Determining rotor azimuth angle of a wind turbine

Abstract

The invention relates to determining rotor azimuth angle of a wind turbine. The invention involves obtaining a rotor speed signal indicative of rotor speed of the wind turbine. The rotor speed signal is for input into a rotor speed integrator. The invention involves receiving an acceleration sensor signal, from an acceleration sensor located in a rotor hub of the wind turbine, indicative of gravitational acceleration of the rotor hub relative to a rotation axis of the rotor hub. The invention involves determining a reference rotor azimuth angle based on the received acceleration sensor signal, and generating a reset pulse signal based on the determined reference rotor azimuth angle. Upon receiving the generated reset pulse signal at the rotor speed integrator, the integrator is reset. The invention involves using the reset rotor speed integrator to determine wind turbine rotor azimuth angle based on the obtained rotor speed signal.

Description

[0001] DETERMINING ROTOR AZIMUTH ANGLE OF A WIND TURBINE

[0002] TECHNICAL FIELD

[0003] The invention relates to determining rotor azimuth angle of a wind turbine. In particular, a reference rotor azimuth angle is determined based on an acceleration sensor signal from an acceleration sensor located in a rotor hub of the wind turbine, and the reference rotor azimuth angle is used to reset a rotor speed integrator used to determine the rotor azimuth angle.

[0004] BACKGROUND

[0005] A wind turbine typically incudes a tower, a nacelle atop the tower for housing one or more electrical components (e.g. converter), a rotor that rotates relative to the nacelle and tower, and a number of rotor blades (typically, three rotor blades) attached to the rotor. The rotational position of a wind turbine rotor - also referred to as the rotor azimuth angle - can be used for various different functions / purposes. These include being used for positioning the rotor and rotor blades during service or maintenance operations of the wind turbine, for calibrating other sensing systems of the wind turbine, and as an input into one or more control routines / process of the wind turbine, e.g. a load reducing control routine. It will be understood, therefore, that it is important to be able to obtain accurate determinations of the rotor azimuth angle of a wind turbine.

[0006] A known approach for obtaining wind turbine rotor azimuth angle is to apply an integrator function / module to an obtained rotor speed signal of the wind turbine, i.e. integrating the obtained speed of the wind turbine rotor with respect to time. The rotor speed signal may be obtained in any suitable manner, e.g. based on a signal obtained from a rotary encoder that measures a speed of a generator of the wind turbine. Such an approach is prone to capture any errors in the rotor speed measurement, accumulating these errors over time.

[0007] A known method for mitigating these accumulated errors is to include a dedicated reset sensor in the wind turbine for sensing when the wind turbine rotor starts a new revolution. Upon this being sensed, a signal is generated to reset the integrator so as to minimise errors being accumulated. Such a method can suffer the drawback that the dedicated sensor may not reliably be able to detect a target magnet protruding from a rotor lock disc of the rotor, and so not able to accurately measure when the rotor starts a new revolution. To address this issue, the sensor may be moved closer to the rotor lock disc; however, this can result in the magnet and sensor colliding under uneven loads of the rotor, which can damage or even destroy the magnet and / or sensor.

[0008] There remains a need to provide improved methods for obtaining rotor azimuth speed in an accurate and reliable manner. It is against this background to which the present invention is set.

[0009] SUMMARY OF THE INVENTION

[0010] According to an aspect of the invention there is provided a method of determining rotor azimuth angle of a wind turbine. The method comprises obtaining a rotor speed signal indicative of rotor speed of the wind turbine. The rotor speed signal is for input into a rotor speed integrator. The method comprises receiving an acceleration sensor signal, from an acceleration sensor located in a rotor hub of the wind turbine, indicative of acceleration, due to gravity, in a radial direction of the rotor hub relative to a rotation axis of the rotor hub. The method comprises determining a reference rotor azimuth angle based on the received acceleration sensor signal. The method comprises generating a reset pulse signal based on the determined reference rotor azimuth angle. The method comprises, upon receiving the generated reset pulse signal at the rotor speed integrator, resetting the rotor speed integrator. The method comprises using the reset rotor speed integrator to determine rotor azimuth angle of the wind turbine based on the obtained rotor speed signal.

