Method for operating a wind turbine

Abstract

A method for operating a wind turbine, in particular at a location characterized by a cold climate is provided. The method includes, specifying an air density at a location of the wind turbine, setting a blade angle of an adjustable rotor blade based on an output power, torque and/or rotor speed. The method includes setting the blade angle as a function of a at pitch characteristic curve which specifies the blade angle as a function of the output power, the torque and/or the rotor speed and as a function of the air density. According to the pitch characteristic curve, the blade angle has a minimum as a function of the air density in a region of a reference density of an atmosphere at the location which is characterized by a cold climate.

Claims

1. A method for operating a wind turbine at a location having an air density, comprising: specifying the air density at the location of the wind turbine, wherein the wind turbine includes: an aerodynamic rotor having a plurality of rotor blades, wherein the plurality of rotor blades have adjustable blade angles and the aerodynamic rotor is capable of being operated with a rotor speed that is variable; and a generator coupled to the aerodynamic rotor, and setting a blade angle based on a pitch characteristic curve, the pitch characteristic curve specifying the blade angle as a function of the air density at the location of the wind turbine and at least one of: an output power, torque, and the rotor speed of the wind turbine, wherein according to the pitch characteristic curve the blade angle has a minimum, as the function of the air density, in a region of a reference density of an atmosphere at the location of the wind turbine.

2. The method as claimed in claim 1, wherein the reference density is a standard density of a standard atmosphere.

3. The method as claimed in claim 1, wherein in the pitch characteristic curve of the blade angle as the function of the air density, the minimum is a mathematical local minimum.

4. The method as claimed in claim 3, wherein at the mathematical local minimum, the blade angle as the function of the air density is lower than a first blade angle at a first air density that is higher than the reference density and that is lower than a second blade angle at a second air density that is less than the reference density.

5. The method as claimed in claim 1, wherein in the pitch characteristic curve of the blade angle as the function of the air density, the minimum of the blade angle is a global minimum and is a smallest blade angle of the pitch characteristic curve.

6. The method as claimed in claim 1, wherein the region of the reference density of the atmosphere at the location includes air densities within +/−10% or +/−5% of the reference density.

7. The method as claimed in claim 1, wherein the blade angle based on the pitch characteristic curve increases starting from the reference density as the air density decreases and as the air density increases.

8. The method as claimed in claim 1, comprising: determining ambient variables of the wind turbine including the air density and temperature which are relevant for the wind turbine; and/or determining air pressure and the air temperature, and determining the air density based on the air pressure and the air temperature; and/or determining the air pressure, the air temperature and air moisture, and determining the air density based on the air pressure, the air temperature and the air moisture.

9. The method as claimed in claim 1, wherein the air density is a current air density at the location of the wind turbine, and the air density is determined in a transient fashion or repeatedly at predetermined intervals.

10. The method as claimed in claim 9, comprising: setting the blade angle dynamically in accordance with the pitch characteristic curve.

11. The method as claimed in claim 1, wherein: the air density is a prevailing air density, the air density is variable and permanently predefined for the location, the air density is determined once or repeatedly, or the air density is predefined as an average air density at the location.

12. The method as claimed in claim 11, wherein the air density is the prevailing air density at the location, and wherein the method comprises: setting the blade angle statically in accordance with the pitch characteristic curve.

13. The method as claimed in claim 1, wherein the pitch characteristic curve is a part of a characteristic curve diagram, and wherein the method comprises: storing a plurality of pitch characteristic curves respectively corresponding to a plurality of different reference densities; and selecting the pitch characteristic curve from the stored plurality of pitch characteristic curves based on an acquired air density.

14. The method as claimed in claim 13, comprising: determining a scaling factor as a ratio of the acquired air density to the reference density; scaling the pitch characteristic curve using the scaling factor; and setting the blade angle in accordance with the pitch characteristic curve.

15. The method as claimed in claim 14, wherein the pitch characteristic curve specifies the blade angle as a function of an acquired current or prevailing air density, and wherein the pitch characteristic curve specifies the blade angle as the function of the air density and at least one of: the output power, the torque, or the rotor speed.

16. The method as claimed in claim 1, comprising: defining a tower clearance and/or a blade deflection of a rotor blade at the reference density of the atmosphere at the location, wherein the tower clearance is a horizontal distance in a region of a blade tip of the rotor blade and the and/or a blade deflection of the rotor blade is with respect to a tower when the rotor blade passes the tower, wherein the minimum of the blade angle is defined such that the tower clearance is at a minimum.

17. The method as claimed in claim 1, wherein the pitch characteristic curve has a progression such that when the blade angle is set, a minimum tower clearance: is maintained or remains constant while predefining at least the pitch characteristic curve at least for an increased air density; and/or is lowered for a reduced air density compared with a linearly decreasing minimum tower clearance as the function of the air density for an identically increased or reduced air density.

