Measuring stress of wind turbine blade and controlling wind turbine

Abstract

A method of determining a value (12) of a stress-dependent quantity of a rotor blade (3) of a wind turbine (1) is described, the method comprising: emitting a primary radar signal (9) towards a portion (21, 22) of the blade (3); receiving a secondary radar signal (10) emanating from the blade (3) as a result of the blade (3) interacting with the primary radar signal (9); analyzing at least the received secondary radar signal (10); and deriving, based on said analysis, a value (12) of the stress-related quantity, said value relating to or indicative of the blade acceleration and / or the first-order time derivative of the blade acceleration and / or the higher-order time derivative of the blade acceleration.

Description

Technical Field

[0001] The invention relates to a method and a corresponding arrangement for determining a value of a stress-related quantity of a rotor blade of a wind turbine. Furthermore, the invention relates to a method and a corresponding controller for controlling a wind turbine, thereby using the determined value of the stress-related quantity of the rotor blade. Furthermore, the invention relates to a wind turbine. Background Art

[0002] During operation, the rotor blades of a wind turbine are subjected to mechanical loads due to impinging wind and rotation and also due to operational influences. In particular, stresses inside the wind turbine blades may be due to pitching the rotor blades, yawing the nacelle housing the rotating shaft to which the rotor blades are connected. Furthermore, stresses inside the wind turbine blades may be due to significant wind loads and other external influences. Stress inside the wind turbine blades may lead to structural damage or may increase wear and tear of the blades.

[0003] US 7246991 uses sensors and radar units inside the rotor blades in order to measure the gap between the rotor blade tip and the tower and the stresses of the rotor blades. EP 2864632 B1 discloses measuring the vibrations of rotor blades by using a radar unit.

[0004] Other conventional methods use strain gauges to measure the deflection of the rotor blades due to the huge loads.

[0005] It has been observed that conventional methods and systems do not measure stresses or loads of a wind turbine during operation in a reliable and fast manner in all cases.

[0006] Therefore, there may be a need for a method and a corresponding arrangement for determining values ​​of stress-related quantities of a rotor blade of a wind turbine. Furthermore, there may be a need for a method of controlling a wind turbine such that loads and / or stresses experienced by the rotor blades are reduced or in particular kept within acceptable limits. There may also be a need for determining stresses or loads, in particular values ​​of stress-related quantities, of rotor blades in an accurate, reliable and fast manner. Furthermore, there may be a need for a wind turbine which can reliably and quickly determine stress-related quantities of rotor blades and / or which can appropriately control the wind turbine such that loads and / or stresses of wind turbine blades are kept within acceptable limits. Summary of the invention

[0007] This need can be met by the subject matter according to the independent claim.The dependent claims describe advantageous embodiments of the invention.

[0008] According to an embodiment of the present invention, a method for determining a value of a stress-related quantity of a rotor blade of a wind turbine is provided, the method comprising: transmitting a primary radar signal towards a portion of the blade; receiving a secondary radar signal emitted from the blade due to the interaction of the blade with the primary radar signal; analyzing at least the received secondary radar signal; and based on the analysis, obtaining a value of the stress-related quantity, the value being related to or indicating the blade acceleration and / or the first-order time derivative of the blade acceleration and / or the higher-order time derivative of the blade acceleration.

[0009] The method may be implemented in software and / or hardware.The method may be at least partially implemented, for example, by a module of a wind turbine controller.

[0010] Stress related quantities of a rotor blade may be indicative of (structural or mechanical) stresses and / or loads to which the rotor blade is subjected, eg due to wind impact and / or pitching and / or yaw. The stress related quantities may be related to mechanical stresses or mechanical loads.

[0011] The method may employ a radar device comprising a transmitter for transmitting a primary radar signal and a receiver for receiving a secondary radar signal. The receiver and / or transmitter may be implemented, for example, by a coaxial cable comprising an opening via which the primary radar signal may be transmitted and via which the secondary radar signal may be received. The coaxial cable may also be referred to as a leakage feeder radar device. The coaxial cable may be mounted around the circumference of the tower so as to measure stress-related quantities in approximately all (horizontal) directions for all yaw positions of a nacelle housing a rotating shaft with a plurality of rotor blades mounted thereon. In other embodiments, the radar device may comprise a number of discrete transmitting (transmitting) and / or receiving units (antennas), which are mounted, for example, at one or more locations around or at the turbine tower.

[0012] The method may for example employ a distance measurement method by measuring the flight time between the emission of a primary radar signal and the reception of a secondary radar signal. In this embodiment, for example, a pulsed radar signal may be emitted and the timing between the emission of a primary radar signal pulse and the reception of a secondary radar signal pulse may be registered. Thereby, the distance between a wind turbine tower and a rotor blade may be determined, in particular continuously, when the rotor blade passes the tower. Thereby, the distance between the tower and the rotor blade may be obtained at different points in time.

