METHOD FOR EVALUATING AN INFLOW ON A ROTOR BLADE OF A WIND TURBINE, METHOD FOR CONTROLLING A WIND TURBINE, AND A WIND TURBINE

Abstract

A method for determining an incident flow at a rotor blade of a wind power installation is provided. The method includes recording at least part of a pressure spectrum of pressure, in particular wall pressure, at the rotor blade at at least one measurement position. The method includes determining at least two characteristic values from the pressure spectrum, determining an indicator value from a relationship between the at least two characteristic values and assessing whether a critical incident flow is present depending on the indicator value.

Claims

1. A method for evaluating an incident flow at a rotor blade of a wind power installation, comprising: recording at least part of a pressure spectrum of pressure at the rotor blade at at least one measurement position, determining at least two characteristic values from the pressure spectrum, determining an indicator value from a relationship between the at least two characteristic values, and determining, based on the indicator value, whether a critical incident flow is present at the rotor blade.

2. The method as claimed in claim 1, wherein: the at least two characteristic values include a first spectral value and a second spectral value, and the first spectral value is a characteristic value of a first frequency range of the pressure spectrum, and the second spectral value is a characteristic value of a second frequency range of the pressure spectrum that is higher than the first frequency range.

3. The method as claimed in claim 1, wherein: the pressure spectrum is a power density spectrum and is subdivided into: a first partial power density spectrum in a first frequency range, and a second partial power density spectrum in a second frequency range, and the at least two characteristic values are first and second spectral components, wherein the method comprises: integrating the first partial power density spectrum over the first frequency range to obtain the first spectral component, and integrating the second partial power density spectrum over the second frequency range to obtain the second spectral component.

4. The method as claimed in claim 3, wherein: the first frequency range lies between a first and a second frequency, and the second frequency range lies between the second and a third frequency, and the method comprises setting at least one of the first, second and third frequencies according to at least one of: setting the second frequency such that the power density spectrum has a maximum in the first frequency range when the critical incident flow is present, setting the first, second and third frequencies such that the frequency range and the second frequency range have the same size, setting the first, second and third frequencies based on a degree of dirtying of the rotor blade, setting the first, second and third frequencies based on sound emission limits at an installation site of the wind power installation, setting the first, second and third frequencies based on sound measurements in a region of the wind power installation, and setting the first, second and third frequencies in a region of 200 Hz, 400 Hz and 600 Hz, respectively.

5. The method as claimed in claim 2, comprising: determining, the indicator value as a quotient of two of the at least two characteristic values, as a quotient of the first and second spectral value, or as a quotient of the first and second spectral components, and determining that the critical incident flow is present if the indicator value is above a specified ratio limit value.

6. The method as claimed in claim 1, wherein the at least one measurement position is: in a region of a rotor blade trailing edge of the rotor blade, on a suction side of the rotor blade, or in a region of the rotor blade the lies longitudinally between 60% to 95% from a connection region of the rotor blade to a blade tip of the rotor blade.

7. A method for controlling a wind power installation having a rotor with at least one rotor blade having an adjustable blade angle, comprising: determining, at the at least one measurement position, a pressure measurement of the at least one rotor blade, determining, based on the pressure measurement, whether a critical incident flow is present at the at least one rotor blade, and adjusting an angle of attack of the at least one rotor blade if the critical incident flow is present.

8. The method as claimed in claim 7, wherein determining whether the critical incident flow is present includes: recording at least part of a pressure spectrum of pressure at the at least one rotor blade at the at least one measurement position, determining at least two characteristic values from the pressure spectrum, determining an indicator value from a relationship between the at least two characteristic values, and determining, based on the indicator value, critical incident flow is present at the at least one rotor blade.

9. The method as claimed in claim 8, comprising: adjusting the angle of attack to reduce the indicator value below a limit value.

10. The method as claimed in claim 9, comprising: determining that the indicator value exceeds an upper hysteresis limit value, in response to determining that the indicator value exceeds the upper hysteresis limit value, beginning adjusting the angle of attack, and continuing adjusting the angle of attack until the indicator value drops below a lower hysteresis limit value that is smaller than the upper hysteresis limit value.

