METHOD FOR CONTROLLING A WIND TURBINE

Abstract

A method for controlling a wind turbine with rotor blades with an adjustable blade angle, comprising: operating the wind turbine in a part-load operation for wind speeds up to a rated wind speed, operating the wind turbine in a full-load operation for wind speeds above the rated wind speed, with the blade angle being increased in full-load operation with increasing wind speed, setting a limit angle as a minimum value of the blade angle, and controlling the wind turbine in such a way that the limit angle is undershot by at most a predetermined difference angle.

Claims

1. A method for controlling a wind turbine having rotor blades with an adjustable blade angle, comprising: operating the wind turbine in a part-load operation for wind speeds up to a rated wind speed, operating the wind turbine in a full-load operation for wind speeds above the rated wind speed, increasing the blade angle in the full-load operation as wind speed increases, setting a limit angle as a minimum value of the blade angle, and controlling the wind turbine such that the limit angle is undershot by an angle that does not exceed a predetermined difference angle.

2. The method as claimed in claim 1, comprising: setting of the limit angle, and wherein controlling the wind turbine such that the limit angle is undershot by the angle that does not exceed the predetermined difference angle () when at least one condition occurs from a list of conditions including: a predetermined gustiness of wind is reached, a predetermined gust frequency of the wind is reached, and a peak rotational speed, higher than a rated rotational speed by more than a predetermined tolerance value, is reached at least once within a predetermined period of time.

3. The method as claimed in claim 1, comprising: setting the limit angle or the predetermined difference angle based on wind gustiness.

4. The method as claimed in claim 1, comprising: selecting a value of the predetermined difference angle from a list including: a value range from 0 to 10, a value range from 3 to 8, and a value that is substantially 5.

5. The method as claimed in claim 1, comprising: determining a mean value or a filtered value of last set blade angles, and setting the limit angle to the determined mean value or filtered value.

6. The method as claimed in claim 5, wherein determining the mean value includes at least one of: forming the mean value of the last set blade angles over a period of time with a length of 5 to 20 seconds, forming the mean value of the last set blade angles over a period of time with a length of 6 to 15 seconds, or forming the mean value of the last set blade angles over a period of time with a length of approximately 8 seconds.

7. The method as claimed in claim 1, comprising: setting the limit angle or the predetermined difference angle based on a gust frequency.

8. The method as claimed in claim 1, wherein a fixed part-load angle is provided for the blade angle during the part-load operation, and wherein the limit angle or the limit angle minus the difference angle is not smaller than the fixed part-load angle.

9. The method as claimed in claim 1, wherein the limit angle decreases over time or decreases with a gradient that is dependent on time.

10. A wind turbine, comprising: at least one adjustable rotor blade, and a controller for adjusting the at least one rotor blade, wherein the controller is configured to control the at least one rotor blade by at least: operating the wind turbine in a part-load operation for wind speeds up to a rated wind speed, operating the wind turbine in a full-load operation for wind speeds above the rated wind speed, increasing a blade angle of the at least one rotor blade in the full-load operation as wind speed increases, setting a limit angle as a minimum value of the blade angle, and controlling the wind turbine such that the limit angle is undershot by an angle that does not exceed a predetermined difference angle.

11. The method as claimed in claim 5, wherein determining the filtered value includes at least one of: low-pass filtering the last set blade angles, low-pass filtering the last set blade angles with a first-order low-pass filter having a time constant in a range of 5 to 20 seconds, low-pass filtering the last set blade angles with a first-order low-pass filter having a time constant in a range of 6 to 15 seconds, or low-pass filtering the last set blade angles with a first-order low-pass filter having a time constant of approximately 8 seconds.

12. The method as claimed in claim 9, wherein the limit angle decreases linearly over time or decreases with a gradient that is linearly dependent on time.

13. The wind turbine as claimed in claim 10, wherein the controller is programmed, with a control program, that causes the controller to control the at least one rotor blade.

14. The wind turbine as claimed in claim 10, wherein the controller is configured to: set the limit angle, and wherein control the wind turbine such that the limit angle is undershot by an angle that does not exceed the predetermined difference angle when at least one condition occurs from a list of conditions including: a predetermined gustiness of wind is reached, a predetermined gust frequency of the wind is reached, and a peak rotational speed, higher than a rated rotational speed by more than a predetermined tolerance value, is reached at least once within a predetermined period of time.

15. The wind turbine as claimed in claim 10, wherein the controller is configured to: set the limit angle or the predetermined difference angle based on wind gustiness.

16. The wind turbine as claimed in claim 10, wherein the controller is configured to: select a value of the predetermined difference angle from a list including: a value range from 0 to 10, a value range from 3 to 8, and a value that is substantially 5.

17. The wind turbine as claimed in claim 10, wherein the controller is configured to: determine a mean value or a filtered value of last set blade angles, and set the limit angle to the determined mean value or filtered value.