[0013] In another embodiment, the Doppler shift may be determined as a frequency or wavelength shift between a transmitted primary radar signal and a secondary radar signal received as a signal emanating from the blades due to the interaction of the rotor blades with the primary radar signal.

[0014] Thus, at least analyzing the received secondary radar signal may include measuring and registering a time interval between transmitting the primary radar signal and receiving the secondary radar signal in order to implement a time-of-flight measurement method. In another embodiment, at least analyzing the received secondary radar signal may include determining a frequency shift or a wavelength shift or a difference between the transmitted primary radar signal and the received secondary radar signal. For this embodiment, the received secondary radar signal and the transmitted primary radar signal may be analyzed in combination or compared to each other with respect to frequency and / or wavelength.

[0015] Thus, the analysis may, for example, result in the distance between the rotor blade and the tower, and / or may result in the frequency shift. These quantities may be determined for subsequent time points, for example for separate sampling time intervals or analysis time intervals. The corresponding quantity, for example the distance and / or the Doppler shift, may also be processed with respect to its temporal variation. Thus, a stress-related quantity or a value of the stress-related quantity may be obtained, which is related to or indicates a blade acceleration, for example fore-aft acceleration and / or lateral acceleration, and / or a first-order time derivative of the blade acceleration and / or a higher-order time derivative of the blade acceleration. Instead of the corresponding time derivative, it is also possible to consider only the temporal variation as a stress-related quantity. The blade acceleration and / or the time derivative of the blade acceleration and / or the higher-order time derivative of the blade acceleration may suitably and advantageously indicate the loads and / or stresses to which the wind turbine blade is subjected. For example, the second derivative of the distance between the rotor blade and the tower and / or the third derivative of the distance can be regarded as a stress-related quantity. Additionally or alternatively, in another embodiment, the first derivative of the Doppler shift and / or the second derivative of the Doppler shift can be regarded as a stress-related quantity. Thereby, the load and / or stress experienced by the blade can be reliably obtained.

[0016] According to an embodiment of the present invention, at least analyzing the received secondary radar signal comprises: determining the frequency shift of the secondary radar signal relative to the primary radar signal; and obtaining a stress-related quantity based on a time variation of the frequency shift, in particular a time variation of the frequency shift per time interval.

[0017] Instead of or in addition to the frequency shift, a wavelength shift may also be determined and utilized (i.e. based on the temporal variation of the wavelength shift) to derive the stress-related quantity. The frequency shift or wavelength shift may be due to a movement of the blade, which then causes a Doppler shift. The frequency shift may be proportional to the respective (radial) velocity of the rotor blade, in particular in the direction towards and away from the radar transmitter and / or receiver. The temporal variation of the frequency shift (and / or wavelength shift) may, for example, be determined as the difference between an actually determined frequency shift at a first point in time and a previously determined frequency shift at a previous point in time. In particular, a differential quotient may be determined or formed between the temporal variation of the frequency shift (or wavelength shift) and the time interval between two determined frequency shifts.

[0018] The temporal variation of the frequency shift may be or may be correlated to the acceleration experienced by the rotor blade, and in particular may be correlated to the temporal variation of the speed (radial, i.e. towards and away from the tower center) of the rotor blade. This quantity may reliably indicate the stresses and / or loads experienced by the rotor blade.

[0019] According to an embodiment of the invention, the stress-related quantity of the rotor blade comprises a blade acceleration, in particular determined as a time derivative or a time differential quotient of a frequency shift.

[0020] In other embodiments, the stress-related quantity may comprise a combination of blade acceleration and higher order time derivatives or time variations of blade acceleration.When determined as a derivative or time differential quotient of a frequency shift, the stress-related quantity may be determined in a simple manner.

[0021] According to an embodiment of the invention, the stress-related quantity of the rotor blade comprises jerk, in particular determined as a second time derivative or a second time differential quotient of a frequency shift, wherein obtaining the value of the stress-related quantity in particular also comprises determining the sign of the value of the jerk.

[0022] The jerk may be proportional to the time change of the acceleration of the rotor blade or may indicate the time change of the acceleration of the rotor blade. The jerk may in particular be determined as a derivative or differential quotient of a derivative or differential quotient of the frequency shift, thus a second time derivative or second time differential quotient of the frequency shift or wavelength shift. The sign of the value of the jerk may indicate the direction in which the load or stress acts or is applied. Thereby, control of the wind turbine may be improved or achieved.

