CA3009204C

CA3009204C - Method for operating a wind turbine

Info

Publication number: CA3009204C
Application number: CA3009204A
Authority: CA
Inventors: Soren Bilges
Original assignee: Senvion GmbH
Current assignee: Siemens Gamesa Renewable Energy Service GmbH
Priority date: 2017-06-30
Filing date: 2018-06-22
Publication date: 2020-04-28
Anticipated expiration: 2038-06-22
Also published as: CA3009204A1; DK3421784T3; ES2927015T3; EP3421784A1; EP3421784B1; DE102017006191A1

Abstract

The invention relates to a method for operating a wind turbine (1), and to a wind turbine (1) designed for performing this method and to a corresponding computer program product. In the case of the method for operating a wind turbine (1) comprising a rotor (2) with a number of rotor blades (4) that can be set with regard to the blade angle and a detection of the formation of ice on the rotor blades (4), the blade angle setting in normal operation is performed on the basis of a standard characteristic curve, in dependence on a characteristic number that can be ascertained during the operation of the wind turbine (1) (step 90), and in the case where the formation of ice is detected takes place according to the following steps (step 95): a) operating the wind turbine (1) on the basis of an initial special characteristic curve for the blade angles of a rotor blade (4) or all the rotor blades (4), in dependence on a characteristic number that can be ascertained during the operation of the wind turbine (1) (step 100); b) recording a first power curve for a prescribed time period (step 105); c) changing the special characteristic curve (step 110); d) recording a further power curve for a predetermined time period (step 110); and e) checking whether the last ascertained further power curve represents an optimum (step 125): - if so, operating the wind turbine (1) on the basis of the optimum special characteristic curve on which the last ascertained further power curve is based; - if not, iteration from step c). The wind turbine (1) according to the invention and the computer program product according to the invention are designed for carrying out the method according to the invention.

Description

METHOD FOR OPERATING A WIND TURBINE
The invention relates to a method for operating a wind turbine, and to a wind turbine designed for performing this method and to a corresponding computer program product.
Wind turbines are known from the prior art. They generally comprise a rotor, which is arranged rotatably on a nacelle, the nacelle in turn being arranged rotatably on a tower. The rotor drives a generator, possibly via a rotor shaft and a gear mechanism. A
wind-induced io rotational movement of the rotor can thus be converted into electrical energy, which can then be fed via converters and/or transformers ¨ depending on the type of construction of the generator also at least partially directly ¨ into an electric grid. The rotor comprises a number (generally three) of rotor blades, which extend substantially radially from the rotor axis and are fastened pivotably with respect to a rotor hub in order to set the angle of attack of the rotor blades.
The rotor blades of the wind turbines are often adjustable with regard to their blade angle (pitch adjustment), whereby the angle of attack of the individual rotor blades can also be changed during operation. In a partial load range between the cut-in speed, from which the rotor can begin to turn, and the rated wind speed, from which the wind turbine feeds its rated power into the electric grid, the blade angle is chosen so as on the one hand to obtain the maximum possible wind yield and on the other hand to be certain of avoiding a stall at one or more rotor blades. In the full load range, with a wind speed beyond the rated wind speed, the blade angles are set such that the rotor only rotates at the prescribed maximum speed.
In the partial load range, the blade angles are often controlled on the basis of a characteristic curve, in dependence on the dimensionless tip speed ratio A, as it is known, which is obtained from the quotient of the speed of the rotor blade tips and the wind speed. The characteristic curve is chosen such that, when it is maintained in the partial load range, the optimum power coefficient of the rotor or the rotor blades is achieved, which in turn means that there is an optimum wind yield. In the full load range, on the other hand, a blade angle deviating from this characteristic curve is set, whereby the power coefficient of the rotor is reduced.