18. The method as claimed in claim 1, wherein the blade angle is lowered and/or a rated rotational speed is increased during rated operation in the region of the reference density for the location, wherein the region is between a first blade angle of a first air density that is greater than the reference density of a standard atmosphere and a second blade angle of a second air density that is less than the reference density of the standard atmosphere.

19. The method as claimed in claim 1, wherein: the pitch characteristic curve has a flatter form around the minimum at a first power than at a second power that is lower than the first power; and/or a region of the pitch characteristic curve, having a lowered blade angle, around the minimum at the first power is wider than the region at the second power.

20. The method as claimed in claim 1, wherein the wind turbine is operated in the region of the reference density in the region around the minimum of the pitch characteristic curve with an increased rotational speed.

21. The method as claimed in claim 1, wherein the pitch characteristic curve specifies the blade angle to be set for a partial load operation and/or for a transition from the partial load operation into a rated load operation, wherein in the partial load operation wind is weak that the wind turbine is not yet operated with a maximum output power.

22. The method as claimed in claim 1, wherein the location is characterized by a cold climate and the reference density is a cold climate air density of the location.

23. The method as claimed in claim 22, wherein the location of the wind turbine characterized by the cold climate has: an average temperature over a year below 0° C., and/or a minimum temperature over the year below −15° C.

24. The method as claimed in claim 23, wherein the location has the average temperature over the year below −15° C. and/or the minimum temperature over the year below −20° C.

25. The method as claimed in claim 1, wherein the reference density is 1.3 kg/m.sup.3 or greater.

26. A device for performing open-loop and/or closed-loop control of a wind turbine at a location having an air density, comprising: an operational control system configured to: specify the air density at the location of the wind turbine, the wind turbine having an aerodynamic rotor with a plurality of rotor blades having a respective plurality of blade angles that are adjustable, and the wind turbine having a generator that is coupled to the aerodynamic rotor, and the aerodynamic rotor being capable of being operated with a rotor speed that is variable; retain a pitch characteristic curve; and set a blade angle of a rotor blade of the plurality of rotor blades based on the pitch characteristic curve, the pitch characteristic curve specifying the blade angle as a function of the air density at the location of the wind turbine and at least one of: an output power, torque and the rotor speed of the wind turbine, wherein according to the pitch characteristic curve, the blade angle, as the function of the air density, has a minimum in a region of a reference density at the location of the wind turbine.

27. A wind turbine, comprising: the aerodynamic rotor; the generator; and the device as claimed in claim 26 including the operational control system.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) Further advantages, features and details of the invention emerge from the following description of preferred embodiments and with reference to the drawing; in which:

(2) FIG. 1 shows a preferred embodiment of a wind turbine;

(3) FIG. 2 shows a preferred embodiment of an aspect of a pitch characteristic curve (relationship between the blade angle (pitch angle) and power), wherein the pitch characteristic curve at a constant power as a function of the air density at a reference density ρ0, which is intended to correspond here to the standard density ρ_norm of a standard atmosphere, can in general however be a reference density ρ0 as “cold climate” air density (that is to say, for example, a reference density ρ0 where ρ>=1.3 kg/m.sup.3) has a global minimum (α0) here;

(4) FIG. 3 shows two pitch characteristic curves which are illustrated by way of example as a typical relationship between a blade angle (pitch angle) and power in comparison with a pitch characteristic curve according to the prior art at a standard density;

(5) FIG. 4 shows three pitch characteristic curves which are illustrated by way of example as a typical relationship between a blade angle (pitch angle) and power level at different densities;

(6) FIG. 5 shows a preferred embodiment of a pitch characteristic curve diagram comprising a multiplicity of pitch characteristic curves taking into account the aspect that the pitch characteristic curves (relationship between the blade angle (pitch angle) and power) as a function of the air density at a reference density ρ0—which is intended to correspond here to the standard density ρ_norm of a standard atmosphere but can generally be a reference density ρ0 as “cold climate” air density (that is to say, for example, a reference density ρ0 where ρ>=1.3 kg/m.sup.3)—have a global minimum (α0) here;

(7) FIG. 6 shows a comparison of a tower clearance (TF) which is illustrated schematically and by way of example, with the illustration of a reduced tower clearance at relatively low densities and an essentially constantly low tower clearance even at relatively high densities according to a pitch characteristic curve;

(8) FIG. 7 shows a family of pitch characteristic curves for different powers, wherein the minimum is all the more pronounced the lower the power;

(9) FIG. 8 shows a preferred embodiment of an open-loop and closed-loop control device in a simplified structure for carrying out setting of a blade angle during partial load operation up to the rated load, as a function of the output power, and if appropriate rotational speed, and the acquired air density.

