive .

[0054] In an optional example , the method comprises determining i f a stop or shut down procedure or shut down procedure of the wind turbine has been initiated, wherein the mitigation 2024PF00525 PCT subsequent filing

[0055] 10 action is taken only i f it is determined that the stop or shut down procedure has been initiated . Such stop procedure , for example initiated by a grid drop, may lead to a sudden reduction or loss of thrust applied to the wind turbine rotor which leads to signi ficant tower loading and vibrations , risking tower integrity . The mitigation action may prevent negative tower bottom bending caused by the reduction of thrust after a stop procedure is initiated while the tower starts swaying upwind .

[0056] Preferably, counteracting the change of thrust applied to the wind turbine rotor comprises counteracting a reduction of thrust applied to the wind turbine rotor . The blade load measurements taken by the sensors may also be used for initiating a mitigation action during an increase of thrust which may be indicated by a positive time derivative .

[0057] Preferably, the mitigation action comprises at least one of the following : blocking a functionality that contributes to the reduction of thrust such as increasing the pitch angle of the rotor blades , adding a negative pitch of fset to a collective pitch reference of the rotor blades , adding a positive of fset to a rotor speed reference of the wind turbine rotor, and adj usting a pitch out rate of the rotor blades .

[0058] Preferably, a first mitigation action comprises blocking a or any functionality that contributes to the reduction of thrust acting on the wind turbine rotor ( e . g . , functionalities that increase the pitch angle ) .

[0059] This first mitigation action may be combined with a second mitigation action that aims to slow down the reduction of thrust experienced by the wind turbine rotor ( i . e . , adding a negative pitch of fset to the collective pitch reference , adding a positive of fset to the rotor speed reference , scaling the allowed pitch out rate , etc . ) . 2024PF00525 PCT subsequent filing

[0060] 11

[0061] Alternatively, only one the first and second mitigation actions may be executed .

[0062] Preferably, the at least one sensor is a load sensor . A blade moment ( e . g . bending moment ) may be measured by measuring a loading or strain of the blade using a respective sensor and processing the sensor data to derive the respective blade moment . A respective strain gauge may be employed . For example , a sensor may comprise one or more optical fibers extending along section of rotor blade , preferably a temperature-compensated optical fiber, that measure strain in the rotor blade by changes to their optical characteristics caused by the strain .

[0063] Any other types of respective load sensors may additionally or alternatively be employed . Respective sensors and blade moment measurements are known in the art , and any of these may be employed here .

[0064] Such measurement of blade load measurements may be based on a respective calibration . A control system ( see below) may store such calibration data .

[0065] The load sensor may enable a more precise determination of the moments acting on the rotor blades and thus of the thrust acting on the wind turbine . Accelerometers may only provide indirect indications that may further be delayed . The method may thus result in a more precise control to counteract the change of thrust applied to the wind turbine rotor by taking the mitigation action .

[0066] According to a further aspect of the invention, a control system configured to control the operation of a wind turbine is provided . The wind turbine comprises a wind turbine rotor having at least three rotor blades , wherein at least one sensor is arranged at one of the rotor blades and wherein the control system is configured to perform any of the methods disclosed herein . The control system may for example comprise 2024PF00525 PCT subsequent filing

[0067] 12 a processing unit and a memory, and the memory may store control instructions which, when executed by the processing unit , cause the control system to perform any of the disclosed methods . Such control system may comprise at least one load sensor on each rotor blade to measure a quantity, such as strain, from which the moment measure is derived .

[0068] According to a further aspect , a wind turbine comprising such control system is provided . The control system may be provided as part of a control system of the wind turbine , or may be coupled to such control system of the wind turbine .

[0069] According to a further aspect of the invention, a computer program for controlling the operation of a wind turbine by a control system is provided . The wind turbine comprises a wind turbine rotor having at least three rotor blades , wherein at least one sensor is arranged at one of the rotor blades . The computer program comprises control instructions which, when executed by a processing unit of the control system, cause the processing unit to perform any of the methods disclosed herein . The computer program may be provided on a volatile or non-volatile carrier or storage medium and / or may be provided via a communication connection, such as a wired or wireless network connection .

[0070] By such control system, wind turbine or computer program, advantages similar to those outlined further above with respect to the method may be achieved .