[0011] The reset pulse signal may be generated when the determined reference rotor azimuth angle satisfies a prescribed condition indicating that the wind turbine rotor is at a prescribed rotational position. The prescribed rotational position may correspond to (a particular) one of the rotor blades of the wind turbine pointing substantially directly downwards. This rotational position may correspond to a rotor azimuth angle of zero radians.

[0012] The prescribed rotational position may correspond to a minimum value of the reference rotor azimuth angle. The prescribed condition may be that the determined reference rotor azimuth angle falls below a prescribed threshold value greater than the minimum value. Typically, the minimum value may be zero radians (and may correspond to one of the wind turbine rotor blades pointing directly downwards). The prescribed threshold value may therefore be a value that is only slightly greater than the minimum value, e.g. only slightly greater than zero radians. For instance, the prescribed threshold value may therefore be any of a value that is less than or equal to 0.1 radians, less than or equal to 0.2 radians, less than or equal to 0.3 radians, etc., or any other suitable value.

[0013] In some examples, resetting the rotor speed integrator may comprise setting instantaneous rotor azimuth angle in the rotor speed integrator to be equal to the reference rotor azimuth angle. Resetting the rotor speed integrator may be regarded as restarting a calculation of the integrator from zero, where this value represent a phase / position of the rotor.

[0014] The reset pulse signal may be generated once per revolution of the wind turbine rotor.

[0015] The received acceleration sensor signal may be indicative of acceleration, due to gravity, of the rotor hub in first and second mutually orthogonal radial directions.

[0016] In some examples, determining the reference rotor azimuth angle may comprise applying a trigonometric function to components of the received acceleration sensor signal in the first and second mutually orthogonal radial directions.

[0017] The reference rotor azimuth angle may be determined in dependence on an orientation and position of the acceleration sensor in the rotor hub.

[0018] The method may comprise applying an offset to the received acceleration sensor signal to compensate for a centrifugal force or centripetal acceleration contribution included therein. The offset may be proportional to the square of the rotor speed or the angular rotor speed in the obtained rotor speed signal. The reference rotor azimuth angle may be determined based on the centrifugal-compensated or the centripetal-compensated acceleration sensor signal.

[0019] The offset may be applied to the components of the transformed acceleration sensor signal, i.e. the acceleration sensor signal which has been transformed from the rotating reference frame into a stationary reference frame. The rotor speed in the obtained rotor speed signal may be determined based on a received generator speed signal indicative of generator speed of the wind turbine. The generator speed signal may be received from a rotary encoder of the wind turbine.

[0020] The rotor speed in the obtained rotor speed signal may be determined as the generator speed compensated by a gear ratio of a gearbox of the wind turbine.

[0021] The rotor speed in the obtained rotor speed signal may be determined by performing steps of: receiving an angular velocity sensor signal, from an angular velocity sensor located in the rotor hub of the wind turbine, indicative of an angular velocity of the rotor hub; determining a first estimated rotational speed of the rotor hub based on the received angular velocity sensor signal; determining a second estimated rotational speed of the rotor hub based on the received acceleration sensor signal; determining a correction value based on a difference between the first and second estimated rotational speeds; and applying the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine.

[0022] According to another aspect of the invention there is provided a non-transitory, computer- readable storage medium storing instructions thereon that when executed by one or more processors cause the one or more processor to perform a method as defined above.