DETAILED DESCRIPTION

(10) FIG. 1 shows as an example a preferred embodiment of a wind turbine 100 with a tower 102 and a nacelle 104. A rotor 106 with three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. The rotor 106 is displaced by the wind into a rotational movement during operation and thereby drives a generator in the nacelle 104.

(11) A wind turbine in FIG. 1 or any wind turbine of a wind park or else the wind park is equipped here with a device 200 for performing open-loop and/or closed-loop control (open-loop and closed-loop control device 200) as part of an operational control system with one or more regulators 220 which comprise here at least one power regulator 222. These regulators 220 control an actuating device 300 with a corresponding actuator system or actuating elements 301, 302, 303, for, as an example, a blade angle of a rotor blade or of the rotor blades according to a pitch characteristic curve and/or an exciter current of the generator in the nacelle and/or an azimuth angle of the nacelle of the wind turbine 100.

(12) The open-loop and closed-loop control device 200 receives according to FIG. 1 measurement information from a sensor system 230 via a signal line 231 which approach a measuring module 210 of the open-loop and closed-loop control device 200. The sensor system 230 for the wind turbine acquires relevant ambient variables comprising at least the air density and/or temperature relevant to the wind turbine and/or an air pressure and an air temperature for the air density as further relevant ambient variables in the surroundings of the wind turbine, in order to determine the air density therefrom.

(13) The sensor system 230 and/or this measuring module 210 therefore has at least one first determining unit 211 for determining a density, and the second determining unit 212 for determining the rotational speed n of a rotor of the wind turbine. Parameters such as the blade deflection can also be measured, for example, with a strain gauge and/or a tower clearance of the wind turbine can be measured with a distance sensor over the distance between the outer wall of the tower and a blade tip of the rotor blade.

(14) Furthermore, in addition to the power regulator 222 here, a pilot control unit 221, e.g., a computing unit or the like with one or more stored operating characteristic curves comprising at least one air-density-dependent pitch characteristic curve Kα (P,n, ρ) with a pitch characteristic curve Kα,ρ which is dependent on the air density is provided here as part of the regulator 220, which pitch characteristic curve Kα (P, n, ρ) is able, in particular, to predefine an adapted rotational speed n in accordance with a density-adapted operating characteristic curve diagram Kα (P, n, ρ) for the power regulator 222 here.

(15) The rotational speed n′ which has been adapted and/or corrected in this way—that is to say a rotational speed n′ which has been adapted from a rotational speed n to a rotational speed n′ according to the pilot control unit 221 and which can additionally or alternatively be corrected according to a density-adapted power P, can be fed via a further signal line 232 to the wind turbine 100 and the corresponding actuating device 300 thereof.

(16) As already mentioned at the beginning, power calculations for a wind turbine are currently carried out with the assumption of a standard atmosphere. The standard density ρ_norm which is used here for the reference density ρ0 is ρ_norm=1.225 kg/m.sup.3. At any rate at locations at a high altitude and/or under “cold climate” conditions with on average relatively low temperatures this assumption is, however, no longer quantitatively correct and deviations in the density of up to 30% from the design conditions based on the standard density ρ_norm used can certainly occur. In contrast to the prior art there is provision that the pitch characteristic curve is a pitch characteristic curve Kα,ρ which is dependent on the air density, wherein the pitch characteristic curve has a local minimum α0 as a function of the air density at a standard density of a standard atmosphere. Basically there is provision that a minimum is provided in the region of a reference density ρ0 of an atmosphere at the location which applies to the location, which is intended to correspond here in this exemplary embodiment to the standard density ρ_norm of a standard atmosphere, but can generally be a reference density ρ0 as a “cold climate” air density (that is to say, for example, a reference density ρ0 where ρ>=1.3 kg/m.sup.3).

(17) FIG. 2 shows in this exemplary embodiment a pitch characteristic curve Kα,ρ which is dependent on the air density, here as part of a characteristic curve diagram Kα (P, n, ρ) which is mentioned with respect to FIG. 1 and is also explained with respect to FIG. 5. The characteristic curve diagram Kα (P, n, ρ) which specifies the blade angle as a function of output power and/or of the torque and/or of the rotor speed, serves especially for setting a blade angle α of a wind turbine 100 in FIG. 1 while predefining this air-density-dependent pitch characteristic curve Kα,ρ. In order to take into account the different air densities at a location which is characterized by a cold climate, the wind turbine 100 has implemented precisely this pitch characteristic curve Kα,ρ, dependent on the air density, in its operational control system. The pitch characteristic curve Kα,ρ is specified here by way of example as part of a characteristic curve diagram Kα (P, n, ρ) whose details are also explained in FIG. 3 and FIG. 4.