Depending on the location where a wind turbine is installed, there is the risk of ice forming on the rotor blades, which changes the properties of the flow passing around the rotor blades. In particular, there is an increase in the risk of stalls ¨ if the control remains the same. Such a stall means that there is a sudden drop in the energy generation and, as a consequence, a considerable mechanical loading for the wind turbine. Also, such a sudden drop in the energy that is fed in also has an adverse effect on the stability of the electric grid. A stall must therefore be avoided as far as possible.
US Patent US 8,096,761 B1 has discovered that, in cases of poorer aerodynamics of the rotor blades ¨ for example due to the formation of ice ¨ a characteristic curve used for io controlling the blade angles in the partial load range can be used to ascertain a comparable (if not identical) curve that indicates for each of the different operating states a minimum blade angle below which there is an increased risk of a stall occurring. As a consequence, the wind turbine control is designed such that, when there are poorer aerodynamics of the rotor blades, the angle of the blades does not go below the minimum blade angle for the respective operating state.
A disadvantage of this prior art is that, although a stall at the rotor blades when there are poorer aerodynamics can be avoided, it may be that the wind turbine cannot deliver the maximum possible yield for the turbine because of the minimum blade angle fixed as a lower limit. In particular in the case of turbines installed at locations where the formation of ice on zo the rotor blades can be expected over a longer period of time, the loss of yield due to maintaining a minimum blade angle, which is only concerned with the certain avoidance of stalls, may be considerable.
The object of the present invention is to provide a method for operating a wind turbine, and also a wind turbine and a computer program product, with which the disadvantages of the prior art no longer occur, or at least only to a reduced extent.
This object is achieved by a method according to the main claim, and also a wind turbine and a computer program product according to the alternative independent claims.
Advantageous developments are the subject of the dependent claims.
Accordingly, the invention relates to a method for operating a wind turbine comprising a rotor with a number of rotor blades that can be set with regard to the blade angle and a detection of the formation of ice on the rotor blades, the blade angle setting in normal operation being

2 performed on the basis of a standard characteristic curve, in dependence on a characteristic number that can be ascertained during the operation of the wind turbine, and in the case where the formation of ice is detected taking place according to the following steps:
f) operating the wind turbine on the basis of an initial special characteristic curve for the blade angles of a rotor blade or all the rotor blades, in dependence on a characteristic number that can be ascertained during the operation of the wind turbine;
g) recording a first power curve for a prescribed time period;
h) changing the special characteristic curve;
i) recording a further power curve for a prescribed time period; and j) checking whether the last ascertained further power curve represents an optimum:
- if so, operating the wind turbine on the basis of the optimum special characteristic curve (that is to say, depending on the definition of the special characteristic curve in step (a), either for the one rotor blade or all the rotor blades) on which the last ascertained further power curve is based;
- if not, iteration from step c).
The invention also relates to a wind turbine comprising a rotor with a number of rotor blades zo .. that can be set with regard to the blade angle, which is arranged rotatably on a nacelle arranged rotatably on a tower, and with a generator arranged in the nacelle for the conversion of wind energy acting on the rotor into electrical energy, and a control device for controlling the wind turbine and its components, the control device being designed for carrying out the method according to the invention.

3 The invention also relates to a computer program product comprising program parts which, when loaded in a computer, are designed for carrying out the method according to the invention.
First of all, some of the terms used in connection with the invention are explained.
The term "power curve" refers to the quantitative relationship between the wind speed and the electrical power generated.
For "recording a power curve", values registered for wind speed and generated electrical power are brought together in pairs to form data points. If the recording is performed over a sufficiently long time period with changing wind speeds, a power curve is obtained from the io individual data points. However, it is also possible that only a small number of data points are registered, from which a power curve can then be extrapolated if need be. In an extreme case ¨ in particular when the wind stays the same over a longer time period ¨
it is also possible that for the recording of a power curve only a single data point is registered, from which ¨ if required ¨ a theoretical power curve can be approximated.
The invention has discovered that, in the case of the formation of ice on the rotor blades of a wind turbine, and the deterioration in the aerodynamics of the rotor blades that often accompanies it, even though the control of the blade angles has to be changed to avoid stalls, the changing of the blade angle control can be optimized iteratively in order to minimize as far as possible the loss of yield resulting from the formation of ice. This applies in particular because it has been found that, with the formation of ice on the rotor blades, completely different ice formations occur in each case, correspondingly changing the aerodynamics of the rotor blades individually. A basic static changing of the blade angle control, which ultimately is also what happens when a minimum blade angle is prescribed, is therefore disadvantageous with regard to the yield of a wind turbine. The method according .. to the invention is relevant in particular for those wind turbines on which the permanent formation of ice over a relatively long time period can be expected, or turbines in areas with low or negative temperatures over several months.
According to the invention, it is provided that, when the formation of ice is found, a wind turbine for which the blade angle setting is performed during normal operation on the basis of a standard characteristic curve, in dependence on a characteristic number that can be ascertained during the operation of the wind turbine, is operated at first on the basis of a