[0023] According to another aspect of the invention there is provided a controller for determining rotor azimuth angle of a wind turbine. The controller is configured to obtain a rotor speed signal indicative of rotor speed of the wind turbine. The rotor speed signal is for input into a rotor speed integrator. The controller is configured to receive acceleration sensor data, from an acceleration sensor located in a rotor hub of the wind turbine, indicative of acceleration, due to gravity, in a radial direction of the rotor hub relative to a rotation axis of the rotor hub. The controller is configured to determine a reference rotor azimuth angle based on the received acceleration sensor data. The controller is configured to generate a reset pulse signal based on the determined reference rotor azimuth angle. The controller is configured to, upon receiving the generated reset pulse signal at the rotor speed integrator, reset the rotor speed integrator. The controller is configured to use the reset rotor speed integrator to determine rotor azimuth angle of the wind turbine based on the obtained rotor speed signal. According to another aspect of the invention there is provided a wind turbine comprising a controller as defined above.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Examples of the invention will now be described with reference to the accompanying drawings, in which:

[0026] Figure 1 is a schematic diagram of a wind turbine in accordance with an example of the invention;

[0027] Figure 2 schematically illustrates a controller of the wind turbine of Figure 1 in accordance with an example of the invention;

[0028] Figure 3 shows the steps of a method performed by the controller of Figure 2 in accordance with an example of the invention;

[0029] Figure 4 schematically illustrates a rotor plane of the wind turbine of Figure 1 , as well as example positions of an accelerometer in a rotor hub of the wind turbine of Figure 1 ;

[0030] Figure 5 is a plot of rotor position of the wind turbine of Figure 1 against time determined using the method of Figure 3 and compared against rotor position obtained according to a previous, known method; and,

[0031] Figure 6 is a plot of a rotor integrator reset pulse against time generated using the method of Figure 3 and compared against a generated rotor integrator reset pulse obtained according to a previous, known method.

[0032] DETAILED DESCRIPTION

[0033] Figure 1 illustrates, in a schematic view, an example of a wind turbine 10. The wind turbine 10 includes a tower 102, a nacelle 103 disposed at the apex of, or atop, the tower 102, and a rotor 104 operatively coupled to a generator housed inside the nacelle 103. In addition to the generator, the nacelle 103 houses other components required for converting wind energy into electrical energy and various components needed to operate, control, and optimise the performance of the wind turbine 10. The rotor 104 of the wind turbine 10 includes a central hub 105 and three rotor blades 106 that project outwardly from the central hub 105.

[0034] The rotor 104 and rotor blades 106 rotate about a rotor axis and define a rotor plane. In particular, wind impinging on the rotor blades 106 drives rotation of the rotor 104 and rotor blades 106, thereby capturing wind energy that can be converted to electrical energy, e.g. for supply to the grid.

[0035] The wind turbine 10 includes an acceleration sensor in the form of an accelerometer 107 located in the rotor hub 105. In the described example, the accelerometer 107 is a three- axis accelerometer configured to detect acceleration of the rotor hub in three mutually orthogonal directions. The orientation and position of the accelerometer 107 within ( / relative to) the rotor hub 105 will be known. The accelerometer 107 may be part of a sensing device I (inertial) measurement unit that includes further sensors. For instance, such a measurement unit may additionally include a (three-axis) gyroscope configured to measure angular motion / velocity of the rotor hub 105.

[0036] The present invention provides a method and controller for determining a rotational position of the wind turbine rotor 104 - hereinafter referred to as the rotor azimuth angle - in an accurate and reliable manner. The invention is advantageous in that it provides for accurate and reliable determinations of rotor azimuth angle without needing to use a dedicated reset sensor for resetting a rotor speed integrator used to determine rotor azimuth angle. The invention beneficially makes use of existing hardware of the wind turbine - namely, the accelerometer 107 in the rotor hub 105 - to determine when to reset the rotor speed integrator. The removal of the need to use a dedicated reset sensor reduces the electrical and mechanical complexity of implementing a rotor azimuth angle determination, which in turn can reduce component costs, reduce maintenance costs / time, and free up space in the rotor hub. The approach of the present invention is more reliable than previous approaches that use a dedicated reset sensor as the risk of said sensor being damaged / destroyed is removed. Further advantages associated with the present invention will become apparent in the following.