11. The method as claimed in claim 7, comprising: recording at least part of a pressure spectrum of pressure at the at least one rotor blade, spectrally evaluating the at least part of the pressure spectrum, subdividing the at least part of the pressure spectrum into a first and second partial power density spectra, calculating a first and second spectral component by integrating the first and second partial power density spectra, respectively, obtaining an indicator value as a quotient of the first and second spectral components, comparing the indicator value to a ratio limit value, determining that the critical incident flow is present if the indicator value exceeds the ratio limit value, reducing the angle of attack of the at least one rotor blade if the critical incident flow is determined to be present, and repeating the steps of recording, spectrally evaluating, subdividing, calculating, obtaining, comparing determining and reducing.

12. The method as claimed in claim 7, comprising: recording a sound measurement at the wind power installation, determining whether infrasound having an amplitude above a infrasound limit value is present in the sound measurement, and modifying at least one operational setting of the wind power installation if the infrasound having the amplitude above the infrasound limit is present in the sound measurement.

13. The method as claimed in claim 12, wherein adjusting the operational setting includes at least one of: adjusting the angle of attack of the at least one rotor blade to improve incident flow, modifying a rotor rotational speed of the at least one rotor blade, and modifying a power produced by the wind power installation.

14. The method as claimed in claim 13, comprising: adjusting the angle of attack of the at least one rotor blade only when the wind power installation has a rotor rotational speed above a rotational speed limit.

15. The method as claimed in claim 8 comprising: rotating, by a rotor, the at least one rotor blade, recording the pressure over at least one revolution of the rotor, for recording the at least part of the pressure spectrum, performing a plurality of pressure measurements successively during the at least one revolution, and determining a current pressure spectrum of a plurality of current pressure spectra for each pressure measurement of the plurality of pressure measurements, respectively, by averaging the plurality of current pressure spectra of the plurality of pressure measurements of the at least one revolution.

16. The method as claimed in claim 15, comprising: determining an angle position of the rotor, and multiplying each current pressure spectrum is multiplied by a cosine of the angle position before averaging the plurality of current pressure spectra, wherein the angle position is 0 when a rotor blade is at a 12 o'clock position.

17. A wind power installation having a rotor with a plurality of rotor blades that have adjustable angles of attack, comprising: at least one sensor for recording, at a measurement position, at least part of a pressure spectrum of a wall pressure at at least one rotor blade of the plurality of rotor blades, wherein the wind power installation is configured to: evaluate the at least part of the pressure spectrum, determine whether a critical incident flow is present at the at least one rotor blade based on evaluating at least part of the pressure spectrum, and adjusting an angle of attack of the at least one rotor blade if the critical incident flow is determined to be present.

18. (canceled)

19. The wind power installation as claimed in claim 17, wherein the at least one sensor is integrated into a rotor blade surface of the at least one rotor blade as a potential-free sensor.

20. The method as claimed in claim 1, wherein the pressure is wall pressure.

21. The wind power installation as claimed in claim 19, wherein the at least one sensor is an optical sensor or an fiber-optical sensor.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0098] Now, the invention will be explained in more detail below on the basis of exemplary embodiments, with reference being made to the attached figures.

[0099] FIG. 1 shows a wind power installation in a perspective illustration.

[0100] FIG. 2 is a diagram for explaining separation phenomena at the rotor blade.

[0101] FIG. 3 shows two power density spectra for different angles of attack.

[0102] FIG. 4 shows curves for indicator values in the case of different boundary conditions.

[0103] FIG. 5 shows a diagram for illustrating a control sequence for controlling a wind power installation.

DETAILED DESCRIPTION

[0104] FIG. 1 shows a wind power installation 100 having a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is arranged at the nacelle 104. During operation, the rotor 106 is put into a rotational movement by the wind and thereby drives a generator in the nacelle 104.

[0105] FIG. 2 shows a profile 2 of a rotor blade at a position relevant to the disclosure. The profile, and hence also the rotor blade, has a blade leading edge 4 and a blade trailing edge 6. Moreover, the profile, and, naturally, the rotor blade as well, has a suction side 8 and a pressure side 10. During the operation of the installation in the case of laminar flow conditions, a boundary layer 12 and 14, respectively, forms on both the suction side 8 and the pressure side 10, which can also be referred to as upper and lower side, respectively. These two illustrated boundary layers 12 and 14 belong to an incident flow, which sets in with substantially laminar flow during a desired operation and which is illustrated as a normal incident flow 16. In relation to a comparison direction 18, which, in particular, is parallel to the chord of the rotor blade (not plotted here), a normal angle of attack 20 sets in. Such an angle of attack, i.e., the normal angle of attack 20 and also a critical angle of attack 22, which is explained in more detail below, arise from a vector addition of a vector reproducing the wind speed and a vector corresponding to the movement of the rotor blade with a negative sign.