[0071] It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention . In particular, the features of the di f ferent aspects and examples of the invention can be combined with each other unless noted to the contrary .

[0072] BRIEF DESCRIPTION OF THE DRAWINGS 2024PF00525 PCT subsequent filing

[0073] The forgoing and other features and advantages of the invention will become further apparent from the following detailed description read in conj unction with the accompanying drawings . In the drawings , like reference numerals refer to like elements .

[0074] Fig . 1 is a schematic drawing showing a wind turbine including a control system according to an embodiment .

[0075] Fig . 2 is a flow diagram illustrating a method of operating a wind turbine according to an embodiment .

[0076] Fig . 3 is a schematic graph illustrating the dependency between a thrust change parameter and an activation level parameter according to an embodiment .

[0077] DETAILED DESCRIPTION

[0078] In the following, embodiments and / or examples of the invention will be described in detail with reference to the accompanying drawings . It is to be understood that the following description of the embodiments is given only for the purpose of illustration and is not to be taken in a limiting sense . It should be noted that the drawings are to be regarded as being schematic representations only, and elements in the drawings are not necessarily to scale with each other . Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art . As used herein, the singular forms "a, " "an, " and "the" are intended to include the plural forms as well , unless the context clearly indicates otherwise . The terms "comprising, " "having, " " including, " and "containing" are to be construed as open-ended terms ( i . e . , meaning " including, but not limited to , " ) unless otherwise noted . 2024PF00525 PCT subsequent filing

[0079] 14

[0080] Fig . 1 schematically illustrates a wind turbine 100 comprising a wind turbine tower 101 and a wind turbine rotor 120 . The rotor 120 has three rotor blades 121 , 122 and 123 . Wind turbine 100 may be an onshore , an of fshore , or a floating wind turbine . Such wind turbines are generally known in the art , and wind turbine 100 may have any of the known configurations .

[0081] To measure blade loads , a control system 10 according to an embodiment is provided . The control system 10 may comprise at least one sensor ( e . g . load sensor ) being arranged at a respective blade 121 , 122 , 123 of the wind turbine rotor 120 . As shown in Fig . 1 , the control system preferably comprises sensors 21 , 22 and 23 , each being provided at a respective blade 121 , 122 , 123 . The sensors 21 to 23 may each comprise one or more optical fibers that measure strain in a blade by changes to their optical characteristics caused by the strain . Any other types of respective load sensors may additionally or alternatively be employed . Measurement of a blade moment may include obtaining a respective sensor signal from the corresponding load sensor and processing the sensor signal to derive the blade moment . Such measurement of blade moment may be based on a respective calibration . Control system 10 may store such calibration data .

[0082] Control system 10 may include a processing unit 11 coupled to a memory 12 . Processing unit 11 may be a micro-processor, a digital signal processor, an application-speci fic integrated circuit , or the like . Memory 12 may comprise RAM, ROM, Flash- Memory, a hard disk drive and other types of memory . Memory 12 may store control instructions which, when executed by processing unit 11 , cause the control system 10 to perform any of the methods disclosed herein . Control system 10 may be implemented in one or more controllers and may accordingly comprise plural processing units and plural memories . Control system 10 may comprise further components common to such control system, for example input / output interfaces , a system bus , a user interface and the like . 2024PF00525 PCT subsequent filing

[0083] The control system 10 may form part of a wind turbine control system 15 of the wind turbine , or may be coupled to such control system . The control system 10 may control the wind turbine 100 to , e . g . , reduce the change of thrust experienced by the rotor 120 i f an excessive reduction of thrust is detected .

[0084] The control system 10 is configured to detect a change of thrust applied to the wind turbine rotor, which may for example be due to a stop or shut down procedure of the wind turbine . Such stop or shut down procedure of the wind turbine may be initiated by a grid drop, a disconnection from the grid, an emergency stop or the like . An embodiment of a method for operating the wind turbine and in particular for monitoring a change of thrust applied to the wind turbine rotor that may be implemented on the control system 10 is illustrated in the flow diagram of Fig . 2 .

[0085] In step S 10 , measurements of a blade load are obtained for each rotor blade 121 , 122 , 123 , in particular by means of the sensors 21 to 23 . Based on the load signals generated by the sensors 21 to 23 , blade moments can be calculated .