(18) The reference density ρ0 of an atmosphere at the location is preferably a standard density of a standard atmosphere or the reference density ρ0 at a location which is characterized by a cold climate can be a “cold climate” air density.

(19) Generally, the wind turbine, in particular a blade of the wind turbine, is configured for a specific air density at the location, specifically the reference density. However, a location also experiences a bandwidth of air densities around the reference density, especially owing to temperature fluctuations. The reference density can be, for example, an average air density at the location. For example, the region of the minimum at the reference density of the atmosphere at the location comprises an air density which deviates by +/−10%, in particular by +/−5% from the reference density. If the reference density at the location is the standard density of a standard atmosphere, the region of the minimum at the standard density of the standard atmosphere comprises an air density which deviates by +/−10%, in particular +/−5%, from the standard density.

(20) In the present case of exemplary embodiments, for the sake of simplicity, it is assumed that the reference density ρ0 of the density corresponds to “cold climate” conditions. That is to say at a location of the wind turbine which is characterized by a cold climate. For example, the average temperature of said location lies below 0° C. over the year, in particular below −15° C. and/or its minimum temperature over the year lies in a range below −15° C., in particular below −20° C. For example, a reference density ρ0 where ρ>=1.3 kg/m.sup.3 can occur at a location of the wind turbine which is characterized by a cold climate.

(21) The statements as explained below for a standard density ρ_norm apply generally to a reference density ρ0, wherein reference is made to the reference density ρ0 in the figures.

(22) Firstly, referring to FIG. 2, said figure shows a pitch characteristic curve Kα,ρ which is plotted at a constant power, here at the rated power, is dependent on the air density and has a progression as a function of the air density, which progression has a local minimum α0 at a reference density ρ0 (here, for example, corresponding to a standard density of a standard atmosphere). The pitch characteristic curve Kα,ρ which is dependent on the air density shows in this respect a blade angle α which is relevant during partial load operation, as a function of the air density ρ, which blade angle α is also relevant at the transition from the partial load operation into the rated load operation, and wherein α0 shows said blade angle α at a standard density ρ_norm, that is to say 1.225 kg/m.sup.3, or, for example, generally at a reference density ρ0 (e.g., a “cold climate” air density where, for example, ρ>=1.3 kg/m.sup.3) at a location of the wind turbine which is characterized by a cold climate.

(23) The local minimum of the blade angles is distinguished by the fact that the blade angle α0 at a reference density ρ0 is smallest in comparison with a first blade angle α> and also in comparison with a second blade angle α<. The blade angle α> is here that blade angle α at the rated power P_N which is provided at an air density ρ> which is slightly increased with respect to the standard atmosphere, and the second blade angle α< is that blade angle which is provided at an air density ρ< which is slightly reduced in comparison with the standard atmosphere, at a rated power P_N.

(24) The first and the second blade angles α>, α< is shown here by way of example. That is to say specifically the progression of the pitch characteristic curve Kα,ρ which is dependent on the air density ensues such that the blade angle α, at the rated power P_N on the pitch characteristic curve Kα,ρ which is dependent on the air density, rises starting from a reference density ρ0 with a decreasing and with an increasing air density ρ, that is to say the blade angle α rises from α0 to α< in the direction of relatively low densities (in comparison with the reference density ρ0) and to α> in the direction of relatively high densities (in comparison with the reference density ρ0).

(25) This is at any rate the case for relatively low and relatively high air densities ρ<, ρ>. A changed air density ρ<, ρ>, is considered here generally to be an air density in the range from 5% up to, under certain circumstances, 10% which deviates from the standard density ρ_norm (or for example generally an air density which deviates from the reference density ρ0 (e.g., where ρ=1.3 kg/m.sup.3) at a location of the wind turbine which is characterized by a cold climate).

(26) The pitch characteristic curve Kα,ρ which is dependent on the air density is shown here in a continuous line in FIG. 2, in comparison with a pitch characteristic curve Kα which is illustrated by a dashed line, according to the prior art. The progression of the customary pitch characteristic curve Kα in the region of the standard density and in the rest of the region outside the standard density is continuously monotonously falling. That is to say a blade angle α_StdT according to the prior art has the progression of a local minimum neither at a reference density ρ0 (e.g., standard density ρ_norm or “cold climate” air density such as, e.g., ρ>=1.3 kg/m.sup.3 or the like) nor in the range between ρ< and ρ> but rather is also monotonously falling toward relatively high air densities.

(27) A location of a wind turbine which is characterized by a cold climate is understood here to be a location which in the exemplary embodiment has an average temperature over the year of 0° C. and/or its minimum temperature over the year lies in a region below −20°, and with a “cold climate” air density where, for example, ρ>=1.3 kg/m.sup.3. At such a location the dependence of an air density which is customary according to the prior art as a continuous strictly monotonously falling function, with, under certain circumstances, a blade angle which is kept constant starting from the reference density, in particular standard air density, with rising densities, is disadvantageous with respect to the following problems.