[0037] Figure 2 schematically illustrates a controller 20 of the wind turbine 10. The controller 20 is for determining rotor azimuth angle of the wind turbine rotor 104. The determined rotor azimuth angle may be used for any suitable purpose. For instance, the determined rotor azimuth angle may be used for positioning the rotor and rotor blades during service or maintenance operations of the wind turbine, for calibrating other sensing systems of the wind turbine, and as an input into one or more control routines / process of the wind turbine, e.g. a load reducing control routine. The controller 20 may be placed inside the rotor hub 105, in the nacelle 103, in the tower 102 or distributed at a number of locations inside (or externally to) the turbine 10 and communicatively connected to one another.

[0038] The controller 20 is configured to receive a rotor speed signal 201 indicative of a measurement / determination of a (rotational) speed of the wind turbine rotor 104. The rotor speed signal 201 may be obtained from any suitable source. The rotor speed signal may be determined based on a measured speed of a generator of the wind turbine 10. The wind turbine 10 may be provided with a rotary encoder configured to measure generator speed. The rotor speed may then be determined as the measured generator speed divided by a gearbox speed ratio of the wind turbine 10 (generator speed compensated by gear ratio). This may be regarded as a particularly accurate method for obtaining the rotor speed signal 201.

[0039] An alternative method for determining the rotor speed may be based on sensor signals obtained only from the inertial measurement unit of the wind turbine 10, in particular from the accelerometer 107 and the gyroscope. Specifically, a first estimated rotational speed of the rotor hub 105 may be determined based on the angular velocity signal from the gyroscope, and a second estimated rotational speed of the rotor hub 105 may be determined based on the acceleration signal from the accelerometer 107. A correction value may be determined based on a difference between the first and second estimated rotor speeds, and the rotor speed is determined by applying the correction to the first estimated rotor speed. This method may be regarded as providing a reliable and accurate rotor speed measurement that is robust against external noise.

[0040] In some examples, the controller 20 may include a rotor speed selector module. In such examples, the controller 20 may be configured to receive more than one rotor speed signal, in particular from different sources I obtained via different methods (such as the ones outlined above). The selector module may then be configured to select which of the received rotor speed signals is to be used by other modules of the controller 20 to determine rotor azimuth angle (as described below). Beneficially, if the rotor speed signal from one source is not received, e.g. because of a fault, then the rotor speed selector module can select another source to provide the rotor speed signal in order to ensure continued operation of the controller 20. In some examples, the rotor speed selector module may combine rotor speed measurements / estimations from a plurality of sources to obtain a rotor speed signal to be used by the controller 20 to determine rotor azimuth angle, which may increase the accuracy of the determination.

[0041] The rotor speed signal 201 is input into an integrator module 202 of the controller 20. The integrator 202 is configured to integrate the received rotor speed signal 201 with respect to time to obtain a rotor azimuth angle signal 203. The integrator module 202 may be any suitable functional control module such as a proportional integral (PI) controller or proportional integral derivative (PID) controller.

[0042] As outlined above, the integrator 202 can suffer from drift away from a correct / accurate determination of rotor azimuth angle as a result of accumulated errors from the rotor speed signal. To address this, a reset signal may be generated to reset the integrator 202 at the start of each (new) revolution of the rotor 104, meaning that errors do not accumulate over multiple revolutions of the rotor 104. To implement this, the controller 20 includes a reset estimation unit / module 204 for determining when the integrator 202 is to be reset, i.e. at the start of a new revolution of the rotor 104. When the unit 204 determines the integrator 202 is to be reset, a reset signal 205 is generated and sent to the integrator 204 to reset the integrator 202. It is important that accurate determination of when the rotor starts a new revolution is performed so that the reset signal is generated at the appropriate time.

[0043] As described above, whereas in previous approaches such a reset signal may be generated based on the output of a dedicated reset sensor, in the described example the reset estimation unit 204 is configured to receive an acceleration signal 206 from the accelerometer 107 in the rotor hub 105, and is configured to generate the reset pulse signal 205 based on the received acceleration signal 107. This will be described in greater detail below.