[0086] Optionally, in step S i l , a moment measure is determined for each rotor blade 121 to 123 from the blade load measurements obtained for the respective blade . The moment measure may be determined from the blade load measurements obtained over at least one revolution of the wind turbine rotor . The moment measure can be determined by a statistical processing of the blade moment measurements , such as by calculating an average of the measurement values . Two or more moment measures may be derived for each blade . It is beneficial i f the determined moment measures for each blade include at least a mean flapwise moment , and a mean edge-wise moment . The parameter P can also be determined from the blade load measurements without determining moments such as the moment measure . For example , out-of-plane blade loads may be determined for the rotor 2024PF00525 PCT subsequent filing

[0087] 16 blades from the blade load measurements and the parameter P indicative of a change of thrust may be determined from the out-of-plane blade loads .

[0088] In step S 12 , a ( e . g . thrust change ) parameter P ( see Fig . 3 ) indicative of a change of thrust applied to the wind turbine rotor 120 is determined based on the blade load measurements obtained for the rotor blades ( in particular based on the moment measures determined for each rotor blade ) . In particular, step 12 of determining the parameter P may comprise at least some of the following steps S 13 to S 16 .

[0089] In step S 13 , for each rotor blade 121 , 122 , 123 based on the determined moment measure and a respective blade pitch angle of the respective rotor blade , an out-of-plane moment is determined .

[0090] In step S 14 , the out-of-of plane moments are processed by averaging the out of plane moments determined for each rotor blade . The occurrence of outliers and incorrect detection of load measurements , from which the out-of-plane moments are determined, may be reduced by averaging over the out-of-plane moments of each rotor blade . This averaged out-of-plane moment of the rotor blades may be a good approximation of the average out-of-plane moment applied on the rotor . Alternatively, the out-of-of plane moments are processed by selecting the maximum out-of-plane moment from the out-of- plane moments of each rotor blade 121 , 122 , 123 . By selecting the maximum out-of-plane moment , it is possible to prevent a local overload on one of the rotor blades from being disregarded .

[0091] Optionally, in step S 15 , the averaged or maximum out-of-plane moment is filtered with one or more filters . The averaged or maximum out-of-plane moment may be filtered by at least one band-stop filter and / or at least one low-pass filter . Preferably, the averaged or maximum out-of-plane moment may be filtered with a first and second band-stop filter and a 2024PF00525 PCT subsequent filing

[0092] 17 low-pass filter . Thus , undesired frequency content such as high frequency content like noise and / or harmonics originated from rotation of the wind turbine rotor and the rotor blades can be removed . Therefore , these undesired frequencies may not af fect the determination of the parameter P ( or the time derivative ) , making it possible to determine based on the determined parameter more accurately and reliably whether a mitigation action needs to be taken or not . Further, unnecessary usage of components of the turbine which are used for the mitigation action can be avoided .

[0093] In step S 16 , a time derivative of the average or maximum out- of-plane moment , preferably a filtered average or maximum out-of-plane moment , is determined . The time derivative preferably may indicate how much the out-of-plane moment changes per time . The parameter P preferably corresponds to the time derivative . In case of a stop procedure , the stop procedure may lead to a sudden reduction or loss of thrust applied to the wind turbine rotor which may lead to signi ficant tower loading and vibrations , risking tower integrity . Reduction in thrust may result in a reduction of the out-of-plane moments acting on the blades . Thus , the time derivative having a negative value may indicate a reduction of thrust acting on the wind turbine rotor which is to be counteracted by the method . The reduction of thrust may lead to substantial tower loading in the upwind direction .

[0094] In step S 17 , the parameter P is employed to determine i f a change of thrust ( e . g . a reduction of thrust ) applied to the wind turbine rotor 120 exceeds a predefined limit .

[0095] Exceeding the predefined limit may comprise comparing the parameter P having a negative value with the predefined limit being a threshold with a negative value . In this case , exceeding the predefined negative threshold may mean that the parameter P falls below the negative threshold, i . e . the parameter P becomes even more negative as the predefined threshold . 2024PF00525 PCT subsequent filing

[0096] Alternatively, exceeding the predefined limit may comprise comparing the parameter P having a positive value with the predefined limit being a (first) threshold 31 having a positive value (as shown in Fig. 3) . The parameter P having a positive value may be obtained by multiplying the (negative) time derivative with -1. In this case, exceeding the predefined positive threshold 31 may mean that the parameter P becomes larger than the positive threshold 31.