[0106] If there now is an increase in the wind speed in the case of an unchanging movement of the rotor blade, i.e., of the profile 2, there is also a change in the incident flow in terms of its direction up to a critical incident flow 24, plotted in FIG. 2, which has the aforementioned critical angle of attack 22. A critical incident flow should be assumed when there is a change in the upper boundary layer, in particular, i.e., the boundary layer 12 of the suction side 8, and a tendency to separate arises. Such a critical situation is plotted in FIG. 2 with a correspondingly modified boundary layer 26, which is assigned to the critical incident flow 24. The flow noises change and also increase in such a situation.

[0107] On the basis of power density spectra, FIG. 2 also explains a noise characteristic underlying the different situations. To this end, a pressure sensor 30 on the suction side 8 and in the vicinity of the blade trailing edge 6 on this relevant profile 2 records pressure signals, specifically sound, in particular. Consequently, the pressure sensor 30 can be a microphone.

[0108] These recorded pressure or sound signals can be converted into a power density spectrum by means of an FFT, i.e., a Fourier transform, and the diagram in FIG. 2 shows power density spectra for three situations, specifically a normal power density spectrum 32 that sets in in the case of a normal incident flow, in particular in the case of the normal incident flow 16, a critical power density spectrum 34 that can set in in the case of a critical incident flow, in particular the critical incident flow 24, and a power spectrum in the case of separation 36 that can set in when the flow separates.

[0109] These three power density spectra are plotted in a log-log diagram as a power density spectra G.sub.PP over frequency f.

[0110] In any case, it is possible to identify that there are significant changes in the power density spectra in the various situations. In addition to an increase from the normal to the critical state, it is also possible to recognize a shift in the frequency.

[0111] This is now exploited, as explained by FIG. 3. The normal power density spectrum 32 and the critical power density spectrum 34 of FIG. 2 are plotted in separate diagrams in FIG. 3. Here, both power density spectra are subdivided into a low frequency range 42 and a high frequency range 44. The spectral components contained therein in each case are referred to as low spectral component P.sub.1 and high spectral component P.sub.2, respectively.

[0112] It is clear that the low spectral component P.sub.1 forms the smaller component during the normal incident flow 16 and forms the greater component during the critical incident flow 24. For evaluation purposes, integrating the partial power density spectra in each case and forming a quotient, which can then be used as an indicator value I, is now proposed. Accordingly, a quotient of the low spectral component P.sub.1 and the high spectral component P.sub.2 according to the following formula is proposed for calculating the indicator value I:

[00001]$I=\frac{P_1}{P_2}={}_{f_1}^{f_2}.Math.G_{\mathrm{pp}}\left(f\right).Math.d.Math.f/{}_{f_2}^{f_3}.Math.G_{\mathrm{pp}}\left(f\right).Math.\mathrm{df}.$

[0113] Accordingly, one embodiment proposes dividing the spectrum into the low and high frequency range 42 and 44, respectively. The two power partial density spectra, which emerge from this subdivision, should be integrated in each case and a quotient should be calculated therefrom for the purposes of forming the indicator value. Now, a ratio limit value can be based on a previously calibrated threshold for this indicator value.

[0114] If this previously calibrated threshold is exceeded by the indicator value, the rotor blade or rotor blades are rotated slightly out of the wind, for example by initially 1, which a person skilled in the art and also refers to as pitching out.

[0115] The curves of such indicator values, i.e., of the described quotients I, are plotted in FIG. 4 as a function of the angle of attack for different wind speeds and for clean and dirtied rotor blades. These curves have been gathered from trials in a wind tunnel.