4 prescribed initial special characteristic curve, but in the further course of the method this special characteristic curve is iteratively adapted in order in spite of the formation of ice to nevertheless achieve the maximum possible yield. The initial special characteristic curve generally deviates from the standard characteristic curve.
For detecting the formation of ice on the rotor blades, various methods are known in the prior art. For example, the detection of the formation of ice may take place by finding there is a power deficit of the wind turbine with respect to a reference power curve, in dependence on the wind speed found. The wind speed may in this case be found by means of an anemometer. Preferably, the outside temperature is also taken into consideration, such that a formation of ice is only detected if the temperature is below 3 C. If the power fed in by the wind turbine drops in comparison with the reference power expected in view of the wind conditions, formation of ice on the rotor blades can be presumed, in particular at temperatures below 3 C. Alternatively or in addition, the formation of ice on one of the rotor blades can be presumed if it is found that an imbalance is occurring at the temperatures mentioned.
The special characteristic curve initially used for operating the wind turbine in the case of a detected formation of ice may be a fixed prescribed special characteristic curve. However, it is also possible that the initial special characteristic curve is a special characteristic curve that was optimized in the case of a previous formation of ice on the wind turbines by the method according to the invention explained still further below. If it can be assumed that the formation of ice on a particular wind turbine always takes place in a similar way, the optimization described below can possibly be carried out more quickly, i.e.
with fewer iterations, by referring back to a special characteristic curve that was optimized in a previous case of the formation of ice.
Both the standard characteristic curve and the special characteristic curve preferably indicates the blade angle, in dependence on the tip speed ratio A of the wind turbine. The dimensionless tip speed ratio A is the ratio of the circumferential speed of the rotor blades to the wind speed and ¨ if not already calculated for other reasons ¨ can be readily calculated from the measured values with regard to the wind and the status of the wind turbine that are registered by the control of the wind turbines.

5 After the changeover of the operation of the wind turbine on the basis of an initial special characteristic curve for the blade angle, first a first power curve is recorded for a prescribed time period. The power curve in this case reflects the ratio of power fed in to wind speed.
Given a sufficiently long time period in which different wind speeds can often be expected, sufficient data points can be obtained to ascertain the power curve sufficiently accurately.
The length of the time period for recording the first power curve, but also the power curves subsequently to be recorded, may in principle be chosen as desired, and may even be several days, for example 3 days. It is preferred, however, if the length of the time period in question is preferably less than 24 hours, preferably less than 12 hours, more preferably less io .. than 6 hours. It is also possible that the recording of the first power curve is only performed for a time period of about 30 minutes, preferably of about 10 minutes.
Although the number of ascertainable data points in a correspondingly short time period may not be sufficient for the complete determination of a power curve, it can generally be extrapolated or approximated by means of theoretical models if a complete power curve is indeed required.
Subsequently, the special characteristic curve is changed. For this, it is particularly preferred if the special characteristic curve is parameterized by a parameter. A
changing of the special characteristic curve can then be achieved simply by changing the parameter.
The parameter may in this case be changed for example by a fixed prescribed increment. It is however also possible that the increment is changed in dependence on the characteristic number that can zo be ascertained during the operation of the wind turbine, in particular therefore for example the tip speed ratio A. Thus, the increment may for example be formed from the characteristic number that can be ascertained during the operation of the wind turbine multiplied by a constant factor.
After appropriate changing of the special characteristic curve, a further power curve is recorded for a prescribed time period. The recording is in this case performed analogously to the recording of the first power curve, for which reason reference is made to the statements given above.
It is subsequently checked whether the last ascertained further power curve represents an optimum. This check may be performed by using comparisons with the previously ascertained power curves on the basis of the initial special characteristic curve and/or changed special characteristic curves.

