DETECTING ROTOR BLADE CLEARANCE IN A WIND TURBINE USING DOPPLER SHIFT AND A MATHEMATICAL MODEL

Abstract

A method of determining a blade clearance during operation of a wind turbine is provided, the blade clearance corresponding to a distance between a rotor blade and a tower of the wind turbine. The method includes (a) detecting a rotor blade velocity, (b) emitting a first signal from an observer location, the first signal having a first frequency, (c) receiving a second signal at the observer location, the second signal being reflected from the rotor blade when the first signal impinges on the rotor blade, (d) determining a Doppler shift of the second signal relative to the first signal, and (e) determining the blade clearance based on the first frequency, the Doppler shift, the observer location, and the rotor blade velocity, wherein the step of determining the blade clearance utilizes a mathematical model. A corre-sponding system and a wind turbine comprising such a system are also provided.

Claims

1. A method of determining a blade clearance during operation of a wind turbine, the blade clearance corresponding to a distance between a rotor blade and a tower of the wind turbine, the method comprising: detecting a rotor blade velocity; emitting a first signal from an observer location, the first signal having a first frequency; receiving a second signal at the observer location, the second signal being reflected from the rotor blade when the first signal impinges on the rotor blade; determining a Doppler shift of the second signal relative to the first signal; and determining the blade clearance based on the first frequency, the Doppler shift, the observer location, and the rotor blade velocity; wherein the step of determining the blade clearance utilizes a mathematical model.

2. The method according to claim 1, wherein the step of determining the blade clearance comprises a lookup in a data array stored in a memory unit, the data array being computed using the mathematical model.

3. The method according to claim 1, further comprising determining a relative blade velocity based on the blade velocity and the observer location.

4. The method according to claim 2, wherein the data array comprises a blade clearance value for each of a plurality of combinations of first frequency values, Doppler shift values and relative blade velocity values.

5. The method according to claim 1, wherein the first signal is emitted by a leaky feeder arrangement at the observer location, and wherein the second signal is received by the leaky feeder arrangement.

6. A system for determining a blade clearance during operation of a wind turbine, the blade clearance corresponding to a distance between a rotor blade and a tower of the wind turbine, the system comprising: a velocity detector configured to detect a rotor blade velocity; a signal emitter configured to emit a first signal from an observer location, the first signal having a first frequency; a signal receiver configured to receive a second signal at the observer location, the second signal being reflected from the rotor blade when the first signal impinges on the rotor blade; a signal processor coupled to the signal emitter and the signal receiver, wherein the signal processor is configured to determine a Doppler shift of the second signal relative to the first signal; and a determination device configured to determine the blade clearance based on the first frequency, the Doppler shift, the observer location, and the rotor blade velocity utilizing a mathematical model.

7. The system according to claim 6, wherein the determination device comprises a memory unit storing a data array computed using the mathematical model, and wherein the determination device is configured to determine the blade clearance by performing a lookup in the data array.

8. The system according to claim 6, wherein the determination device further comprises a velocity calculation unit configured to calculate a relative blade velocity based on the blade velocity and the observer location.

9. The system according to claim 7, wherein the data array comprises a blade clearance value for each of a plurality of combinations of first frequency values, Doppler shift values and relative blade velocity values.

10. The system according to claim 6, wherein the signal emitter and the signal receiver comprise a leaky feeder arrangement.

11. The system according to claim 10, wherein the leaky feeder arrangement comprises at least one leaky feeder forming a loop shape around the tower of the wind turbine.

12. The system according to claim 11, wherein the at least one leaky feeder is a leaky coaxial cable or a leaky waveguide.

13. A wind turbine comprising a tower and a least one rotor blade, the wind turbine further comprising the system according to claim 6 for determining the blade clearance during operation.

14. The wind turbine according to claim 13, wherein the observer location is positioned on a loop shape extending around a circumference of the tower.