[0097] If the parameter P exceeds the predefined limit, a mitigation action that comprises controlling the wind turbine 100 to counteract the change of thrust may be carried out. The mitigation action may prevent negative tower bottom bending caused by the reduction of thrust after a stop procedure is initiated while the tower starts swaying upwind. If the parameter P is equal to or exceeds the predefined limit, no mitigation action may be executed and the method may start again at method step S10.

[0098] Optionally, in step S18, if the parameter P exceeds the predefined limit (e.g. positive threshold 31 in Fig 3) , an activation level parameter A (see Fig. 3) indicating how much the parameter P exceeds the predefined limit may be determined .

[0099] In step S19, a mitigation action that comprises controlling the wind turbine 100 to counteract a change (preferably a reduction) of thrust applied to the wind turbine rotor is executed dependent on the determined activation level parameter A. The intensity and / or type of the mitigation action may depend on the value of the activation level parameter .

[0100] For example, Fig. 3 shows a graph 33 illustrating the dependency between the parameter P (e.g. time derivative of the average or maximum out-of-plane moment) and the activation level parameter A according to an embodiment. The 2024PF00525 PCT subsequent filing

[0101] 19 parameter P is for example measured in kNm / s . The predefined limit may be a first threshold 31 corresponding to a first activation level of 0% and a second threshold 32 may correspond to a second activation level of 100% indicating a maximum mitigation action . The activation level parameter A may have a value from 0 to 100 % . The activation level parameter A may indicate how much the parameter P exceeds the first threshold 31 . An intensity and / or type of the mitigation action may depend on the value of the activation level parameter A. An activation level parameter value of 0 % may mean that no mitigation action is executed .

[0102] As shown in Fig . 3 , the parameter P and the first and second thresholds 31 and 32 are illustrated as positive values . The parameter P having a positive value may be obtained from the negative time derivative by multiplying the negative time derivative with - 1 . However, alternatively, the parameter P may correspond to the time derivative being negative , i . e . the parameter P and the limits 31 and 32 ( to which the parameter P is compared) may have negative values .

[0103] The method shown in Fig . 2 may further comprise determining i f a stop or shut down procedure of the wind turbine 100 has been initiated, and taking the mitigation action only i f it is determined that the stop procedure has been initiated . The mitigation action may comprise at least one of the following : blocking a functionality that increases the pitch angle of the rotor blades 121 , 122 , 123 , adding a negative pitch of fset to a collective pitch reference of the rotor blades 121 , 122 , 123 , adding a positive of fset to a rotor speed reference of the wind turbine rotor 120 , and adj usting a pitch out rate of the rotor blades 121 , 122 , 123 .

[0104] The method may improve a reduction of tower loading and vibrations during a stop or shut down procedure of the wind turbine and may allow to avoid interference with other control methods that for example mitigate or reduce tower loads induced by a gust . By using the ( load) sensors arranged 2024PF00525 PCT subsequent filing

[0105] 20 at or on the rotor blades an improved counteracting of a change of thrust acting on the wind turbine rotor can be ensured . While speci fic embodiments are disclosed herein, various changes and modi fications can be made without departing from the scope of the invention . The present embodiments are to be considered in all respects as illustrative and non- restrictive , and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein .

Claims

2024PF00525 PCT subsequent filing21Patent claims1. A method of operating a wind turbine (100) , wherein the wind turbine (100) comprises a wind turbine rotor (120) having at least three rotor blades (121, 122, 123) , wherein a sensor (21, 22, 23) is arranged on at least one of the rotor blades (121, 122, 123) , wherein the method comprises:- obtaining, from the sensor (21, 22, 23) , measurements of a blade load of the rotor blade (121, 122, 123) ;- determining a moment measure for the rotor blade (121, 122, 123) from the obtained blade load measurements;- determining for the rotor blade (121, 122, 123) based on the determined moment measure and a respective blade pitch angle of the rotor blade an out-of-plane moment, which is a moment perpendicular to a rotor plane of the wind turbine rotor (120) ;- determining a parameter (P) indicative of a change of thrust applied to the wind turbine rotor (120) based on the out-of-plane moment;- employing the parameter (P) to determine if a change of thrust applied to the wind turbine rotor (120) exceeds a predefined limit (31) ;- determining if a stop or shut down procedure of the wind turbine (100) has been initiated; and- if the parameter (P) indicates that the change of thrust applied to the wind turbine rotor (120) exceeds the predefined limit (31) , taking a mitigation action that comprises controlling the wind turbine (100) to counteract the change of thrust, wherein the mitigation action is only taken if it is determined that the stop or shut down procedure has been initiated.