[0044] Figure 3 shows the steps of a method 30 performed by the controller 20 to determine rotor azimuth angle of the wind turbine 10. At step 301 , the method 30 involves obtaining the rotor speed signal 201 indicative of rotor speed of the wind turbine 10. As described above, the rotor speed signal may be obtained from any suitable source, and is for input into the rotor speed integrator 202.

[0045] At step 302, the method 30 involves receiving the acceleration sensor signal 206 from the acceleration sensor 107 located in the wind turbine rotor hub 105. The acceleration signal 206 is indicative of acceleration, due to gravity, in a radial direction of the rotor hub 105 relative to a rotation axis of the rotor hub 105. In the described example, it is desired to obtain the acceleration in two mutually orthogonal directions in the rotor plane, i.e. in the radial direction relative to the rotor axis, as this will then be used to determine an indication of the azimuth angle of the rotor. The estimation unit 204 may perform a coordinate transform of the acceleration signal 206 from a rotating reference frame into a stationary reference frame in order to obtain the mutually orthogonal directions in the rotor plane. Thus, the one or more components of the acceleration sensor signal 206 may be transformed from a rotating reference frame rotating with the rotor 104 into a stationary reference frame.

[0046] It is particularly desired to obtain the acceleration caused by gravity in the two mutually orthogonal directions. The accelerometer 107 in the rotor hub 105 is excited primarily by gravity; however, the acceleration signal 206 will also include contributions as a result of centrifugal force as well as rotor rotational speed accelerations and vibrations of the rotor hub 105 and nacelle 103. The method 30 may involve steps to compensate for ( / remove) the centrifugal force contribution in the acceleration signal 206. This may be performed by the estimation unit 204 of the controller 20.

[0047] The centrifugal force contribution induces an offset in the acceleration signal 206 that is proportional to the rotor hub speed squared in the two measurement axes of the accelerometer 107 that are perpendicular to the main shaft of the rotor 104, i.e. the two mutually orthogonal axes in the rotor plane. The method 30 may therefore include a step of applying an offset to the received acceleration sensor signal 206 to compensate for a centrifugal force contribution included therein. In particular, the offset may be proportional to the square of the rotor speed in the obtained rotor speed signal 201 , in which case the rotor speed signal 201 is received by the estimation unit 204 as shown in Figure 2.

[0048] With reference to Figure 4, one approach for determining the offset to be applied to the received acceleration signal 206 to compensate for centrifugal force contributions in the signal 206 is described. The offset is determined based on the (known) position and orientation of the accelerometer 107 in the rotor hub 105 relative to the axis of rotation. Figure 4 schematically illustrates the rotor plane 401 of the wind turbine 10. In particular, Figure 4 shows the rotor plane 401 when (a specific) one of the rotor blades 106 is pointing directly downwards, which corresponds to a rotor azimuth angle, 6, of zero radians, i.e. 0 = 0. Figure 4 schematically illustrates two possible positions A,Anof the accelerometer 107. In position A the accelerometer 107 is located directly below (vertically downwards from) the rotor axis 402 (i.e. centre of the rotor plane 401), i.e. on a centre line 403 of the rotor plane 401 , at a distance rafrom the centre 402 of the rotor plane 401. On the other hand, in position Anthe accelerometer 107 is offset from the centre line 403 by an angle 0off, and located a distance rnfrom the centre 402, as shown in Figure 4. Note that 0off= 0 at position A. As illustrated in Figure 4, the positioning of the accelerometer 107 will impact the manner in which the centrifugal force Fcis captured on the accelerometer axis. The accelerometer 107 measures acceleration ax, ayin the mutually orthogonal directions x and y in the rotor plane 401 (as well as parallel to the rotor axis in direction z). Here, the measured acceleration ax, ayis the sensor signal 206 transformed from the rotating reference frame to the stationary reference frame.