Description

BRIEF DESCRIPTION

[0045] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0046] FIG. 1 shows a block diagram of a wind turbine control system utilizing an embodiment of the present invention;

[0047] FIG. 2 shows a block diagram of another wind turbine control system utilizing an embodiment of the present invention;

[0048] FIG. 3 shows a side view of a wind turbine comprising a system according to an embodiment of the present invention;

[0049] FIG. 4 shows a schematic front view of a rotating rotor blade, in particular with regard to a relation between tangential and radial velocity of the rotating rotor blade in accordance with a mathematical model utilized by embodiments of the present invention; and

[0050] FIG. 5 shows a schematic side view of a wind turbine model as used to establish a mathematical model utilized by embodiments of the present invention.

DETAILED DESCRIPTION

[0051] The illustrations in the drawings are schematic. It is noted that in different figures, similar or identical elements are provided with the same reference numerals or with reference numerals which differ only within the first digit.

[0052] FIG. 1 shows a block diagram of a wind turbine control system utilizing an embodiment of the present invention. More specifically, FIG. 1 shows a control system for blade clearance, i.e. for assuring that a rotor blade of a wind turbine keeps a certain distance (clearance) from the tower of the wind turbine during operation.

[0053] The depicted system comprises a subtraction point 110, a controller 120, a pitch drive 130, a load (or disturbance, e.g. wind) 140, a Doppler shift determining device 150 (or radar unit), and a lookup array device 160. The system receives a clearance set point d.sub.ref at one input of subtraction unit 110. The other input of the subtraction unit 110 receives a measured blade clearance d. The difference (dref−d) is supplied to controller 120 which provides a corresponding control signal to pitch drive unit 130. The load 140 represents an indication of the current load on the wind turbine.

[0054] The Doppler shift determining device 150 (which will be described in more detail below) is configured to determine a Doppler shift f.sub.D based on a signal emitted towards a rotor blade and a signal reflected from the rotor blade. The determined Doppler shift is supplied to lookup array device 160 which also receives inputs relating to the rotor speed v, an observer location P.sub.1 and a frequency f.sub.t of the signals emitted to determine the Doppler shift. Using these parameter values, the lookup array device 160 determines the value d of the blade clearance and supplies it to the subtraction unit 110 as described above. The subtraction unit 110 thus calculates the difference between the clearance set point d.sub.ref and the actual (measured clearance) d.

[0055] FIG. 2 shows a block diagram of another wind turbine control system utilizing an embodiment of the present invention in a different way in comparison to the system shown in FIG. 1 and discussed above. More specifically, the control system depicted in FIG. 2 comprises an empirical lookup unit 161 (as known in the art and discussed in the introduction) to determine a clearance value based on the Doppler shift obtained by Doppler shift determining device 150. Additionally, the lookup array unit 160 is used to check the values provided by the empirical lookup unit 161. Although the drawing shows that values for both d and f.sub.D are output by the lookup array unit 160 and supplied to respective summation points 163, 162, it should be understood that only one of these values are used in a specific implementation. The idea is to have the mathematical model 160 running in the background such that the expected values of either d or f.sub.D can be compared with the measured value output by either the Doppler shift determining device 150 or the empirical lookup unit 161. In case of a deviation exceeding a predetermined threshold, an error message may be output. In other words, the model based device 160 provides a kind of plausibility check.

[0056] FIG. 3 shows a side view of a wind turbine 1 comprising a system according to an embodiment of the present invention.

More specifically, the wind turbine 1 comprises a tower 2, which is mounted on a non-depicted fundament. A nacelle 3 is arranged on top of the tower 2. In between the tower 2 and the nacelle 3, a yaw angle adjustment device (not shown) is provided, which is capable of rotating the nacelle around a vertical yaw axis Z. The wind turbine 1 further comprises a wind rotor 5 having one or more rotor blades 4 (in the perspective of FIG. 1 only two blades 4 are visible). The wind rotor 5 is rotatable around a rotational axis Y. In general, when not differently specified, the terms axial, radial and circumferential in the following are made with reference to the rotational axis Y. The rotor blades 4 extend radially with respect to the rotational axis Y. The wind turbine 1 comprises an electric generator 6 having a stator 11 and a rotor 12. The rotor 12 is rotatable with respect to the stator 11 about the rotational axis Y to generate electrical power. The electric generator 6 and the generation of electrical power through embodiments of the present invention is not a specific aspect of embodiments of the present invention and will therefore not be described in further detail.