6 In order that the comparison of the power curves is sufficiently meaningful, the check as to whether the last ascertained further power curve represents an optimum is preferably only carried out if the turbulences in the time period of the recording of this power curve are comparable to the turbulence in the time period of the recording of the previous power curve.
If not, the recording of the further power curve is repeated. The turbulence can in this case be depicted for example by way of the turbulence intensity, that is to say the ratio of the standard deviation of the wind speed to the mean value of the wind speed.
Alternatively, it is possible to convert the ascertained power curves to reference conditions and perform the check as to whether the last ascertained further power curve represents an io optimum on the basis of these power curves converted for reference conditions. The conversion of the power curves to reference conditions provides a direct comparability.
The check as to whether the last ascertained further power curve represents an optimum may be performed for example on the basis of yields that can be ascertained from the power curves and a prescribed wind distribution. For this purpose, the yield to be expected during operation with the corresponding power curve is calculated for a prescribed wind distribution and used as a criterion for comparison. If the power curves only comprise a small number of data points, or even only one data point each, the check may even be confined to a direct comparison of the electrical power generated ¨ in particular if the wind speeds of the individual data points of the individual power curves are comparable. In this case, it is not zo necessary to extrapolate or approximate a power curve for the data points registered.
If it is found that the last ascertained further power curve is an optimum, the wind turbine is operated on the basis of the optimum special characteristic curve, which is based on the last ascertained further power curve. If an optimum has still not been reached yet, the special characteristic curve is changed once again and the method subsequently repeated iteratively.
In the case of the method described, the optimization of the special characteristic curve may be carried out for each individual blade one after the other, after ascertaining the optimum special characteristic curve for one rotor blade, the aforementioned steps a) to e) being repeated one after the other for the further rotor blades. If, for example, it is presumed because of an imbalance that ice has formed on a rotor blade and this rotor blade can be

7 identified on the basis of the imbalance, it is preferred to carry out the method for this rotor blade first.
Alternatively, it is possible to set the blade angle of all the rotor blades uniformly by means of the initial and subsequently optimized special characteristic curve. In particular in this case, it is however preferred that, after an optimization of the common special characteristic curve, a blade angle correction is carried out for each rotor blade individually one after the other.
For this purpose, first the special characteristic curve is changed for an individual rotor blade.
The blade angle of this one rotor blade is therefore controlled differently from the special characteristic curve ascertained previously as common to all the rotor blades.
The changing io of the special characteristic curve for an individual rotor blade may in this case be performed analogously to the previously described changing of the special characteristic curve for all the rotor blades, that is to say in particular by changing the parameter of a parameterized special characteristic curve. Alternatively, it is also possible that the changing of the special characteristic curve for an individual rotor blade is achieved by prescribing a blade angle deviation. The blade angle of the rotor blade is in this case set by a constant Delta in comparison with the special characteristic curve for all the rotor blades.
Subsequently, a power curve is again ascertained for a prescribed time period and the special characteristic curve for the individual rotor blade is changed iteratively as long as it takes until an optimum is achieved. The preconditions and/or possibilities for checking for the presence of an optimum for the special characteristic curve of an individual rotor blade, what was said with regard to the optimization of the special characteristic curve for all the rotor blades applies analogously.
If an optimum for the special characteristic curve of an individual rotor blade is found, the blade-individual optimization is carried out for the other rotor blades of the wind turbine one after the other.
After completion of the method described, a repetition of the method may be envisaged. The repetition may be initiated by ambient conditions that have perhaps changed decisively (for example temperature or atmospheric humidity) or for example take place on the basis of a prescribed time interval.

