Method for operating a wind turbine, wind turbine and wind park

Abstract

Provided is a method for operating a wind turbine, an associated wind turbine and a wind park. The method comprises a) providing an indicator for the occurrence of a flow separation on a pressure side of a rotor blade of a rotor of the wind turbine, and b) changing an operational management of the wind turbine using the indicator, wherein the indicator comprises a pitch angle of the rotor blade. By using the pitch angle as an indicator, a flow separation on the pressure side of the rotor blade can be effectively prevented.

Claims

1. A method, comprising: operating a wind turbine, the operating comprising: providing an indicator for an occurrence of a flow separation on a pressure side of a rotor blade of a rotor of the wind turbine; and changing an operational management of the wind turbine based on the indicator, wherein the indicator includes a pitch angle of the rotor blade, wherein the wind turbine is operated with reduced nominal power level in a reduced power mode, and wherein changing the operational management comprises increasing a rotational speed of the rotor.

2. The method according to claim 1, wherein the pitch angle is measured or determined based on a set position of a pitch adjuster.

3. The method according to claim 1, wherein for a plurality of operating points of the wind turbine, a critical value of the indicator is provided at which the occurrence of flow separation on the pressure side begins, the method further comprising: comparing a current indicator value with the critical value of the indicator for the current operating point, wherein changing the operational management takes place when a difference between the current indicator value and the critical value of the indicator falls below a predefined safety value.

4. The method according to claim 3, wherein the plurality of operating points of the wind turbine is a plurality of rotational speeds of the wind turbine.

5. The method according to claim 3, wherein the critical value of the indicator is determined based on a distribution of angles of attack of the rotor blade and a critical angle of attack.

6. The method according to claim 3, wherein the critical value of the indicator corresponds to a critical pitch angle.

7. The method according to claim 3, wherein the critical value of the indicator diminishes as a rotational speed of the rotor increases.

8. The method according to claim 3, wherein the critical value of the indicator is established independently of a level of reduced nominal power in a restricted operating mode or rises as a degree of restriction increases or both.

9. The method according to claim 3, wherein the critical value of the indicator is a function of a profile surface state of the rotor blade.

10. The method according to claim 9, wherein the profile surface state of the rotor blade is soiling.

11. The method according to claim 1, wherein the indicator indicates an occurrence of the flow separation in an outer region of the rotor blade.

12. The method according to claim 11, wherein the outer region lies more than 70% of the rotor radius away from a center of the rotor.

13. The method according to claim 1, wherein changing the operational management is implemented as a control strategy.

14. A wind turbine comprising: an aerodynamic rotor configured to be operated with a rotation speed, a rotor blade coupled to the aerodynamic rotor, the rotor blade having a longitudinal axis and comprising: a pressure side and a suction side opposite the pressure side, a pitch adjuster for rotational movement of the rotor blade about the longitudinal axis for setting a pitch angle, and a controller configured to: provide a critical value indicator for an onset of a flow separation on the pressure side of the rotor blade, wherein the indicator includes the pitch angle, control an operating point of the wind turbine such that the indicator remains below the critical value indicator, operate the wind turbine in a reduced power mode with reduced nominal power level, and increase the rotation speed in a reduced power mode such that the indicator remains below the critical value indicator.

15. The wind turbine according to claim 14, wherein the controller is configured to control the pitch angle such that the critical value indicator is not exceeded.

16. The wind turbine according to claim 14, wherein the controller is configured to: provide the critical value indicator as a function of a rotation speed of the rotor, and control the pitch angle or the rotation speed or both such that the critical value indicator is not exceeded.

17. A wind park comprising at least one wind turbine according to claim 14.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Further advantages and particular embodiments are described below with reference to the appended figures. The drawings show:

(2) FIG. 1 schematically as an example, a wind turbine;

(3) FIG. 2 schematically as an example, a rotation speed—power curve;

(4) FIG. 3 schematically as an example, a diagram of the pitch angle and angle of attack over the wind speed;

(5) FIG. 4 schematically as an example, a further diagram; and

(6) FIG. 5 schematically as an example, a further diagram.