18. The wind turbine as claimed in claim 17, wherein the controller is configured to determine the mean value by at least one of: forming the mean value of the last set blade angles over a period of time with a length of 5 to 20 seconds, forming the mean value of the last set blade angles over a period of time with a length of 6 to 15 seconds, or forming the mean value of the last set blade angles over a period of time with a length of approximately 8 seconds.

19. The wind turbine as claimed in claim 10, wherein the controller is configured to: set the limit angle or the predetermined difference angle based on a gust frequency.

20. The wind turbine as claimed in claim 10, wherein a fixed part-load angle is provided for the blade angle during the part-load operation, and wherein the limit angle or the limit angle minus the difference angle is not smaller than the fixed part-load angle.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0043] The invention is explained in more detail below in an exemplary manner, with reference being made to the attached figures.

[0044] FIG. 1 shows a wind turbine in a perspective view.

[0045] FIG. 2 shows a schematic diagram that illustrates relationships between rotational speed, wind speed and blade angle.

DETAILED DESCRIPTION

[0046] FIG. 1 shows a wind turbine 100 with a tower 102 and a nacelle 104. A rotor 106 with a three rotor blades 108 and a spinner 110 are arranged on the nacelle 104. During operation, the wind puts the rotor 106 into a rotational movement and this drives a generator in the nacelle 104.

[0047] In FIG. 2, the wind speed V.sub.wind, the blade angle and the rotor rotational speed n are plotted over time. Although the axis label of the ordinate provides units for the corresponding variables, the exact values are not illustrated to facilitate explanation of the basic principles.

[0048] In addition to the actual blade angle , the possible curve of a limit angle .sub.G and a curve of the limit angle .sub.G minus a difference angle are plotted (denoted as .sub.G- in this case). Moreover, two dotted partial curves are illustrated, namely a modified angle .sub.mod specifying the intended blade angle profile, and a rotational speed n.sub.mod showing the resultant rotational speed in this respect.

[0049] In the illustration of FIG. 2, the assumption is made that the wind speed rises at the time T.sub.1 and that a gust is present at that time. At first, as the wind speed increases, there likewise is an increase in the rotational speed n. The blade angle is also increased in order to counteract this. It is possible to identify that the blade angle can vary slightly even before the time T.sub.1 in order to keep the rotational speed n approximately constant. In this respect, keeping the rotational speed n constant prior to the time T.sub.1 is also quite successful. However, the gust occurring immediately after the time T.sub.1 leads to a noticeable increase in the rotational speed n.

[0050] The limit angle .sub.G, which forms a mean value of the angle , can also be seen there in addition to the curve of the angle . Accordingly, there is a comparatively small change in the curve of the limit angle .sub.G.

[0051] Moreover, the difference angle is plotted for the limit angle .sub.G. The limit angle .sub.G is allowed to be undershot by no more than the difference angle . Accordingly, a limit to be observed, which is plotted as .sub.G-, arises. Said limit starts shortly after the time T.sub.1, and this beginning is denoted by Start. There was a detection at this time that a certain gustiness of the wind is present, and consequently the difference angle was switched to be active. In this example, the limit angle .sub.G is always recorded, namely as a mean value of the angle . So that the difference angle now finds use, the corresponding feedback control is also switched to be active, said feedback control monitoring whether there is observation of the limit angle .sub.G being undershot by no more than the difference angle .

[0052] Then, with increasing time, the gust passes and the wind speed V.sub.Wind once again approximately assumes the value prior to the time T.sub.1. The rotational speed n could also be regulated to its initial value, namely the rated rotational speed n.sub.N, in the meantime. The blade angle has also been reduced correspondingly to a value approximately equal to that before the time T.sub.1. Variations can still be identified; these are unavoidable as the wind also varies slightly. These variations can hardly still be identified in the limit angle .sub.G, which, as stated previously, forms a mean value of this blade angle .

[0053] The wind speed starts to drop at the time T.sub.2. Whether this is a start of a fundamental reduction in the wind speed or a gust trough cannot be identified. In any case, the wind speed drops comparatively strongly, and so there also is reduction in the rotational speed n at first. The blade angle is likewise reduced in order now to keep the rotational speed n as constant as possible, namely to counteract the drop in rotational speed n. On account of the averaging, the limit angle .sub.G follows this curve of the blade angle only weakly at first.

[0054] However, the wind speed V.sub.Wind continues to drop and, at the time T.sub.3, the blade angle then reaches a value that lies below the limit angle .sub.G by the difference angle . The proposed feedback control would now start here.