(28) On the one hand, it is therefore the case that at air densities below the reference density ρ0, that is to say at an air density ρ<, the tendency of flow separations at the rotor blade rises owing to the decreasing air temperature. This this is counteracted by α0 starting with an increased blade angle for low air densities ρ<.

(29) Furthermore, it has been realized that for air densities above the reference density ρ0 with increasing air densities ρ> the loads on the rotor blade rise and in this respect there is a risk of excessive blade deflection, possibly toward a small tower clearance, owing to the increased air density. This is counteracted with a blade angle which rises starting from the reference density ρ0. That is to say according to the pitch characteristic curve Kα,ρ which is dependent on the air density, the rotor blade is set with an increasing air density, starting from the local minimum at (ρ0, α0) with a blade angle which increases at any rate up to a value ρ>. That is to say the rotor blade is moved in the direction of a feathered position owing to the initially increasing blade angle between α0 and α> starting from the local minimum.

(30) This behavior which is provided, according to the pitch characteristic curve Kα,ρ which is dependent on the air density, therefore counteracts, on the one hand, a flow separation at the rotor blade at air densities below the reference data ρ0, and on the other hand, loads are limited for air densities above the reference density ρ0 and ensure sufficient tower clearance.

(31) Furthermore, the concept herein has realized that with such a measure of an air-density-dependent pitch characteristic curve Kα,ρ with a local minimum at (ρ0, α0) there is the possibility of a gain in production of energy with blade angles α0 which are reduced in such a way, in comparison with a blade angle according to the prior art α_StdT.

(32) FIG. 2 shows a computational example at an average wind speed of 6.5 m/s. This would bring about an annual gain in production of energy of 1.67%, according to a comparison calculation, owing to this measure.

(33) The concept herein therefore offers, within the scope of a further adaptation according to demand, not only aerodynamic advantages and a reduction in load on the rotor blade, but furthermore also a gain in production of energy which is achieved in this combination by the pitch characteristic curve with is dependent on the air density, with a local minimum at (α0, ρ0) in the region of the reference density ρ0 (that is to say generally a density such as, e.g., where ρ>=1.3 kg/m.sup.3 at a location of the wind turbine which is characterized by a cold climate or as, e.g., a standard density ρ_norm).

(34) The concept herein is explained here within the scope of such a pitch characteristic curve Kα,ρ which can be complied with as a dependent characteristic curve for controlling the wind turbine according to FIG. 1. It is nevertheless possible to predefine this pitch characteristic curve as a pilot control, in order then to continue to perform open-loop and/or closed-loop control of the wind turbine on the basis of this pitch characteristic curve.

(35) FIG. 3 shows in this respect, for example for a method for adapting a blade angle, the progression of a pitch characteristic curve Kα. Here, pre-pitching with increasing power P from the partial load region occurs below P_N to the rated load region at and above P_N. In this context, the minimum blade angle αT of the partial load region is increased starting from a specific power P(αT) or P(αT′) up to a blade angle α_StdT at the rated load P_N at the reference density ρ0 which is assumed here by way of example.

(36) In the present embodiment here there is provision that, as characterized by the arrow in FIG. 3, the value of the blade angle α_StdT is decreased to a value α0 at the reference density ρ0 (that is to say generally a density such as, e.g., where ρ>=1.3 kg/m.sup.3 at a location at the wind turbine which is characterized by a cold climate or as, e.g., a standard density ρ_norm, as has been explained at the start of FIG. 2).

(37) Furthermore, the same can also be provided in a range between ρ< and ρ>, that is to say with blade angles α< and α> in a region around the reference density ρ0. The progression of the pre-pitch process can also occur starting from an αT or some other angle αT′ in such a way that either the threshold value of the power is changed to an increased threshold value P(αT′) (progression of α_1) or else the positive gradient starting from the first threshold value P(αT) is lower (progression α_2); a combination of these measures is also possible.

(38) In both cases, the progression of the pitch characteristic curve, that is to say the adaptation of the blade angle as a function of the power of the wind turbine P, ends such that at the rated load P_N the blade angle α0 which is present at the standard density lies below that of the prior art α_StdT owing to the local minimum of the air-density-dependent pitch characteristic curve.

(39) FIG. 4 shows in this respect a family of characteristic curves Kα,ρ, that is to say progressions of blade angles α as in FIG. 2, for example, also as progressions of blade angles α< and α> at the densities which deviate from the reference density ρ0 (e.g., a density such as, e.g., where ρ=1.3 kg/m.sup.3 at a location of the wind turbine which is characterized by a cold climate or as, e.g., a standard density ρ_norm), i.e., a lower density ρ< and higher density ρ>, in order to clarify prepitching from the partial load region at a power P below the rated power P_N. Therefore, the progression a provides the progression already given in FIG. 3, as a function of the power. The same also applies to the progressions α< and α> at the locations α< and α> characterized in FIG. 2.