[0023] According to an embodiment of the invention, deriving the value of the stress-related quantity comprises determining values ​​of the jerk at a first surface of the rotor blade and at a second surface of the rotor blade, the first surface being an outer surface of the blade facing the tower, the second surface being an inner surface of the blade facing the tower and spaced apart from the first surface.

[0024] The primary radar signal may be reflected on one or more interfaces or surfaces of the rotor blade. For example, the first surface may be an airfoil surface, i.e., an outer surface of the rotor blade. The second surface may be, for example, an inner surface of a wall delimiting the rotor blade. The first surface and the second surface may be, for example, spaced apart in the thickness direction of the rotor blade by, for example, the thickness of the blade minus the thickness of the blade wall. Thus, in this embodiment, the jerk or, in general, a stress-related quantity may be determined on different parts of the rotor blade, such as wall parts, which may allow a more reliable determination of the effective stress or load experienced by the rotor blade as a whole.

[0025] According to an embodiment of the invention, the frequency shift is caused by a blade movement, the blade movement comprising at least one of: a movement away from and towards the tower, in particular away from and towards a radar device mounted at the tower, in particular a movement out of the plane of rotation, and / or an edgewise movement substantially in the plane of rotation.

[0026] Movement away from and towards the tower may also be referred to as fore-aft vibration or oscillation or movement, and sideways movement may also be referred to as sideways vibration or oscillation.Thereby, typically occurring vibrations or oscillations and load conditions of wind turbine blades may be supported.

[0027] According to an embodiment of the invention, the method further comprises: determining a further value of the stress-related variable at a later time, in particular after at least one further revolution; comparing the value with the further value in order to determine the stress of the rotor blade.

[0028] Thereby, the time development of the loads / stresses of the rotor blades can be registered and also processed. Furthermore, the comparison can allow a plausibility check, for example allowing outliers to be ignored. Depending on the comparison result, the control of the wind turbine can be modified.

[0029] The determined value of the stress-related quantity may advantageously be utilized for controlling the wind turbine.

[0030] According to an embodiment of the present invention, there is provided a method of controlling a wind turbine comprising a plurality of rotor blades mounted on a rotating shaft, the method comprising: implementing the method of determining a value of a stress-related quantity of a rotor blade of the wind turbine according to one of the previous embodiments; and controlling the wind turbine based on the value of the stress-related quantity.

[0031] When controlling a wind turbine taking into account the values ​​of stress-related quantities, wear of wind turbine blades as well as loads and / or stresses may, for example, be kept within acceptable limits. Furthermore, the performance of the wind turbine may be improved and the life of components may be extended.

[0032] The method may also be implemented in software and / or hardware and / or may be performed by a wind turbine controller module.

[0033] According to an embodiment of the invention, controlling a wind turbine comprises: yaw a nacelle accommodating a rotating shaft on which a plurality of rotor blades are mounted; and / or pitching at least one rotor blade; and / or controlling generator torque and / or power based on values ​​of stress-related quantities of the rotor blades.

[0034] Therefore, different conventionally available control actions can be implemented according to the determined stress-related quantity value. Some or all of these control measures may affect the loads experienced by the blades. Thereby, the loads can be kept within acceptable limits. In addition, the generator speed can be controlled based on the value of the stress-related quantity.

[0035] According to an embodiment of the invention, the wind turbine is controlled such that the value of the stress related quantity of the rotor blade is substantially at a stress reference value or below a stress threshold, in particular including smooth control of pitch and / or smooth control of yaw and / or reduced pitch.

[0036] In particular, the pitching may be smoothed in such a way that the rate of change of the pitch angle is reduced compared to a conventionally used pitch rate or normal pitch rate. Furthermore, the yaw rate may be reduced compared to a nominal yaw rate in order to provide smooth control of yaw. Reduced pitching may involve keeping the pitch angle, for example, below an angle threshold.

[0037] According to an embodiment of the invention, controlling a wind turbine comprises adopting a cascade control, which comprises: an inner control loop with feedback of measured internal control variables, the internal control variables comprising pitch angle or yaw position or generator torque or generator power; and an outer control loop with feedback of measured external control variables, the external control variables comprising stress-related quantities, in particular jerk, wherein the outer control loop receives a difference between a reference value of the external control variable, in particular a stress reference value or a jerk reference value, and an actual value of the external control variable as input to an external controller, from which the external controller obtains a reference value or a reference offset value of the internal control variable, in particular a reference pitch angle or a reference yaw angle or a reference torque, wherein the difference between the reference value of the internal control variable and the actual value of the internal control variable is provided to the internal controller.