2. The method according to claim 1, wherein determining the moment measure comprises determining, based on the blade load measurements, one or more of the following:- a flap-wise moment; and- an edge-wise moment.2024PF00525 PCT subsequent filing223. The method according to claim 1 or 2, wherein at least one sensor (21, 22, 23) is arranged on each of at least two rotor blades, preferably on each of the rotor blades (121, 122, 123) , and wherein the method comprises:- obtaining from each of the sensors (21, 22, 23) , measurements of a blade load of the respective rotor blade (121, 122, 123) , and- determining a moment measure for each of the rotor blades (121, 122, 123) from the respective obtained blade load, wherein the parameter (P) is determined from the moment measures determined for each rotor blade.

4. The method according to any of the preceding claims, wherein at least one sensor (21, 22, 23) is arranged on each of at least two, preferably on each, of the rotor blades (121, 122, 123) , wherein the moment measure and the out-of- plane moment are determined for each rotor blade (121, 122, 123) , and wherein determining the parameter (P) comprises processing the out-of-of plane moments by averaging the out of plane moments determined for each rotor blade, or selecting the maximum out-of-plane moment from the out-of- plane moments of each rotor blade (121, 122, 123) .

5. The method according to any of the preceding claims, wherein determining the parameter (P) comprises filtering the out-of-plane moment, in particular the averaged or maximum out-of-plane moment, with one or more filters such as bandstop filter and / or low-pass filter, and wherein the parameter is determined from the filtered out-of-plane moment.

6. The method according to any of the preceding claims, wherein determining the parameter (P) comprises determining a time derivative of the out-of-plane moment, wherein preferably, the parameter is the time derivative of the filtered out-of-plane moment.

7. The method according to any of the preceding claims, further comprising determining from the parameter (P) an2024PF00525 PCT subsequent filing23 activation level parameter (A) indicating how much the parameter (P) exceeds the predefined limit (31) , and taking the mitigation action dependent on the determined activation level parameter (A) .

8. The method according to claim 7, wherein the predefined limit is a first threshold (31) corresponding to a first activation level of 0% and a second threshold (32) corresponds to a second activation level of 100% indicating a maximum mitigation action, wherein the activation level parameter (A) has a value from 0 to 100 %, and wherein an intensity or a type of the mitigation action depends on the value of the activation level parameter (A) .

9. The method according to any of the preceding claims, wherein counteracting the change of thrust applied to the wind turbine rotor (120) comprises counteracting a reduction of thrust applied to the wind turbine rotor.

10. The method according to any of the preceding claims, wherein the mitigation action comprises at least one of the following :- blocking a functionality that increases the pitch angle of the rotor blades (121, 122, 123) ;- adding a negative pitch offset to a collective pitch reference of the rotor blades (121, 122, 123) ;- adding a positive offset to a rotor speed reference of the wind turbine rotor (120) ; and- adjusting a pitch out rate of the rotor blades (121, 122, 123) .

11. The method according to any of the preceding claims, wherein the at least one sensor (21, 22, 23) is a load sensor .

12. A control system (10) configured to control the operation of a wind turbine (100) , wherein the wind turbine (100) comprises a wind turbine rotor (120) having at least three2024PF00525 PCT subsequent filing24 rotor blades (121, 122, 123) , wherein at least one sensor (21, 22, 23) is arranged at one of the rotor blades (121, 122, 123) , and wherein the control system (10) is configured to perform the method according to any of the preceding claims.

13. A computer program for controlling the operation of a wind turbine (100) by a control system (10) , wherein the wind turbine comprises a wind turbine rotor (120) having at least three rotor blades (121, 122, 123) , wherein at least one sensor (21, 22, 23) is arranged at one of the rotor blades (121, 122, 123) , and wherein the computer program comprises control instructions which, when executed by a processing unit (11) of the control system, cause the processing unit (11) to perform the method according to any of claims 1-11.

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