(40) FIG. 5 shows schematically in combination a characteristic curve diagram Kα (P, n, ρ), wherein the pitch characteristic curve Kα,ρ shown in FIG. 2 is part of this characteristic curve diagram Kα (P, n, ρ). The characteristic curve Kα,ρ shown in FIG. 5 at a constant rated power P_N corresponds to the pitch characteristic curve Kα,ρ illustrated in FIG. 2.

(41) Furthermore, the characteristic curve diagram Kα (P, n, ρ) generally comprises characteristic curves Kα,ρ not only for a reference density ρ0 but also for densities ρ< and ρ>above and below that, with corresponding progressions α< and α> which are plotted by way of example in FIG. 5 for the characteristic curve diagram Kα (P, n, ρ).

(42) In all the cases between a lower and a higher density α< and ρ> which can be seen in the hatched area of the characteristic curve diagram, the rated rotational speed in this respect can generally be increased and/or at the transition from a partial load region into the rated load region, which can be beneficial for the increase in energy production at rated load or also in the region above it. Increasing the rated rotational speed in a region below the reference density ρ0 is basically advantageous.

(43) Increasing the rated rotational speed however also proves possible in a region above the reference density ρ0 by taking into account the boundary conditions relating to a tower clearance TF. Therefore, an increase in the rated rotational speed in the hatched area of the characteristic curve diagram, that is to say generally in the region of a lowered blade angle on the pitch characteristic curve which is dependent on the air density (here at any rate between ρ< and ρ> with corresponding values α< and α>) should occur, in particular, with the measures that the rated rotational speed is basically a strictly monotonously falling function in accordance with an increasing density.

(44) It is basically possible to treat such a characteristic curve diagram in a variety of ways. The operational control system of a wind turbine preferably has for this purpose a rotational speed/power operational characteristic curve (n/P operational characteristic curve), wherein in a first exemplary case the adapted operational characteristic curve is predefined as a function of the air density which is relevant for the wind turbine, by means of the air-density-dependent pitch characteristic curve Kα,ρ which is illustrated here. Then, the wind turbine is operated to generate the power which is to be output on the basis of the adapted rotational speed/power operational characteristic curve (n/P operational characteristic curve) in the operational control system. In one variant, the wind turbine can be operated to generate power on the basis of an adapted rotational speed/torque operational characteristic curve in the operational control system.

(45) In both variants a predefined, adapted current rotational speed n can be predefined in the operational control system and then set by means of an open-loop and/or closed-loop control system. In particular, the increase, illustrated here, in rotational speed is proposed in the range between ρ<, ρ> and α< and α>; at any rate for partial load operation and the transition to rated load.

(46) In a modified procedure, in a second exemplary case firstly the rotational speed of the rotor can be set or predefined taking into account a predetermined reference density ρ0, that is to say, e.g., a density such as, e.g., where ρ=1.3 kg/m.sup.3 at a location of the wind turbine which is characterized by a cold climate or as, e.g., a standard density ρ_norm of a standard atmosphere, and the adapted rotational speed can subsequently be predefined taking into account the air density which is relevant for the wind turbine, if appropriate also an air temperature. This can be done according to the aspect of an air density dependence of a pitch characteristic curve (relationship between the power and blade angle or between the torque and blade angle in one variant) Kα,ρ; that is to say for possibly dynamically changing air densities ρ< or ρ>.

(47) In both specified first and second cases, the wind turbine can therefore be set to generate power which is to be output (or torque in one variant) by predefining the adapted rotational speed n′, which can at any rate be adapted, specifically increased, from n auf n′ in the region of the minimum of the blade angle. The wind turbine can be operated with rotational speed control in the partial load region and/or rated load region.

(48) This can be done, in particular in the rated load region, under certain circumstances by additionally or alternatively setting the generator by setting an exciter current, preferably the generator rotor. This can be additionally or alternatively done by setting one or more rotor blades corresponding to the explained predefined values of a pitch characteristic curve for setting a blade angle. Additionally or alternatively it is also possible to set an azimuth angle for the nacelle of the wind turbine.

(49) With respect to FIG. 8, the open-loop and closed-loop control of a wind turbine is specifically explained further by way of example within the scope of a particularly preferred embodiment using the example of a family of air-density-dependent pitch characteristic curves Kα,β, which are referred to in FIG. 8 as K1, K2, K3 and are to be considered part of a characteristic curve diagram Kα (P, n, ρ).