[0038] The internal control loop may for example control the pitch angle or yaw position of the rotor blades or the generator torque or generator power, which are examples of internal control variables. As examples of external control variables, the external control loop may control stress-related quantities, in particular jerk. The external controller obtains a reference value or a reference offset value of the internal control variable, such as a pitch angle set point or a pitch angle reference value (or a yaw position reference or a generator torque reference or a generator power reference), from a corresponding reference value of the external control variable. The internal controller and the external controller may for example comprise a PI controller.

[0039] In this case, the system may be in steady state, where for example the jerk set point may be zero. Loads (gusts) may induce stress in the blades, which may be measured as the jerk becoming different from zero. The jerk must become zero again, so the controller may induce a pitch change or yaw to reduce the stress / jerk. If we set a new pitch angle for the power output, this signal may come from the turbine controller and result in a dynamic change. Here, it may be necessary to implement a multivariable control.

[0040] According to an embodiment of the invention, controlling the wind turbine comprises shutting down the wind turbine when the value of the stress-related quantity and / or the jerk value exceeds a stress threshold, in particular for a time longer than the stress time interval. Thereby, reliable operation of the wind turbine can be ensured and the life of the components can be extended.

[0041] It shall be understood that features disclosed, described, provided or applied, alone or in any combination, to the method for determining a value of a stress-related quantity for a rotor blade of a wind turbine also apply, alone or in any combination, to the arrangement for determining a value of a stress-related quantity for a rotor blade of a wind turbine according to embodiments of the present invention, and vice versa.

[0042] According to an embodiment of the present invention, an arrangement structure for determining a value of a stress-related quantity of a rotor blade of a wind turbine is provided, the arrangement structure comprising: a radar transmitter, the radar transmitter being configured to transmit a primary radar signal towards a portion of the blade; a radar receiver, the radar receiver being configured to receive a secondary radar signal emitted from the blade due to the interaction of the blade with the primary radar signal; an analysis module, the analysis module being configured to: at least analyze the received secondary radar signal; based on the analysis, obtain a value of the stress-related quantity, the value being related to or indicating the blade acceleration and / or the first-order time derivative of the blade acceleration and / or the higher-order time derivative of the blade acceleration.

[0043] Furthermore, features disclosed, described or applied to the method of controlling a wind turbine, alone or in any combination, may also be applied to the controller for controlling a wind turbine according to embodiments of the invention, alone or in any combination, and vice versa.

[0044] According to an embodiment of the present invention, there is provided a controller for controlling a wind turbine, the wind turbine comprising a plurality of rotor blades mounted on a rotating shaft, the controller comprising: an arrangement structure for determining a value of a stress-related quantity of a rotor blade of the wind turbine according to the previous embodiment; and a control module configured to control the wind turbine based on the value of the stress-related quantity.

[0045] Furthermore, a wind turbine is provided, which includes a rotating shaft on which a plurality of rotor blades are mounted, and a controller according to the previous embodiment.

[0046] Embodiments of the present invention provide methods and systems for measuring and / or monitoring the jerk of a rotor blade. Thus, the jerk can be considered to be directly proportional to the force applied to the blade. By measuring the jerk, it may be possible to take action to reduce stress in a short time. Pitch and / or yaw actions can be smoothed and reduced to acceptable levels. It may be possible to shut down a turbine under heavy loads (indicated by high jerk values).

[0047] According to an embodiment of the invention, a Doppler radar unit is used to measure the jerk of the blade. Thereby, the blade jerk may be directly related to the second order time derivative or second order time variation of the Doppler shift. Measuring the jerk and using the jerk for control actions may have the advantage of directly influencing the stress inside the blade. Furthermore, it may be possible to reduce wear and tear by smoothing the control actions of pitch and / or yaw.

[0048] The blade acceleration may be directly related to the first time derivative or first order time variation of the Doppler shift.

[0049] The radar device may comprise a leaky feed or a device referred to as a leaky feed according to the prior art. However, the leaky feed device is not mandatory and embodiments of the invention may be used without a leaky feed device.

[0050] The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment.The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Embodiments of the present invention will now be described with reference to the accompanying drawings. The present invention is not limited to the embodiments shown or described.

[0052] Figure 1 schematically illustrates a wind turbine according to an embodiment of the present invention;

[0053] Figure 2 schematically illustrates a radar unit comprised in an arrangement for determining a value of a stress-related quantity of a rotor blade according to an embodiment of the invention;

[0054] Figure 3 Schematically illustrates a control method scheme adopted according to an embodiment of the present invention;

[0055] Figure 4schematically illustrates a control circuit diagram as employed according to an embodiment of the present invention; and

[0056] Figure 5 A part of a wind turbine according to an embodiment of the invention is schematically illustrated. DETAILED DESCRIPTION

[0057] Figure 1 The wind turbine 1 schematically illustrated in comprises a rotating shaft 2 on which a plurality of rotor blades 3 are mounted. The rotating shaft 2 is accommodated in a nacelle 4 which is rotatably mounted on top of a wind turbine tower 5. The wind turbine 1 further comprises a controller 6 for controlling the wind turbine according to an embodiment of the invention.