[0057] FIG. 3 further shows a Doppler shift determining device 150 of a system according to embodiments of the present invention. The device 150 comprises a leaky feeder arrangement 20 mounted at the tower 2, and a transmitter 30 and a receiver 40 coupled to the leaky feeder arrangement 20. The transmitter 30 is configured to supply a first signal to the leaky feeder arrangement 20 which emits (leaks) it from the observer location P1. The receiver 40 is configured to receive a second signal from the leaky feeder arrangement 20, e.g. a signal reflected from point P2 on the rotor blade 4 when the first signal hits the rotor blade 4 and received at the observer location P1. The apparatus further comprises a processing unit (not shown) configured to determine the Doppler shift f.sub.D. The leaky feeder arrangement surrounds the entire circumference of the tower 2. The radial distance from the rotor axis Y to the blade tip P2 is R while the radial distance from the rotor axis Y to the leaky feeder arrangement 20 is L1.

[0058] FIG. 4 shows a schematic front view of a rotating rotor blade 4, in particular with regard to a relation between tangential velocity v and radial velocity (also referred to as relative velocity) v.sub.D of the point P2 on the rotor blade 4 in accordance with a mathematical model utilized by embodiments of the present invention. The observer location P1 is located at the bottom of the drawing. Thus, when the rotor blade 4 rotates and the point P2 moves correspondingly, the rotating movement will cause a radial movement of P2 with a relative velocity v.sub.D given as the projection of the tangential velocity v (i.e. the rotational speed of the rotor 4 at P2) on the vector L extending between P1 and P2. This radial movement will influence the measured Doppler shift f.sub.D and will therefore have to be taken into consideration as shown in the following.

[0059] FIG. 5 shows a schematic side view of a wind turbine model as used to establish a mathematical model utilized by embodiments of the present invention. As can be seen, the model depicted in FIG. 5 corresponds essentially to the view presented in FIG. 3 and introduces a number of angles, vectors and other parameters. The angle θ is the rotational angle (around the rotational axis Y) of the rotor while Ψ is the yaw angle (around the axis of the tower 2). The distance between the central tower axis and the rotor plane is h. Similarly, the distance between the central tower axis and the observer location P1 (i.e. the radius of the loop-shaped leaky feeder arrangement 20) is l. The distance from the rotor axis Y to the observer location P1 is L1.

[0060] For given values of transmitter frequency (first frequency) f.sub.t, radial (or relative) velocity v.sub.D and speed of light c.sub.0, the Doppler shift f.sub.D is given as

[00001]$f_D=2f_t\frac{v_D}{c_0}$

The relative velocity v.sub.D is given as

[00002]$v_D=\frac{\stackrel{.fwdarw.}{L}.Math.\stackrel{.fwdarw.}{v}}{.Math.\stackrel{.fwdarw.}{L}.Math.}$

It can be shown that the scalar product in the numerator of the above equation is given as

{right arrow over (L)}.Math.{right arrow over (v)}=−Rω(L.sub.1 sin θ cos.sup.2ψ+L.sub.1 sin θ sin.sup.2ψ−l cos θ sin ψ)

and that the denominator is given as

[00003]$.Math.\stackrel{.fwdarw.}{L}.Math.=\sqrt{{\left(L_1+R\cos \theta \right)}^2+{\left(h\sin \theta +R\sin \theta \cos \psi \right)}^2+{\left(h\cos \psi -l-r\sin \theta \sin \psi \right)}^2}$

Referring still to FIG. 4 and FIG. 5, the following relation between Doppler shift f.sub.D and radial velocity v.sub.D is established:

[00004]$f_D=2f_t\frac{v}{c_0}\cos \alpha =2f_t\frac{v_D}{c_0}$

[0061] Using the above formulas, a data array for looking up the blade clearance d for a large number of combinations of values for Doppler shift f.sub.D, radial velocity v.sub.D, and transmission frequency f.sub.t can be calculated and stored for a given wind turbine (where the observer location P1 is given by the distances L.sub.1 and l in FIG. 5). The resulting data array is stored in a lookup array device 160 (see FIG. 1 and FIG. 2) and allows easy, cheap and precise determination of the blade clearance during operation of a wind turbine.

[0062] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0063] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.