8 For an explanation of the wind turbine according to the invention and the computer program product according to the invention, reference is made to the statements given above.
The invention is now described by way of example on the basis of an advantageous embodiment with reference to the accompanying drawings, in which Figure 1 shows a wind turbine designed for carrying out the method according to the invention; and Figure 2 shows a flow diagram of a first embodiment of the method according to the invention.
In Figure 1, a wind turbine 1, which is designed for carrying out the method according to the invention, is outlined.
The wind turbine 1 comprises a rotor 2 with a number of rotor blades 4 that are adjustable in blade angle by means of blade adjusting drives 3, which is arranged rotatably on a nacelle 5.
The nacelle 5 is in turn arranged rotatably on a tower 6.
The rotor 2 drives via the rotor shaft a gear mechanism 7, which is connected on its output side to a generator 8. A wind-induced rotational movement of the rotor 2 can thus be converted into electrical energy, which can then be fed, possibly via converters (not shown) and/or transformers 9, into an electric grid 10.
The wind turbine 1 also comprises a control device 11, which is connected by means of control lines (not shown) to the various components of the wind turbine 1 in order to control them. As known from the prior art, the control device 11 also registers the measured values not only from sensors (not shown) of the wind turbine 1, which register operational characteristic numbers such as the rotor speed, but also from sensors 12 for registering the wind speed and temperature in the region of the nacelle. The control device 11 is also designed to derive further characteristic numbers, such as the tip speed ratio A, from the data registered.

9 The control device 10 is designed inter alia to set the blade angle of the rotor blades 4. In this respect, the control device 11 is designed for running a computer program product with which the method explained below is carried out.
In normal operation, the setting of the blade angles of all the rotor blades 4 is performed on the basis of a standard characteristic curve, by means of which the optimum blade angle for the operation of the wind turbine is ascertained on the basis of the tip speed ratio A (step 90).
It is regularly checked on the basis of the tip speed ratio A, the measured wind speed in the region of the nacelle and the power actually fed in whether the momentary yield deviates from the theoretical yield of the wind turbine that can be ascertained by means of a reference io power curve (step 95). If this is the case and the temperature in the region of the nacelle is still below 3 C, the otherwise imminent further operation of the wind turbine is interrupted on the basis of the standard characteristic curve and a changeover is made to step 100 of the method. Yet other methods suitable for detecting a formation of ice on wind turbines that can be used equally well at this point are possibly known in the prior art.
However, the chosen method has the advantage that the relies exclusively on measured variables already registered for other reasons concerning the control of a wind turbine, and to this extent no additional components are required.
In step 100, the operation of the wind turbine 1 is carried out of a special characteristic curve for the blade angles of the rotor blades 4 that is stored in a fixed state in the control device 11. The special characteristic curve deviates from the standard characteristic curve, but also reproduces a relationship between the blade angle and the tip speed ratio A.
The special characteristic curve is in this case parameterized, i.e. the profile of the special characteristic curve can be adapted by a parameter. For the initial special characteristic curve, an initial value for this parameter is established.
Subsequently, a first power curve of the wind turbine 1 is recorded in a known way for a prescribed time period of 12 hours (step 105). The recording of a corresponding power curve is known from the prior art.
Subsequently, the special characteristic curve is changed by changing its parameter (step 110). The parameter is changed by an increment that is obtained from a product of the tip speed ratio A and a constant factor, the algebraic sign of the change being obtained from rules that are known for iterative methods.