[0049] The offset angle 0offmay be determined as 0off= tan-1(dx / dy), where dx, dyare the known distances of the accelerometer 107 from the rotor plane centre 402 in the x and y directions, respectively. The radius rnis determined as

[0050] Also, the centrifugal force Fcis then determined as

[0051] Fcvrot / rn where vrotis the angular rotor speed, and the centrifugal force components along the accelerometer axes x and y, e.g. in the stationary frame, would be Fcsin 0offand Fccos 0off. For convenience, the mass is considered to be one in the above formula. These components can then be removed from the total measurement of the accelerometer 107 along the respective axis, thereby providing the centrifugal force offset to the received accelerometer signal 206.

[0052] In practice, the accelerometer 107 is affected by and may measure the centripetal acceleration (and additional accelerations, e.g. gravity). The centripetal acceleration measured by the accelerometer is given wA2-rn, where omega is the angular velocity. Thus, the accelerometer 107 may not measure Fc, but instead the centripetal acceleration which is related to Fc. The components of the measured centripetal acceleration along the accelerometer axes correspond to those of the centrifugal force Fc. Thus, instead of using the measurements from the accelerometer 107 directly, by mapping the sensor signal 206 into the stationary rotor frame and then applying the centrifugal force compensation, or centripetal acceleration compensation, the position and orientation of the accelerometer 107 can be chosen arbitrarily. Thus, mounting locations of accelerometers in the hub can be chosen based on considerations of locations which provide the simplest mounting.

[0053] The other disturbances in the received acceleration signal 206, e.g. caused by vibrations of the rotor hub 105 and nacelle 103, may be neglected as they have no continuous impact on the acceleration signal 206 and in any case are negligible in magnitude relative to the contribution from gravity. The resulting acceleration signal is therefore two components that are 90 degrees out of phase, providing sinusoidal signals with an amplitude close to 1G and a frequency proportional to the rotor hub speed.

[0054] Returning to Figure 3, at step 303 the method 30 involves, at the estimation unit 204 of the controller 20, determining a reference rotor azimuth angle based on the received acceleration sensor signal 206. This may be achieved by applying an inverse tangent function (arctangent function) to the mutually orthogonal components ax, ayin the rotor plane 401 of the acceleration signal 206 (or the offset components ax, aythat have been compensated for centrifugal force). This operation may take into consideration the signs of the axes / components so that a result in the range -TT / 2 to TT / 2 is obtained, and this may further be transformed to be in the range 0 to 2n. A further offset may be applied to ensure that the reference rotor azimuth angle is obtained in a correct quadrant of the rotor plane 401.

[0055] At step 304, the method 30 involves, at the estimation unit 204, generating the reset pulse signal 205 based on the determined reference rotor azimuth angle. In the described example, the estimation unit 204 generates the reset signal 205 when the determined reference rotor azimuth angle is zero, which corresponds to (a specific) one of the rotor blades 106 pointing directly downwards. More generally, the reset signal 205 may be generated when the determined reference rotor azimuth angle satisfies a prescribed condition indicating that the rotor 104 is at a certain rotational position. When operating in the range 0 to 2n, the determined reference rotor azimuth angle will increase monotonically from zero - when the specific rotor blade 106 is pointing downwards - to 2n over the course of one full revolution of the rotor 104. The reference rotor azimuth angle will then fall instantaneously back to zero at the start of the next rotor revolution. The acceleration signal 206 may be sampled at the sampling rate of the controller 20, and the reference rotor azimuth angle may be determined at each sampling point. Depending on when the acceleration signal 206 is sampled, the determined reference rotor azimuth angle may not be exactly zero at any given sample as the rotor 104 finishes one revolution and starts the next. To ensure that the reset pulse signal 205 is nonetheless generated at the start of a new rotor cycle, the estimation unit 204 may in some examples be configured to generate the reset pulse signal 205 when the determined reference rotor azimuth angle changes from being above a defined threshold value close to zero to being below the defined threshold value. This will capture the point at which the rotor 104 moves from completing one revolution - when the determined reference rotor azimuth angle at a sampling point is nearly 2n - to starting the next revolution - where the determined reference rotor azimuth angle at the subsequent sampling point may be just greater than zero. The threshold value can be defined as appropriate, e.g. based on the sampling rate. As a purely indicative example, the threshold value may be set to be one of 0.1 radians, 0.2 radians, 0.3 radians, etc.; however, it will be understood that any suitable value may be used for this purpose.