[0058] The controller 6 for controlling a wind turbine 1 comprises an arrangement 7 for determining a value of a stress related quantity of a rotor blade 3 of the wind turbine 1. In addition to the arrangement 7, the controller 6 comprises a control module 13 configured to control the wind turbine 1 based on the value 12 of the stress related quantity.

[0059] Thus, the arrangement 7 comprises a radar unit 8, the radar unit 8 comprising a radar transmitter, the radar transmitter being configured to transmit a primary radar signal 9 towards a portion of the blade 3. The radar unit 8 further comprises a radar receiver, the radar receiver being configured to receive a secondary radar signal 10 emanating from the blade 3 as a result of the blade 3 interacting with the primary radar signal 9. The arrangement 7 further comprises an analysis module 11, the analysis module 11 being configured to analyze at least the received secondary radar signal 10 (and in particular further also the primary radar signal 9). The analysis module 11 is further configured to derive a value 12 of a stress-related quantity based on the analysis, the value being related to or indicative of the blade acceleration and / or the first time derivative of the blade acceleration and / or the higher time derivative of the blade acceleration.

[0060] The radar unit 8 may be configured to measure in different radial directions around the tower, for example 360 ​​degrees around the tower, in order to measure blade movement / distance for each nacelle position.

[0061] exist Figure 1 In the figure, the stress-related quantity (the value) is marked with the reference numeral 12.

[0062] The wind turbine comprises a yaw system 14 configured to yaw the nacelle 4 relative to the wind turbine tower 5, i.e. to rotate the nacelle 4 about a substantially vertically oriented axis relative to the wind turbine tower 5. To control the wind turbine, the control module 13 may provide a yaw control signal 15 to the yaw system 14, which may be derived based on the value 12 of the stress-related quantity.

[0063] The wind turbine 1 further comprises a pitch system 16 configured to pitch at least one wind turbine blade 3 about a substantially longitudinal axis, i.e. to rotate the wind turbine blade about the longitudinal axis. Thus, for controlling the wind turbine 1, the control module 13 may provide a pitch control signal 17 to the pitch system 16, wherein the pitch control signal 17 is derived at least based on the value 12 of the stress-related quantity.

[0064] The wind turbine 1 comprises a generator system 18, which is driven by the rotating shaft 2 and outputs alternating current at an output terminal 19. To control the wind turbine 1, the control module 13 may also provide a generator system control signal 20 to the generator system 18. The generator control signal 20 may, for example, be indicative of a generator torque and / or a generator power, wherein the control signal 20 may in particular be provided to a converter.

[0065] Thereby, the control system 13 may be configured to control the yawing of the nacelle 4 and / or the pitching of at least one rotor blade 3 and / or control the generator system 18. The control module is configured to implement a method of controlling a wind turbine 1, wherein first a method for determining a value of a stress-related quantity 12 of a rotor blade 3 is implemented and then the wind turbine 1 is controlled based on the value 12 of the stress-related quantity.

[0066] The arrangement 7 can continuously determine the value 12 of the stress-related variable and can register and retrieve different values ​​determined for different points in time and in particular compare them in order to also improve the control of the wind turbine.

[0067] The primary radar signal 9 may interact with the first surface 21 of the blade 3 and / or the second surface 22 of the blade 3. In particular, the primary radar signal 9 may be reflected at the first surface 21 and / or the second surface 22, thereby generating the secondary radar signal 10. Depending on which surface it is (i.e., the first surface 21 is the outer surface of the blade facing the tower, or the second surface 22 is the inner surface of the blade facing the tower and spaced apart from the first surface by a distance d), the stress at the first surface 21 or the stress at the second surface may be obtained according to an embodiment of the present invention. Two stress values ​​may be employed or considered to control the wind turbine 1. Therefore, the control of the wind turbine 1 may depend on one or two stress-related values ​​associated with the first surface 21 and / or the second surface 22.

[0068] Embodiments of the present invention can determine the frequency shift Δf (or wavelength shift Δλ) of the secondary radar signal 10 relative to the primary radar signal 9. The stress-related quantity 12 can then be obtained, for example, as the time variation of the frequency shift or the time variation of the frequency shift per time interval, i.e., the differential quotient.

[0069] The time derivative of the frequency shift may be proportional to the blade acceleration.The stress-related quantity 12 may also include jerk, for example determined as the second order time derivative of the frequency shift or wavelength shift.