Subsequently, a further power curve of the wind turbine 1 is once again recorded in a known way for a time period of 12 hours (step 115).
In the subsequent step 120, it is checked whether the turbulence intensity during the recording of the further power curve in step 115 is comparable to the turbulence intensity during a corresponding previous recording, for example the recording of the first power curve in step 105. This can be ascertained on the basis of wind data recorded by the control unit 11. If the turbulence intensity is not comparable, once again a power curve of the wind turbine 1 is recorded on the basis of the changed special characteristic curve (step 115). It goes without saying that a completely new 12-hour time period does not have to be begun io for this. Rather, it is possible to continue the already previously begun recording of the power curve, and to limit the results in each case to the last 12 hours of the recording, until the turbulence intensity in these 12 hours specifically is comparable to the turbulence intensity during a corresponding previous recording.
If the turbulence intensity for the last recorded power curve corresponds to the prescribed values, it is checked whether the last recorded power curve represents an optimum. For this purpose, a theoretical yield is ascertained with the aid of a prescribed wind distribution on the basis of the last recorded power curve and is compared with the corresponding yields of the previously recorded power curves, including the first power curve. If the yield of the last recorded power curve is not optimum, a jump is made back to step 110, where the special zo characteristic curve is changed once again, whereupon the subsequent steps 115 to 125 are performed as described.
If the check in step 125 finds that the last ascertained power curve represents an optimum, the operation of the wind turbine is continued on the basis of the special characteristic curve taken as a basis for this power curve (step 130).
The optimized special characteristic curve thus ascertained is first used for setting the blade angles of all the rotor blades 4 of the wind turbine 1, whereby a yield that is often higher in comparison with the initial special characteristic curve can already be achieved by the wind turbine 1. In order to optimize this yield still further, an individual blade angle correction is also carried out individually for each rotor blade 4 on the basis of the optimized special characteristic curve found.

For this purpose, in step 135, the special characteristic curve for an individual rotor blade is changed by prescribing a constant blade angle deviation, which is applied to the blade angles for this one rotor blade actually ascertained by means of the special characteristic curve. It is however also possible to change the special characteristic curve by means of the adaptation of the respective parameters of the parameterizable special characteristic curve for this particular blade, in order in this way to optimize the energy yield.
Subsequently, a further power curve is recorded for a time period of 12 hours (step 140). If the turbulence intensity in this time period does not coincide with the turbulence intensity of the previously recorded power curve, the recording of the power curve is repeated or io continued until a power curve based on a time period of 12 hours with a comparable turbulence intensity could be found (step 145).
If there is a suitable power curve, it can be checked in step 150 in a way comparable to step 125 whether this power curve represents an optimum. If this is not the case, the special characteristic curve for the rotor blade 4 in question is changed once again according to step 135 and the subsequent steps are run through as described. If the power curve ascertained is the optimum, the control of the blade angle of the rotor blade in question is carried out on the basis of the ascertained individual special characteristic curve.
Subsequently, the method is repeated as from step 135 for each rotor blade 4 of the wind turbine 1, until there is an individual special characteristic curve for each rotor blade 4.
The operation of the wind turbine 1 then takes place on the basis of the special characteristic curves optimized individually for each rotor blade 4.
In the case of the exemplary embodiment described above, the power curves are recorded in each case over a time period of 12 hours. It is however also possible to limit the recording to or 10 minutes in each case. Even if a complete power curve generally cannot be 25 ascertained in such a time period, the data point or points thereby obtained may be sufficient to carry out the optimization of the special characteristic curve.

Claims

1. Method for operating a wind turbine comprising a rotor with a number of rotor blades settable with regard to the blade angle and a detection of the formation of ice on the rotor blades, the blade angle setting in normal operation being performed on the basis of a standard characteristic curve, in dependence on a characteristic number that is ascertainable during the operation of the wind turbine (step 90), and whenformation of ice is detected taking place according to the following steps (step 95):
a) operating the wind turbine on the basis of an initial special characteristic curve for the blade angles of a rotor blade or all the rotor blades, in dependence on a characteristic number that is ascertainable during the operation of the wind turbine (step 100);
b) recording a first power curve for a prescribed time period (step 105);
c) changing the special characteristic curve (step 110);
d) recording a further power curve for a prescribed time period (step 115);
e) checking whether a last ascertained further power curve represents an optimum (step 125):
- if so, operating the wind turbine on the basis of the optimum special characteristic curve on which the last ascertained further power curve is based;
- if not, iteration from step c); and f) after ascertaining the optimum special characteristic curve (step 130) for one rotor blade, steps a) to e) are repeated one after the other for the further rotor blades.