[0116] Here, five curves 51-55 are plotted, the following boundary conditions applying thereto: [0117] 51: 40 m/s wind speed in the case of a clean blade [0118] 52: 60 m/s wind speed in the case of a clean blade [0119] 53: 80 m/s wind speed in the case of a clean blade [0120] 54: 60 m/s wind speed in the case of a dirtied blade [0121] 55: 80 m/s wind speed in the case of a dirtied blade.

[0122] For the cases with a clean, i.e., non-dirtied, rotor blade, i.e., for the cases with a very smooth profile surface, the curves of different incident flow speeds, namely 51, 52 and 53, almost coincide. The indicator value, which can also be referred to as the quotient of the power density spectra or as spectral energy coefficient, would consequently always detect starting of the separation very well for these clean cases. To this end, only this coefficient would be required and, in particular, knowledge of the incident flow speed and of the rotational speed are not required to this end. For illustration purposes, a clean separation limit 56 is plotted to this end, said separation limit, for instance, denoting an angle of attack, namely approximately 8.5 in this case, in which separation would arise in the case of a clean and hence very smooth profile surface, and said separation limit also arising in trials in a wind tunnel.

[0123] For the dirtied case, i.e., the curves 54 and 55, the critical angle of attack is lower than in the clean case. This, too, is mapped by the indicator value, i.e., the indicator values 54 and 55 in this case. However, a slight dependence on rotational speed, namely a dependence on the incident flow wind speed, is visible in this case. For illustration purposes, a dirtied separation limit 58 is also plotted for dirtied rotor blades.

[0124] Such an influence of the rotational speed or the wind speed and the dirtying situation can be reduced by choosing suitable limit frequencies. Such limit frequencies, namely the lower, mid and upper frequency f.sub.1, f.sub.2 and f.sub.3, respectively, can be accordingly ascertained in advance and programmed into the corresponding evaluation algorithm. It is also possible for four frequencies to be present, two of which in each case defining a frequency range. Of these, two frequencies could correspond and accordingly form the mid frequency f.sub.2, or, in fact, four different frequencies could be chosen.

[0125] Moreover, or alternatively, the described regulation could also be set to be exact only above a sound-critical rotational speed, above which the indicator value operates reliably. Thus, pitching-out on the basis of the indicator value can be proposed only to be carried out once a predetermined minimum rotational speed is present.

[0126] Consequently, FIG. 4 shows the relationship of the low spectral component P.sub.1 and the high spectral component P.sub.2 for different boundary conditions. To this end, different limit frequencies were selected, namely the lower, mid and upper frequency or limit frequency f.sub.1, f.sub.2 and f.sub.3, respectively, which also supply a meaningful indicator value in relation to a ratio limit value for different boundary conditions, i.e., in particular, different incident flow wind speeds, even in the case of dirtied rotor blades. In the case of a ratio limit value 60, which has a value of 2 in this case, it is consequently possible to recognize separation tendencies well, even for the different conditions, by way of the indicator value. The frequencies chosen to this end are: f.sub.1=200 Hz, f.sub.2=400 Hz and f.sub.3=600 Hz.

[0127] For implementation purposes, attaching the sensor or sensors in the outer region of the rotor blade, on the suction side and in the direct vicinity of the trailing edge, is proposed. From there, fiber-optical lines can be installed in the direction of the hub, where possible along a neutral fiber, for example along a web in the support structure of the rotor blade. There, the sensor or the sensors can be connected to an evaluation unit in the rotor blade, particularly if only one sensor is present, or in the hub, in particular if three sensors are present, namely one sensor per rotor blade. The laser signals cast back by the sensor or sensors, to name but one example, can be evaluated at the evaluation unit.

[0128] Then, linking such an evaluation unit, in particular an evaluating microprocessor unit used to this end, to the installation controller and installation regulator of the wind power installation is proposed. As a result, such an evaluated measurement value, i.e., in particular, the indicator value, can cause a displacement of the blade adjustment angle motors toward smaller angles of attack if a threshold calibrated in advance is exceeded. Such a calibrated threshold is plotted in FIG. 4 as a ratio limit value 60. The effect of such a control measure will be a reduction in the indicator value. In relation to the diagram in FIG. 4, this would correspond to a reduction in the angle of attack , and so the values on the relevant curve change in accordance with this modified angle of attack.