[0070] Frequency shifts may be caused by blade movement, e.g. Figure 1 Movement away from and towards the tower is indicated with reference numeral 23. The arrangement 7 may implement a method of determining a value of a stress-related quantity of a rotor blade 3 of a wind turbine 1 according to an embodiment of the invention.

[0071] like Figure 1 As illustrated in FIG. 1 , the analysis module 11 and the control module 13 may be included as components of a wind turbine controller, or may be implemented at separate distinct modules.

[0072] Figure 2 A radar unit 8 is schematically shown, which can be used, for example, Figure 1 . The radar unit 8 comprises a voltage controlled oscillator (VCO) 24 which receives a drive signal 25 from a processing unit 26 comprising a respective D / A port 27 and an A / D port 28. The voltage controlled oscillator 24 generates an oscillation signal 29 which is provided to a separation element 30 which separates the radar signal into two parts. One part is provided to an amplifier 31 which provides the radar signal to a transmitter antenna 32 for transmitting the radar signal as a primary radar signal 9 towards the rotor blade 3. The secondary radar signal 10 returning from the rotor blade 3 is received by a receiver antenna 33 which is provided to a receiving module 34 which provides an output to a mixing element 35. The mixing element 35 mixes the received secondary radar signal 10 with a part of the primary signal and provides the result to a filter unit 36. The processing unit 26 receives the filtered mixed secondary radar signal 10. According to an embodiment of the invention, the processing unit 26 may, for example, implement a time-of-flight measurement and / or a frequency shift measurement.

[0073] Figure 3 The control module 13 according to an embodiment of the present invention is shown in FIG. Figure 1In the wind turbine 1 shown in FIG. 1 , a jerk (or stress in general) reference value 37 is received at a differential element 38, which also receives an actual value 40 of the jerk to which the rotor blade 3 is subjected. A jerk sensor 41 measures the actual value 40 of the jerk (the jerk sensor 41 may, for example, comprise Figure 1 The control module 13 provides an output 47.

[0074] Figure 4 The control module 13a according to an embodiment of the present invention is schematically illustrated, which may for example include Figure 1 In the wind turbine 1 shown in FIG. Figure 4 The control module 13a shown in FIG. 1 is implemented as a cascade control. Figure 4 The control module 13a illustrated in FIG. 1 comprises an inner control loop 48 with feedback 49 of measured internal control variables, including for example pitch angle or yaw position or generator torque or generator power. The control module 13a also comprises an outer control loop 50 with feedback 51 of measured external control variables, including stress-related quantities, in particular jerk. Thus, the measured feedback signal 51 may be substantially equal to Figure 3 The measured jerk 40 is shown in FIG. The outer loop 50 receives a reference value 52 of an external control variable. In particular, the outer loop 50 receives a stress reference value 52 or a jerk reference value (eg, 37, as shown in FIG. 3). Figure 3 ).

[0075] The external control loop 50 receives the difference 53 between the reference value 52 of the external control variable and the actual value 51 of the external control variable as input to the external controller 54 (the external controller 54 may be configured, for example, as Figure 3 The external controller 54 thereby obtains a reference value 55 or a reference offset value 55 of the internal control loop (ie a reference value of the internal control variable), wherein a difference 58 between the reference value 55 and the actual value 49 of the internal control variable is provided to the internal controller 56.

[0076] The internal controller 56 may be, for example, a pitch controller. The reference value 55 may be, for example, a pitch angle reference value. The actual value of the internal control variable 49 may be, for example, an actual pitch angle. The differential element 57 obtains an error signal 58 as the difference between the reference value 55 and the actual value 49 of the internal control variable.

[0077] The internal controller 56 obtains an actuation signal 59, which may, for example, comprise a set point for a variable frequency drive. The signal 59 is received by a system 60, which may, for example, comprise a (conventional) pitch system, comprising blocks 60, 62, 63. In the illustrated embodiment, the signal 59 is received by a power electronics 60, which comprises an inverter for a variable frequency drive. The system part 60 receiving the signal 59 thereby obtains a current and / or voltage 61, which is supplied to a motor 62 as actuation power. The motor 62 generates a specific torque and rotation speed, which are supplied to a gearbox 63 (output torque 63a), which ultimately actuates the pitch of the rotor blade 3.

[0078] The mechanical torque output by the system 60 is received by the blade system 64 (output jerk 64a). In addition, a disturbance 65 is received that disturbs the jerk measurement. The jerk measurement system 41 measures the jerk of the blade. The jerk measurement system 41 may include, for example, Figure 1 The arrangement 7 is shown in FIG. 8 . The pitch angle measurement system 66 measures the pitch angle in order to provide the actual pitch angle 49 .