DETAILED DESCRIPTION

(7) FIG. 1 shows a diagrammatic depiction of a wind turbine according to the invention. The wind turbine 100 has a tower 102 and a nacelle 104 on the tower 102. An aerodynamic rotor 106 with three rotor blades 108 and a spinner 110 is provided on the nacelle 104. The aerodynamic rotor 106 is set in a rotational movement by the wind during operation of the wind turbine, and thus also turns an electrodynamic rotor of a generator which is coupled directly or indirectly to the aerodynamic rotor 106. The electrical generator is arranged in the nacelle 104 and generates electrical energy. The pitch angle of the rotor blades 108 may be changed by pitch motors at the rotor blade roots of the respective rotor blades 108.

(8) FIG. 2 shows schematically as an example a curve 200, which shows in exemplary fashion a correlation between a rotation speed N on the horizontal axis and a generated electrical power P on the vertical axis, wherein other curves are also possible. Reduced power operating modes were developed in order to meet the requirements of network operators who, e.g., in high wind situations, require only a reduced infeed power from wind turbines 100 or wind parks in order to prevent overload of the power networks. According to the prior art, on an operating curve of an optimized power mode, such as for example curve 200, the rotation speed is reduced until the reduced nominal power level is reached. This necessarily means that reduced power operating modes always have lower nominal rotation speeds than the optimized power mode. FIG. 2 shows schematically examples of operating curves (BKL) of an optimized power operating mode 210, and a reduced power mode 220 for the region until reaching the rated output.

(9) FIG. 3 shows schematically as an example a diagram 300 of an effective angle of attack α.sub.eff and a pitch angle θ on the vertical axis for various free wind speeds v.sub.inf on the horizontal axis. The effective angles of attack 330, 340 are shown as an example on a rotor blade section at 90% of the rotor blade length, since at this point there is a high risk of occurrence of negative stall. The corresponding pitch angles are shown as lines 310, 320.

(10) The exemplary scale for the effective angle of attack α.sub.eff is shown on the left-hand side of the diagram 300, and that for the pitch angle θ on the right-hand side of the diagram 300. The same applies to the further FIGS. 4 and 5.

(11) The diagram 300 is based on a reduced rated power of around 15% of the nominal output of the installation for two different rotation speeds. The solid lines 310 and 340 indicate the conditions at the rotation speed provided according to the prior art for these restricted operating modes, wherein the rotation speed is around 65% of the nominal rotation speed N.sub.soll at nominal power. The dotted lines 320 and 313 show the conditions if the rotation speed were increased to N.sub.soll for the operating mode restricted to 15% of nominal power.

(12) The whole of FIG. 3 therefore refers to restricted mode. Whereas lines 310 for pitch angle θ and 340 for the resulting effective angle of attack α.sub.eff correspond to the conditions at reduced operating speed, wherein the reduced operating speed is in conformity with the usual procedure for power reduction in restricted mode, the rotation speed is increased for the dotted lines 320 and 330 to a value which substantially corresponds to the nominal rotation speed N.sub.soll at nominal power.

(13) The information content of this diagram will be explained with reference to some concrete numerical examples. Initially, let us assume that the critical angle of attack 302 for the occurrence of negative stall at this blade section is −8°. According to the operational management of the prior art, then the critical angle of attack of −8° would be reached at a wind speed of for example v=17 m/s. Over a wide range of wind speeds, above the trigger speed of for example v=5 m/s, the effective angle of attack α.sub.eff diminishes monotonously as the wind speed rises.

(14) Without further measures, now, i.e., on reaching the critical effective angle of attack α.sub.eff, the installation would have to be shut down or at least the rotation speed N further reduced if the occurrence of negative stall leads to undesirable phenomena, such as noise increase or load increases caused by aero-elastic instabilities.

(15) The associated critical pitch angle 304 in this case is for example around 29°. If the rotation speed is raised for example to N.sub.soll, the critical angle of attack of −8° is only reached above 22 m/s, i.e., at higher wind speeds. This is indicated in that the dotted line 330 only reaches a specific value of the angle of attack at a higher wind speed, i.e., further to the right in the drawing, than the solid line 340 for the reduced rotation speed.