(50) FIG. 6 shows a tower clearance TF, illustrated symbolically and by way of example, within the scope of a comparison. In the prior art, a tower clearance TF will normally not be constant at a value ρ> but rather decrease further and ideally only reach its minimum at an extremely high density value of ρ=1.3 kg/m.sup.3. In comparison therefore with a tower clearance for a pitch characteristic curve α_StdT according to the prior art, as illustrated in FIG. 6, a reduced tower clearance can at any rate already be set at relatively low densities; this is indicated below by means of an arrow at a density ρ<. The progression of the tower clearance TF leads to a low, preferably minimum value, with increasing density just before the reference density ρ0, in particular a standard density ρ_norm or possibly a density such as, e.g., where ρ>=1.3 kg/m.sup.3 at a location of the wind turbine which is characterized by a cold climate, here for example in a region of 5% below the reference density ρ0. In this embodiment, the progression of the tower clearance TF also leads at relatively high densities ρ> to an essentially constant low tower clearance, even at the relatively high densities according to a pitch characteristic curve. Accordingly, there is preferably also provision that the pitch characteristic curve which is dependent on the air density has such a progression that the minimum tower clearance when the blade angle is set is maintained by predefining at least the pitch characteristic curve at least for an increased air density. With the measures it is additionally or alternatively also possible to decrease a tower clearance for reduced air density.

(51) FIG. 7 shows a family of pitch characteristic curves for different powers P1, P2 and P3. FIG. 7 is intended to clarify in a schematic form, by means of a basic progression of the three pitch characteristic curves shown for different powers P1, P2 and P3, that the potential for a rated rotational speed increase and/or rated power increase is greater in the region of a reduction in the blade angle. In particular, this can be provided to an advantageously balanced extent, preferably in such a way that the rated rotational speed and/or rated power are/is increased to the same extent. This is at any rate possible for densities below the reference density ρ0. An increase in the rated rotational speed in a region below the reference density ρ0 is basically advantageous. An increase in the rated rotational speed proves, as explained above, possible in a region above the reference density ρ0 taking into account the boundary conditions relating to a tower clearance TF. Therefore, an increase in the rated rotational speed in the hatched region in FIG. 5 of the reduction in the blade angle below the upper, strictly monotonously falling pitch characteristic curve—that is to say generally in the region of a lowered blade angle on the pitch characteristic curve which is dependent on the air density can be carried out with the measure that the rated rotational speed is basically a strictly monotonously falling function in accordance with an increasing density.

(52) It becomes apparent that the minimum α_0 of the blade angle is all the more pronounced the lower the power—this increases from P3 to P1. It can be seen that the density-dependent progression of the pitch characteristic curves for an average power P2 corresponds approximately to the density-dependent progression of the pitch characteristic curve in FIG. 2.

(53) Accordingly there is also provision that the progression M of the blade angle is more pronounced around the minimum at a relatively low power P3 than the progression m of the blade angle around the minimum at a relatively high power P1. In other words, it can be stated that the progression m of the blade angle around the minimum at a relatively high power P1 (in particular the amplitude m of the reduction of the blade angle below the strictly monotonously falling function, indicated by dashed lines, at a relatively high power level P1) has a flatter form than the profile M of the blade angle around the minimum at a relatively low power level P3 (in particular the amplitude M of the reduction in the blade angle below the strictly monotonously falling function, indicated by dashed lines, at a relatively low power level P3).

(54) Additionally or alternatively there can be provision that, as can also be seen in FIG. 7, a region b of the pitch characteristic curve of a lowered blade angle between an increased air density ρ> and a reduced air density ρ< around the minimum at a relatively low power P3 is lower than a region B at a relatively high power P1. In other words, it can be stated that the density region B of the pitch characteristic curve of a decreased blade angle between an increased air density ρ> and a reduced air density ρ< around the minimum below the strictly monotonously falling function, indicated by dashed lines, extends over a larger density interval at a relatively high power P1 than the density region b of the pitch characteristic curve of a decreased blade angle between an increased air density ρ> and a reduced air density ρ< around the minimum below the strictly monotonously falling function, indicated by dashed lines, at a relatively low power P3. It is apparent that the wind turbine can be basically operated with the measures explained above with an increased rotational speed in the region of a reference density ρ0, which applies for the location, of an atmosphere at the location in the region around the minimum α0 of a decreased pitch angle of the pitch characteristic curve (that is to say in the region of a decreased blade angle between an increased air density ρ> and a reduced air density ρ< around the minimum below the strictly monotonously falling function, indicated by dashed lines, as specified by way of example by means of the values (B, m) and (b, M) explained above).

(55) The control structure in FIG. 8 shows in illustrative fashion a generator 401 and a rotor blade 403 which can be adjusted by means of a pitch drive 405. These elements are illustrated only symbolically and, for example, three rotor blades 403 can each be provided with a pitch drive 405, which pitch drive 405 are driven by the wind and as a result drive the generator 401.