[0056] At step 305, the method 30 involves resetting the rotor speed integrator 202 upon reset pulse signal 205 generated at the estimation unit 204 being received at the rotor speed integrator 202. Resetting the rotor speed integrator 202 may be in some examples be regarded as setting instantaneous rotor azimuth angle in the rotor speed integrator 202 to be equal to the reference rotor azimuth angle, e.g. zero. Resetting the rotor speed integrator may be regarded as resetting the calculation of the integrator 202 to zero so as to clear the output of the integrator 202, i.e. the rotor azimuth angle, of accumulated errors. In the described example, the integrator 202 is reset for each revolution of the rotor 104, which means that any errors from the rotor speed signal 201 in the integrator 202 accumulate only for one revolution of the rotor 104. In different examples, the reset pulse signal 205 may be generated more or less frequently that once per rotor revolution, as needed.

[0057] At step 306, the method 30 then involves using the reset rotor speed integrator 202 to determine rotor azimuth angle 203 of the wind turbine 10 based on the obtained rotor speed signal 201. It is noted that, while it is beneficial to use the reference rotor azimuth angle determined based on the rotor hub accelerometer 107 to generate a reset pulse for the rotor speed integrator 202, it remains beneficial to determine rotor azimuth angle that is to be used for instance as input to one or more control routines of the wind turbine based on the obtained rotor speed signal 201. This is because vibrations of the wind turbine 10, such as sideside vibrations of the tower 102 can influence the accuracy of the reference rotor azimuth angle determined based on the acceleration signal 206. For instance, there may be a discontinuity in the signal, e.g. when a rotor blade 106 passes the tower 102. On the other hand, rotor speed measurement is not influenced by tower vibrations and so can use the integral of the rotor speed signal without there being any discontinuity.

[0058] Figures 5 and 6 show comparative results of how the method of the present invention compares against the above-described, known approach for determining rotor azimuth angle when a dedicated reset sensor is used. In particular, Figure 5 shows a plot of rotor position against time, with the rotor position as determined according to the present invention being shown in the solid line and the rotor position according to the previous method using a dedicated azimuth sensor being shown in the dashed line. It is seen that the rotor position (rotor azimuth angle) increases monotonically from zero to 2n, before falling substantially instantaneously back to zero at the start of a new revolution of the rotor. It is seen that the rotor position determined using the present invention is almost indistinguishable from the previous, known approach.

[0059] Figure 6 shows a plot of the generated reset pulse against time and, in particular, includes a first plot 601 illustrating the reset pulse generated according to the present invention and a second plot 602 illustrating the reset pulse generated according to the previous method using a dedicated reset sensor. The time difference between the two reset pulses 601 , 602 is within a couple of samples. The method of the present invention provides a reset pulse that is at least as accurate as the previous solution while being more reliable and cost effective than the previous solution.

[0060] Many modifications may be made to the described examples without departing from the scope of the appended claims.

[0061] The described controller 20 may be in the form of any suitable computing device, for instance one or more functional units or modules implemented on one or more computer processors. Such functional units may be provided by suitable software running on any suitable computing substrate using conventional or customer processors and memory. The one or more functional units may use a common computing substrate (for example, they may run on the same server) or separate substrates, or one or both may themselves be distributed between multiple computing devices. A computer memory may store instructions for performing the methods performed by the controller, and the processor(s) may execute the stored instructions to perform the method.

Claims

CLAIMS1. A method of determining rotor azimuth angle of a wind turbine, the method comprising: obtaining a rotor speed signal indicative of rotor speed of the wind turbine, the rotor speed signal being for input into a rotor speed integrator; receiving an acceleration sensor signal, from an acceleration sensor located in a rotor hub of the wind turbine, indicative of acceleration, due to gravity, in a radial direction of the rotor hub relative to a rotation axis of the rotor hub; determining a reference rotor azimuth angle based on the received acceleration sensor signal; generating a reset pulse signal based on the determined reference rotor azimuth angle; upon receiving the generated reset pulse signal at the rotor speed integrator, resetting the rotor speed integrator; and using the reset rotor speed integrator to determine rotor azimuth angle of the wind turbine based on the obtained rotor speed signal.