(16) The amount of increase in the wind speed, at which the critical angle of attack is reached after increasing the rotation speed, depends on the profile used at this blade section. If the critical angle of attack is for example −9°, the critical wind speed rises from 19.5 m/s to >25 m/s, i.e., in some cases in a region of storm control in which the rotation speed and power are reduced. If the critical angle of attack is just −4°, the critical wind speed rises marginally from 10 m/s to slightly over 11 m/s, so measures such as shut-down are only slightly delayed.

(17) If we return to the original assumption of a critical angle of attack of −8° and assume a still constant wind speed, it is clear from FIG. 3 that by increasing the rotation speed, the angle of attack is increased from −8° to around −6.5° and hence moves away from the negative stall. The pitch angle is reduced for example from 29° to around 22°, as shown by a comparison of lines 310 and 320.

(18) Until the angle of attack at increased rotation speed N.sub.soll and 22 m/s again reaches the critical mark of −8°, the pitch angle rises to around 27°, i.e., the critical pitch angle diminishes slightly as the rotation speed increases, in this numerical example from 29° to 27°. The critical pitch angle is thus preferably at least a function of the rotation speed. The dependency on the power level of the restricted mode will be clarified below.

(19) FIG. 4 shows a diagram 400 which has the same axes as the diagram 300 in FIG. 3. The pitch angle and angle of attack at the rotor blade section at 90% radius are shown for two operating modes, which are restricted to 15% and 25% of rated output, with a rotation speed increase to the nominal rotation speed N.sub.soll. Lines 410 and 420 accordingly correspond to the pitch angle development with operation restricted to 15% and 25% of the nominal power. Lines 430 and 440 show accordingly the development of the effective angle of attack, wherein the solid lines 410, 440 refer to operation reduced to 15% of rated output, and dotted lines 420, 430 refer to operation reduced to 25% of rated output.

(20) In the case of the 15% mode, with the exemplary assumption of the critical angle of attack of −8°, negative stall is reached at a wind speed of 22 m/s and a pitch angle of 27°. In the case of the 25% mode, these values are around 23.5 m/s and a pitch angle of 28°. From this example, it is clear that the critical pitch angle, as an indicator 415 of negative stall in reduced power operating modes, is dependent on the respective power level. If the differences in practical cases are considered slight, the lower of the two critical pitch angles may be used for both modes, which simplifies the control system.

(21) A further uncertainty in using the pitch angle as an indicator is the modified profile properties due to soiling. Firstly, the angle of attack at which negative stall occurs may change. In many practical cases, this change is very small so that a single stall limit may be defined for various degrees of profile soiling. Secondly, a changed pitch control results for the modified profile properties.

(22) FIG. 5 shows a diagram 500 which has the same axes as diagram 300 in FIG. 3. Lines 510 and 520 correspond to a development of pitch angle over wind speed for operation reduced to around 25% of rated output, while lines 530 and 540 show the associated effective angles of attack. The solid lines 510 and 540 belong to a clean profile, and lines 520 and 530 to a soiled profile.

(23) It is clear from FIG. 5 that the critical angle of attack of −8° for clean flow profiles is reached at around 21.5 m/s wind speed and 29.5° pitch angle. For soiled profiles (tripped), these values change to 23.5 m/s and 31°. The difference between 29.5° and 31° can in practice again be ignored since the lower of the two pitch angles can be defined as critical for both cases.

(24) If in addition we assume a shift in the stall limit, i.e., the critical angle of attack, from −8° to −7° due to the soiling of the profiles, the value pair for negative stall then changes to 27.5° pitch at 20 m/s. The critical pitch angle in this case could be established at 27.5° independently of profile property.

(25) The clean blade would then trigger a corresponding control strategy such as speed increase at a wind speed of around 20 m/s instead of 21.5 m/s. It is accordingly preferred that the profile surface (e.g., profile surface 116 in FIG. 1) states such as soiling are taken into account in the choice of definition of critical pitch angle. It is however possible that in practice it is sufficient to define one pitch angle for all profile states.

(26) The invention may be applied to all pitch-controlled, variable speed wind turbines which are able to detect the pitch angle.