[0055] However, with the solid curve of the angle and also the solid curve of the rotational speed n, FIG. 2 shows a curve that would set in without application of the proposed feedback control. Accordingly, the blade angle would continue to drop until the rotational speed n can be adjusted to the original rotational speed, namely the rated rotational speed n.sub.N at the time T.sub.4. By way of precaution, reference is made here to the fact that these assumptions, too, serve illustrative purposes and, by all means, it could also be the case that the wind speed then drops off so strongly that the wind turbine is in part-load operation and the rotational speed n cannot even hold the rated rotational speed n.sub.N due to a lack of wind. However, for illustrative purposes, the assumption is made that the described process plays out completely in full-load operation or for wind speeds that are usually settled in the full-load operation.

[0056] Consequently, it is now possible to keep the rotational speed n at its rated value n.sub.N according to the full line of the blade angle and the rotational speed n.

[0057] Now, the wind speed increases strongly again at the time T.sub.5. This may be typical in the case of a gusty wind speed. Accordingly, the rotational speed n also increases and the blade angle likewise increases again in order to counteract this rise in the rotational speed n.

[0058] Now, the special situation is present where the wind speed is initially comparatively low and the blade angle is also comparatively small but the rotational speed n nevertheless is at the rated value and therefore not far away from the limit rotational speed n.sub.MAX either. As a result of this now quickly increasing wind, the rotational speed also increases to such an extent that the controller of the blade angle is no longer able to sufficiently prevent the rotational speed from an increase that is too strong. Consequently, the rotational speed n then reaches the maximum value of the rotational speed n.sub.max at the time T.sub.6 and consequently an emergency shutdown would have to be implemented at the time T.sub.6; the latter would usually also be carried out because this is a safety aspect that cannot be precluded.

[0059] For illustration purposes, FIG. 2, however, shows the further curve of the rotational speed n, as if this emergency shutdown were deactivated. Accordingly, it is possible to identify that the rotational speed n still continues to rise slightly; however, it can then finally be adjusted too because the blade angle likewise increases further, it can be lowered below the maximum rotational speed n.sub.max and it can finally be regulated to the value of the rated rotational speed n.sub.N as well.

[0060] If the method is now carried out using the proposed feedback control, the blade angle will not be allowed to drop below the value of .sub.G- at the time T.sub.3 This deviating curve is illustrated there in dotted fashion. Consequently, this dotted line initially extends along the limit .sub.G-. The result of this is that, initially, the rotational speed n.sub.mod drops off more strongly than would be the case without this feedback control. Even at the time T.sub.4, this rotational speed n.sub.mod still is significantly lower than the rotational speed n without this proposed protective feedback control. At the time T.sub.5, too, this rotational speed n.sub.mod still is significantly lower than the rotational speed n.

[0061] Then, at the time T.sub.5, i.e., at a time when there is a strong increase in the wind, there is also a strong increase in blade angle, namely .sub.mod. Consequently, this blade angle .sub.mod is already greater than the normal blade angle . The rotational speed n.sub.mod can be regulated correspondingly strongly and an increase that is too high can be prevented. Moreover, what is advantageously additionally the case is that the rotational speed n.sub.mod is lower than the rotational speed n. Thus, the blade angle is greater than for the case without this protective feedback control and the rotational speed is lower than for the case without protective feedback control.

[0062] Consequently, this modified blade angle .sub.mod leaves the limit characteristic .sub.G- in the upward direction at the time T.sub.5.

[0063] There is no peculiarity at the time T.sub.6 for this curve when using the protective feedback control. However, it is possible to identify that the modified rotational speed n.sub.mod has not reached the limit rotational speed n.sub.MAX and a shutdown is consequently avoided.

[0064] Reference should also be made to the fact that the limit angle .sub.G drops off in a comparatively gentle fashion even after the time T.sub.3 because, in the process, it orients itself along the curve of the actual angle according to the modified blade angle .sub.mod.

[0065] Finally, a hatched region has additionally been plotted in the rotational speed characteristic, namely the region between the rotational speed curve without protective feedback control and the rotational speed curve n.sub.mod with protective feedback control. This hatched region illustrates the power losses that can arise due to the protective feedback control. It should be noted here that this only serves illustrative purposes and that, in fact, there would be an emergency shutdown at the time T.sub.6. Then, naturally, the hatched region after this time T.sub.6 would cease or, rather, it would be necessary to plot a hatched region below the dotted rotational speed characteristic n.sub.mod, all the way down to the time axis. Thus, it is easily identifiable that the supposed power losses are low and, in any case, not negative in comparison with the case where an emergency shutdown was able to be in fact prevented. Purely by way of precaution, reference is made to the fact that this hatched region only serves for illustration purposes and that, naturally, the integration of the rotational speed over the time does not yield power, already in view of the units.

[0066] Consequently, a solution for preventing an unwanted emergency shutdown in the case of overspeed could be proposed in a simple manner. No hardware adaptation would be needed to this end. In particular, there is also no need to use stronger pitch motors. The proposed feedback control requires neither additional measurement variables nor additional manipulated variables.