2. A method according to Claim 1 , wherein the reset pulse signal is generated when the determined reference rotor azimuth angle satisfies a prescribed condition indicating that the wind turbine rotor is at a prescribed rotational position.

3. A method according to Claim 2, wherein the prescribed rotational position corresponds to a minimum value of the reference rotor azimuth angle, and wherein the prescribed condition is that the determined reference rotor azimuth angle falls below a prescribed threshold value greater than the minimum value.

4. A method according to any previous claim, wherein resetting the rotor speed integrator comprises setting instantaneous rotor azimuth angle in the rotor speed integrator to be equal to the reference rotor azimuth angle.

5. A method according to any previous claim, wherein the reset pulse signal is generated once per revolution of the wind turbine rotor.

6. A method according to any previous claim, wherein the received acceleration sensor signal is indicative of acceleration, due to gravity, of the rotor hub in first and second mutually orthogonal radial directions.

7. A method according to Claim 6, wherein determining the reference rotor azimuth angle comprises applying a trigonometric function to components of the received acceleration sensor signal in the first and second mutually orthogonal radial directions.

8. A method according to Claim 7, wherein the reference rotor azimuth angle is determined in dependence on an orientation and position of the acceleration sensor in the rotor hub.

9. A method according to any previous claim, the method comprising applying an offset to the received acceleration sensor signal to compensate for a centrifugal force contribution included therein, wherein the offset is proportional to the square of the rotor speed in the obtained rotor speed signal, and wherein the reference rotor azimuth angle is determined based on the centrifugal-compensated acceleration sensor signal.

10. A method according to any previous claim, wherein the rotor speed in the obtained rotor speed signal is determined based on a received generator speed signal indicative of generator speed of the wind turbine.

11. A method according to Claim 10, wherein the rotor speed in the obtained rotor speed signal is determined as the generator speed compensated by a gear ratio of a gearbox of the wind turbine.

12. A method according to any of Claims 1 to 9, wherein the rotor speed in the obtained rotor speed signal is determined by: receiving an angular velocity sensor signal, from an angular velocity sensor located in the rotor hub of the wind turbine, indicative of an angular velocity of the rotor hub; determining a first estimated rotational speed of the rotor hub based on the received angular velocity sensor signal; determining a second estimated rotational speed of the rotor hub based on the received acceleration sensor signal; determining a correction value based on a difference between the first and second estimated rotational speeds; and applying the correction value to the first estimated rotational speed to determine the rotor speed of the wind turbine.

13. A method according to any previous claim, further comprising performing a coordinate transformation of the components of the acceleration sensor signal (206) from a rotating reference frame into a stationary reference frame.

14. A non-transitory, computer-readable storage medium storing instructions thereon that when executed by one or more processors cause the one or more processor to perform a method according to any previous claim.

15. A controller for determining rotor azimuth angle of a wind turbine, the controller being configured to: obtain a rotor speed signal indicative of rotor speed of the wind turbine, the rotor speed signal being for input into a rotor speed integrator; receive acceleration sensor data, from an acceleration sensor located in a rotor hub of the wind turbine, indicative of acceleration, due to gravity, in a radial direction of the rotor hub relative to a rotation axis of the rotor hub; determine a reference rotor azimuth angle based on the received acceleration sensor data; generate a reset pulse signal based on the determined reference rotor azimuth angle; upon receiving the generated reset pulse signal at the rotor speed integrator, reset the rotor speed integrator; and use the reset rotor speed integrator to determine rotor azimuth angle of the wind turbine based on the obtained rotor speed signal.

16. A wind turbine comprising a controller according to Claim 15.

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