[0079] Figure 4 is an example of an electric pitch system. Other embodiments may not include an electric pitch system, and / or the pitch system may additionally or alternatively include a hydraulic system.

[0080] exist Figure 4 In the embodiment of the present invention, the inner control loop 48 can control the pitch angle, and the outer control loop 50 can control the jerk of the blades. For example, there can be additional cascade controls regarding torque control and other control variables that may affect the jerk on the blades.

[0081] Figure 5 A wind turbine or part of a wind turbine control system according to an embodiment of the invention is schematically illustrated. A radar unit 8 performs radar measurements and provides the measurement results to an analysis module 11. As a main path 67, the value 12 of the stress-related quantity can be provided to a main turbine controller 68. The main turbine controller 68 can control or provide corresponding control signals 69, 70 to a pitch controller 71 and a yaw controller 72, respectively.

[0082] According to the alternative path 73 , the processing unit 11 directly provides the value 12 of the stress-related quantity to the pitch controller 71 and the yaw controller 72 , or directly provides corresponding control signals (eg indicating reference values) to the pitch controller 71 and the yaw controller 72 .

[0083] Measuring jerk can be important because it can allow an indication of stresses within the blade. Reducing jerk to an acceptable level can extend the life of the blade by reducing wear and tear. Jerk can be positive or negative and can be directly proportional to the load (force) applied to the blade. By measuring jerk, the stresses applied to the blade can be reduced. Figure 3 As illustrated in , the pitch change can be smoothed by limiting the jerk in the closed loop control unit. The pitch change process itself can be smoothed or, in case of turbulence, the load on the blades can be reduced by pitching it (flag position / angle of attack).

[0084] Conventional methods may have problems in directly measuring low frequency vibrations. Conventional methods may require multiple rotation measurements of the blade to detect such low frequency vibrations. In contrast, according to an embodiment of the present invention, by detecting jerkiness using the Doppler effect of the radar, action can be taken in a shorter time because low frequency vibrations can be detected earlier. Stress or in particular low frequency vibrations can be detected within about 1 / 5 of a revolution of the blade or rotor. Jerkiness may cause a single event, which may be so strong and huge that the blade breaks or some cracks appear. When the blade enters the range of the radar unit or the sensor in general, the jerkiness or in general stress-related quantities can be detected.

[0085] The analysis module 11 or a separate element may comprise storage means or memory, in particular for each blade, wherein in particular the actual response from the blade (and optionally other data) and the state of the last (previous) revolution may be stored for comparison. Close to the tower (i.e. when the rotor blade passes the tower), the value of the jerk may be best detected. Furthermore, sensor fusion may allow 360° measurement or tracking.

[0086] According to an embodiment of the invention, a wind turbine may include at least one Doppler radar unit and a processing unit configured to receive measurement data from the radar unit and determine the jerk of the blade movement and / or edgewise movement in a direction towards or away from the tower by analyzing the Doppler shift of the received radar signal due to the movement of the blade towards or away from the turbine tower. Thereby, the processing unit may be configured to calculate the properties of the blade jerk based on the time derivative of the Doppler shift change caused by the jerk in a given time. The jerk is proportional to the time derivative of each Doppler shift change.

[0087] The Doppler shift may also be referred to as a frequency response, as the system may be configured to determine the frequency response. The jerk of the tower side and the opposite side of the blade may be derived from the corresponding identified signals. The controller may be configured to shut down the wind turbine when the jerk detected in the blade in a direction toward or away from the turbine tower exceeds a predetermined stress value.

[0088] The controller may be configured to reduce the pitch response and / or electrical load (torque) variation to reduce the jerk of the blades. Thus, pitch angle control and / or generator torque control may be implemented.

[0089] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Moreover, elements described in connection with different embodiments may be combined. It should also be noted that the reference signs in the claims should not be interpreted as limiting the scope of the claims.

Claims

1. A method for determining a value (12) of a stress-related quantity of a rotor blade (3) of a wind turbine (1), the method comprising: emitting a primary radar signal (9) towards a portion (21, 22) of the blade (3); receiving a secondary radar signal (10) emitted from the blade (3) as a result of the blade (3) interacting with the primary radar signal (9); analyzing at least a received secondary radar signal (10); as well as Based on the analysis, a value (12) of the stress-related quantity is obtained, which is related to or indicates the blade acceleration and / or the first time derivative of the blade acceleration and / or the higher time derivatives of the blade acceleration.

2. The method according to the preceding claim, wherein: At least analyzing the received secondary radar signal (10) comprises: determining a frequency shift (Δf) of the secondary radar signal (10) relative to the primary radar signal (9); The stress-related quantity (12) is obtained based on the time variation of the frequency shift, in particular the time variation of the frequency shift at each time interval.