2. Method according to claim 1, wherein after ascertaining the optimum special characteristic curve (step 130) for all the rotor blades, the blade angle correction is subsequently carried out for each rotor blade individually one after the other, with the steps of:
g) changing the special characteristic curve for an individual rotor blade (step 135);
h) recording a further power curve for a prescribed time period (step 140);
i) checking whether the last ascertained further power curve represents an optimum (step 150):
- if so, operating the wind turbine on the basis of the optimum special characteristic curve for the rotor blade on which the last ascertained further power curve is based;
- if not, iteration from step f).
j) repetition from step f), until an optimum special characteristic curve is ascertained for each rotor blade (step 155).

3. Method according to claim 2, wherein the changing of the special characteristic curve for an individual rotor blade is achieved by prescribing a blade angle deviation.

4. Method according to any one of claims 1 to 3, wherein the special characteristic curve is parameterized by a parameter and a changing of the special characteristic curve can be achieved by changing the parameter.

5. Method according to claim 3 or 4, wherein the parameter and/or the blade angle deviation is/are changed by a fixed prescribed increment or in dependence on the characteristic number that is ascertained during the operation of the wind turbine.

6. Method according to any one of claims 1 to 5, wherein the checking as to whether the last ascertained further power curve represents an optimum is only carried out if the turbulence in the time period of the recording of this power curve are comparable to the turbulence in the time period of the recording of the previous power curve and if not the further power curve is recorded once again according to step d) for a prescribed time period (steps 120 and 145).

7. Method according to any one of claims 1 to 6, wherein the checking as to whether the last ascertained further power curve represents an optimum is performed on the basis of power curves converted for reference conditions or on the basis of yields that is ascertained from the power curves and a prescribed wind distribution.

8. Method according to any one of claims 1 to 7, wherein the initial special characteristic curve is a prescribed special characteristic curve or the special characteristic curve that was ascertained as the optimum characteristic curve in a previous case of the formation of ice.

9. Method according to any one of claims 1 to 8, wherein the characteristic number that is ascertained during the operation of the wind turbine is the tip speed ratio .lambda..

10. Method according to any one of claims 1 to 9, wherein the prescribed time period for the recording of power curves is 24 hours or less.

11. Method according to any one of claims 1 to 9, wherein the prescribed time period for the recording of power curves is 12 hours or less.

12. Method according to any one of claims 1 to 9, wherein the prescribed time period for the recording of power curves is 6 hours or less.

13. Method according to any one of claims 1 to 9, wherein the prescribed time period for the recording of power curves is 30 minutes or less.

14. Method according to any one of claims 1 to 9, wherein the prescribed time period for the recording of power curves is 10 minutes.

15. Method according to any one of claims 1 to 14, wherein the detection of the formation of ice takes place by finding there is a power deficit of the wind turbine with respect to a reference power curve, in dependence on the wind speed found, and the outside temperature being taken into consideration, such that a formation of ice is only detected if the temperature is below 3°C.

16. Computer program product comprising program parts which, when loaded in a computer, are configured for carrying out a method according to any one of claims 1 to 15.

17. Wind turbine comprising a rotor with a number of rotor blades that are set with regard to the blade angle, which is arranged rotatably on a nacelle arranged on a tower, and with a generator arranged in the nacelle for the conversion of wind energy acting on the rotor into electrical energy, and a control device for controlling the wind turbine , wherein the control device is configured for running the computer program product as defined in claim 16 for carrying out the method according to any one of claims 1 to 15.

18. Wind turbine according to claim 17, wherein the wind turbine has an anemometer and/or a temperature sensor for measuring the wind speed and/or the temperature in the region of the nacelle.