(56) The generator 401 is provided here as an externally excited synchronous generator and is actuated in the structure by means of a current controller 407 which controls the exciter current I.sub.E. As a result, a power control process is performed which is indicated here only in a simplified fashion and can also be performed in a different way. It is also possible to provide other generators. The current controller 407 is also representative here of other power control devices. It receives a power value P as a predefined value, and this power value P is obtained from a rotational speed-power characteristic curve which is stored in a characteristic curve block 409. The characteristic curve block 409 outputs a power value P on the basis of the rotational speed-power characteristic curve as a function of the rotational speed n of the rotor to which the rotor blades 403 belong.

(57) The power value P is input not only into the current controller in order to control the power of the generator 401 via the current controller 407, but also the power value P is also used as an input variable for a blade angle predefining unit 411. The blade angle predefining unit 411 determines, as a function of the power P, a blade angle α which is to be set. In this context, the output power of the wind turbine, that is to say the power which is actually output by the wind turbine, is preferably used as an input variable.

(58) However, for the sake of simplicity and for the purpose of illustration, the output power can be equated here with the power P which a characteristic curve block 409 outputs. The output power is set with a high dynamic so that this simplification for the purpose of illustration is permissible and so that no oscillation problems or risks arise between the power setting, on the one hand, and the adjustment of the blade angle, on the other.

(59) The blade angle predefining unit 411 has a plurality of characteristic curve blocks, of which three characteristic curve blocks K1, K2 and K3 are shown here by way of example; that is to say a family of air-density-dependent pitch characteristic curves Kα,ρ which are referred to below as K1, K2, K3 and are to be considered part of a characteristic curve diagram Kα (P, n, ρ). Each of these characteristic curve blocks therefore has a power-dependent blade angle characteristic curve which together form a family of characteristic curves and are available for selection. It is then proposed to select one of the characteristic curve blocks and therefore one of the characteristic curves in accordance with the air density ρ. The air density ρ can be acquired for this, for example, by means of a measuring unit 413.

(60) The blade angle α can therefore be set as a function of the output power P and the air density ρ. Therefore, the output power P forms the input variable for the blade predefining unit 411, and the air density ρ is input by virtue of the fact that an adapted characteristic curve is selected as a function of the air density ρ. The blade angle α which is determined in this way is then sent to the pitch drive 405 in order to correspondingly set the respective rotor blade 403.

(61) Therefore, a solution is proposed for improving the prior art in which rotor blades are configured in such a way that at a standard air density of ρ_norm=1.225 kg/m.sup.3 and especially below it there can be a flow without separation at all operating points of the wind turbine and nevertheless loads are controlled even in the case of a raised density.

(62) It has been realized that increasingly wind turbines are planned at locations at which the air density lies, in some case significantly, below and especially also above the standard air density. Basically, an increase in an effective attitude angle of the rotor blade can result in flow separations which in turn can lead to considerable power losses. Furthermore, it has been realized that as the air density decreases and the effective attitude angle of the rotor blade increases it becomes more probable that power-reducing flow separations will occur.

(63) The family of air-density-dependent pitch characteristic curves Kα,ρ, which are referred to below as K1, K2, K3 and are to be understood as part of the characteristic curve diagram Kα (P, n, ρ), such flow separations can be counteracted or they can at any rate be reduced or even prevented. By pitching the rotor blades according to the pitch characteristic curves Kα,ρ it is possible to advantageously avoid the flow separations.

(64) It has been proposed here that the pitching of the rotor blades be adapted to the air density. Accordingly it is proposed that the blade angle which is to be set is then a function depending on the electrical output power, specifically the output power and the air density; specifically depending on the air density at a reference density ρ0, which can correspond here to the standard density ρ_norm of a standard atmosphere, but can generally be a reference density ρ0. In particular, the reference density ρ0 can be a “cold climate” air density (that is to say, for example, a reference density ρ0 where ρ>=1.3 kg/m.sup.3, with it being understood that that value ρ>=1.3 kg/m.sup.3 is basically selected by way of example and another value between a standard density ρ_norm and 1.3 kg/m.sup.3 could also be selected as a “cold climate” air density for a reference density).

(65) It is therefore proposed that only a function of the electrical output power is used as the basis for the setting of the blade angle. It is also proposed to measure, if not the air density directly, for example the air pressure and the temperature and, if appropriate, the air humidity on the wind turbine and to calculate the air density therefrom or, if appropriate, in one refinement to take into account the installation altitude of the wind turbine so that the respective blade angle can be determined using a stored function.

(66) Finally, in this way it is therefore also possible to achieve an increase in the annual energy production of a pitch-controlled, rotational-speed-variable wind turbine by means of the proposed use of pitch characteristic curves which are adapted to the air density of the location.