3. A method according to any one of the preceding claims, wherein: The stress-related quantity (12) of the rotor blade comprises a blade acceleration, in particular determined as a time derivative or a time differential quotient of the frequency shift.

4. A method according to any one of the preceding claims, wherein: The stress-related variable of the rotor blade comprises a jerk (51), in particular determined as a second-order time derivative or a second-order time differential quotient of the frequency shift, wherein the value of the stress-related variable is obtained, in particular comprising determining the sign of the value of the jerk (51).

5. A method according to any one of the preceding claims, wherein: Obtaining the value (12) of the stress-related quantity comprises determining values ​​of a jerk (51) at a first surface (21) of the rotor blade (3) and at a second surface (22) of the rotor blade (3), the first surface being an outer surface of the blade facing the tower (5) and the second surface being an inner surface of the blade facing the tower (5) and spaced apart from the first surface.

6. A method according to any one of the preceding claims, when dependent on claim 2, wherein: The frequency shift is caused by a movement of the blade (3), the movement comprising at least one of the following: a movement (23) away from and towards the tower (5), in particular away from and towards a radar device installed at the tower, in particular out of the plane of rotation, and / or Essentially edgewise movement in the plane of rotation.

7. The method according to any one of the preceding claims, further comprising: determining, at a later time, in particular after at least one further revolution, a further value of the stress-related variable; The value is compared with the further value and / or at least one previous value in order to determine the stress of the rotor blade.

8. A method of controlling a wind turbine (1), the wind turbine (1) comprising a plurality of rotor blades (3) mounted on a rotating shaft (2), the method comprising: Implementing a method for determining a value (12) of a stress-related quantity for a rotor blade of a wind turbine according to any of the preceding claims; as well as The wind turbine (1) is controlled based on the value (12) of the stress-related quantity.

9. The method according to the preceding claim, wherein: Controlling the wind turbine comprises: Yawing (14) a nacelle (4) housing a rotating shaft (2) on which the plurality of rotor blades (3) are mounted; and / or Pitching (17) at least one rotor blade (3); and / or Generator torque and / or power is controlled (20) based on the value (12) of the stress-related quantity of the rotor blade.

10. The method according to the preceding claim, wherein: The wind turbine (1) is controlled such that the value of the stress-related quantity of the rotor blade is substantially at a stress reference value or below a stress threshold, in particular including smooth control of pitch and / or smooth control of yaw and / or reduced pitch.

11. The method according to any one of the preceding claims 8 to 10, wherein: Controlling the wind turbine comprises using a cascade control (13a), the cascade control (13a) comprising: an internal control loop (48) with feedback (49) of measured internal control variables, including pitch angle or yaw position or generator torque or generator power; and an outer control loop (50) with feedback (51) of measured external control variables, said external control variables comprising said stress-related quantities, in particular jerk, The external control loop (50) receives the difference (53) between the reference value (52) of the external control variable, in particular the stress reference value or the jerk reference value and the actual value (51) of the external control variable as input to an external controller (54), and the external controller (54) thereby obtains the reference value (55) or the reference offset value of the internal control variable, in particular the reference pitch angle or the reference yaw angle or the reference torque, wherein the difference (58) between the reference value (55) of the internal control variable and the actual value (49) of the internal control variable is provided to the internal controller (56).

12. The method according to any one of the preceding claims 8 to 11, wherein: Controlling the wind turbine comprises: When the value (12) of the stress-related variable and / or the jerk value exceeds a stress threshold, in particular for a time period longer than a stress time interval, the wind turbine (1) is shut down.

13. An arrangement (7) for determining a value (12) of a stress-related quantity of a rotor blade (3) of a wind turbine (1), the arrangement comprising: a radar transmitter (32) configured to transmit a primary radar signal (9) toward a portion of the blade; a radar receiver (33) configured to receive a secondary radar signal (10) emanating from the blade as a result of the blade interacting with the primary radar signal (9); An analysis module (11), wherein the analysis module (11) is configured to: analyzing at least a received secondary radar signal (10); Based on the analysis, a value (12) of the stress-related quantity is obtained, which is related to or indicates the blade acceleration and / or the first time derivative of the blade acceleration and / or the higher time derivatives of the blade acceleration.

14. A controller (6) for controlling a wind turbine, the wind turbine comprising a plurality of rotor blades mounted on a rotating shaft, the controller comprising: Arrangement (7) for determining a value of a stress-related variable of a rotor blade of a wind turbine according to the preceding claim; A control module (13) is configured to control the wind turbine based on the value of the stress-related quantity.

15. A wind turbine (1) comprising: A rotating shaft (2) on which a plurality of rotor blades (3) are mounted; A controller (6) as claimed in the preceding claim.

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