[0129] The response of a sensor, i.e., after carrying out the evaluation of the indicator value, can suffice to trigger such an action, namely the adjustment of the rotor blades. Preferably, a subsequent waiting time is provided, which can be one minute, for example, before the blade angle can be rotated back again if the indicator value always lay below the limit value or below the lower hysteresis value during this time. If the indicator value once again exceeds the threshold, the blade angle should be increased further until the indicator value permanently lies below the threshold.

[0130] Should the indicator value then not be triggered for a relatively long period of time, for example because modified atmospheric conditions are present, it is possible to reduce the blade angle, which can also be referred to as pitch angle, again for the purposes of increasing the power. This increase for elevating the power can then be realized by the installation controller.

[0131] A further embodiment proposes a second, smaller underlying ratio limit value being used as a basis, i.e., a second ratio limit value that is smaller than the ratio limit value 60. As a result, a control hysteresis can be realized in the controller. After the occurrence of an above-described OAM noise, this second ratio limit value would have to be initially (permanently) undershot before the blade angle is reduced again, i.e., before the rotor blade is adjusted again in the direction toward an ideal blade angle.

[0132] A control sequence is shown in FIG. 5. Consequently, FIG. 5 shows a control diagram 70, in which a sensor block 72 represents the recording of a time-dependent pressure p, which is shown in the time-dependent pressure diagram 74. This time-dependent pressure curve according to the pressure diagram 74 is then converted into a power density spectrum G.sub.PP(f) according to the spectral evaluation block 76 and this result is visualized in the power density spectrum block 78.

[0133] Then, the power density spectrum, as illustrated by block 78, is evaluated in the integration evaluation block 80. In this evaluation, a subdivision into two frequency ranges is undertaken on the basis of a lower, mid and upper frequency f.sub.1, f.sub.2 and f.sub.3, respectively. Consequently, the power density spectrum is subdivided into a lower and upper spectral component and these two power density spectra of the low and high spectral component are integrated and a ratio of these two integrated values is formed in order to form an indicator value therefrom.

[0134] This indicator value is then compared to a limit value, namely, in particular, a ratio limit value, and a decision is made dependent thereon in the decision block 82 as to whether the indicator value is low enough to still assume a normal incident flow or whether it has exceeded the ratio limit value and it is hence necessary to assume a critical incident flow, shown as not ok (n. ok) in the decision block 82. Otherwise, the result can be visualized as ok in the decision block. Depending thereon, a control signal for increasing the blade adjustment angle for the purposes of reducing the angle of attack is then produced in the actuator block 84 if a critical incident flow being present was determined in the decision block 82, i.e., if the result was not ok. The actuator block 84 can be realized in the central installation controller, the software of which being accordingly expanded in order to take account of the indicator according to the disclosure.

[0135] Then, this process shown in the control diagram 70 is continuously repeated. Such repetition can lie in the range of approximately 0.01 to 0.2 seconds. A lower value of 0.01 seconds (i.e., 100 Hz) is particularly advantageous when the indicator is subject to low-pass filtering. Such a high evaluation rate is proposed for this case, in particular.

[0136] Consequently, a solution was now proposed here, by means of which an unwanted noise phenomenon, which is also referred to as other amplitude modulation (OAM) in the art, can be prevented or at least reduced. To this end, in particular, sensors integrated into the blade surface, or at least one such sensor, and a control strategy are proposed. By way of a good choice of the parameters, in particular the lower, mid and upper frequency f.sub.1, f.sub.2 and f.sub.3, respectively, it is possible not only to reduce but completely suppress the phenomenon. Moreover, it is particularly advantageous if the evaluation of the measurement signals is independent or at least robust in relation to the calibration, the incident flow speed and the degree of dirtying of the blade or else an erosion of the blade.

[0137] Consequently, it was also possible to create a solution that makes do with as little outlay in terms of measurement technology and with as little sensitivity as possible of the method in relation to environmental influences, which influence the object to be measured and could lead to incorrect results. This includes eddies within a turbulent boundary layer, which are responsible for the surface pressure field of the rotor blade.

[0138] In particular, the proposed solution is also superior over methods which only detect an OAM event in a far field in order to intervene in the regulation so as to remove the problem again. The solution also has advantages over methods that are based on a determination of the angle of attack since the critical angle of attack depends on the properties of the boundary layer around the rotor blade profile and hence depends on the condition of the surface, in particular on dirtying as well.