METHOD FOR CONTROLLING A WIND TURBINE

Description

[0001] The present invention relates to a method for controlling a wind turbine. The present invention also relates to a wind turbine.

[0002] To operate a wind turbine, it is typically operated at partial load, i.e., when the wind speed is below the rated wind speed, at the most optimal operating point possible. For a variable-speed wind turbine with a rotor with adjustable blade pitch, such an operating point is characterized by the speed, the blade pitch, and the resulting power output. Once the optimal operating point is set, the power output for the prevailing wind speed is the maximum power the wind turbine can continuously generate at that wind speed.

[0003] To represent this and other operating points, a tip speed ratio-blade angle diagram is often used, referred to here as a λθ diagram. In this λθ diagram, the tip speed ratio λ of the respective operating point is plotted at the corresponding value of the blade angle θ of the same operating point.

[0004] The tip speed ratio λ is generally defined as the ratio of the rotor blade's tip speed to the wind speed. The resulting tip speed ratio λ is therefore unitless. By considering the relationship between rotational speed and wind speed, the λθ diagram can be used for different wind speeds. Thus, operating points with different wind speeds can be plotted in the same diagram. Ideally, operating points with different wind speeds can even be identical in this diagram.

[0005] Such identical operating points can occur in partial load operation if the blade angle remains constant, i.e., unchanged, at varying wind speeds, and the rotational speed is proportional to the wind speed. This is precisely the desired condition in the ideal design of a wind turbine. If the rotational speed and wind speed change proportionally to each other, the angle of attack at which the air actually flows against the rotor blade remains the same. It therefore makes sense to also keep the blade angle unchanged.

[0006] Of course, the output power does not remain the same, because with increasing wind speed and increasing rotational speed, more power can be generated.

[0007] In addition, it is common and useful to plot power coefficients, also known as Cp values, in the λθ diagram. Such a power coefficient indicates the efficiency with which the corresponding operating point can convert wind power into rotational power. For clarity, operating points with the same Cp value are plotted on a common, closed line. For example, such Cp curves can be plotted in increments of 0.05. These curves are also referred to here as iso-curves. With an optimal design of the wind turbine, its operating point in partial load operation is at the highest possible Cp value, which is slightly greater than 0.45.

[0008] The definition of an operating point is therefore relatively clear, namely such that the power output is as high as possible, which often leads to an operating point that lies within the iso-characteristic curve that has the highest Cp value. Depending on the boundary conditions, however, operating points may not be able to achieve this ideal value. Particularly at very low wind speeds, friction conditions can be so dominant that an operating point with a comparatively high speed, i.e. a higher tip speed ratio, must be selected, which can then also have an increased blade angle. The maximum speed of a wind turbine is also limited. For example, above a certain wind speed, the speed of the turbine must not increase any further, otherwise operating points outside the optimum will arise. However, even such operating conditions can be determined through simulations or, if necessary, on-site adjustments.

[0009] It may now be necessary to meet additional requirements which stipulate a reduced output power. For example, a reduced output power may be required for a period of time, which may be 15 minutes but may also last several hours, in order to cope with a situation in the electrical supply grid. Such a situation may arise when power demand is correspondingly low, but it may also be required to provide standby power. When standby power is provided, the wind turbine is operated with a lower output power than is possible due to the prevailing wind, so that if there is a sudden increase in power demand, it can then increase its power production, namely it can permanently increase it to the maximum power that can be generated from the prevailing wind.However, other requirements may also be considered, such as the requirement that the wind turbine be operated at reduced speed to minimize noise. The reduction can affect both the output power and the speed.

[0010] If a power reduction is required, there are many options for selecting a corresponding operating point. In any case, the optimal operating point, i.e., the operating point at the maximum Cp value, is no longer selected, as this would result in maximum power, which is not desired. The operating point with reduced power can, for example, be selected so that the reduced output power is generated with the least possible wear. The operating point can also be selected so that it is particularly stable, so that small changes in wind speed do not lead to a stall effect, i.e., aerodynamic collapse.

[0011] However, the desire to select an operating point with the lowest possible wear can also lead to various variants. For example, low wear may occur when the wind turbine's speed is as low as possible. However, this can lead to the rotor blades having to be frequently adjusted in their pitch angle when wind speed changes, meaning that this operating point is not low-wear with regard to the rotor blade adjustment drives. It may be possible to define a quality criterion that takes these diverse requirements into account.

[0012] Further situations may arise in which the wind turbine must move from a reduced operating point to a less reduced operating point as quickly as possible. Such a requirement can also arise from the conditions of the electrical supply grid; in particular, a rapid change in the grid frequency can trigger such a need. Especially if the grid frequency of the electrical supply grid drops below a limit, which could be, for example, 0.3% below the nominal grid frequency, it may be necessary for the reduced-speed wind turbine to quickly increase its output power, especially as the grid frequency continues to drop.

[0013] This then presents the problem of finding an operating point for this new situation. It may also be important to find the transition from the reduced operating point to the less—or possibly even no—reduced operating point.

[0014] The technical paper entitled "Comparison of Down-Regulation Strategies for Wind Farm Control and their Effects on Fatigue Loads" (2018-06-27) by Daan van der Hoek, Stoyan Kanev and Wouter Engels is considered to be relevant state of the art.

[0015] The present invention is therefore based on the object of addressing at least one of the aforementioned problems. In particular, a solution is to be found that specifies a reduced operating point. In particular, this reduced operating point is to be selected as advantageously as possible with regard to a further change to a less reduced operating point. In particular, a proposal is also to be made for selecting such a less reduced operating point. At the very least, an alternative to previously known solutions is to be proposed.

[0016] According to the invention, a method for controlling a wind turbine according to claim 1 is proposed. Thus, the method assumes a wind turbine having a rotor with adjustable blade angles, and wherein the rotor can be operated at a variable speed. In this respect, a conventional wind turbine can be assumed.

[0017] The procedure applies to the case where the wind turbine is operated in a partial load range. This partial load range is characterized by insufficient wind to operate the wind turbine at its rated power. Therefore, the wind speed is also below the rated wind speed. The rotational speed of the wind turbine is often also below the rated speed.

[0018] The wind turbine can be operated at a variably specified operating point, and this operating point is characterized by the blade angle, i.e., the value of the blade angle, and a tip speed ratio. This operating point can be represented in a λθ diagram. Such a λθ diagram, which can also be synonymously referred to as a tip speed ratio-blade angle diagram, has already been described above in the introduction. In such a λθ diagram, the tip speed ratio is plotted against the blade angle, and the operating point can then be represented in the λθ diagram as a pair of values ​​representing its blade angle and tip speed ratio.

[0019] The operating point can therefore be represented by a pair of values ​​(θ, λ), where θ denotes the blade angle or the value of the blade angle and λ the tip speed ratio. Furthermore, as already described in the introduction, each operating point is assigned a power coefficient, which can also be referred to as Cp value. Several operating points with the same Cp value can be represented as an iso-characteristic curve in the λθ diagram. Such an iso-characteristic curve is thus composed of many operating points with the same Cp value, whereby these can be fictitious operating points and do not have to be operating points that are also provided for in the actual control system of the wind turbine in question. These iso-characteristic curves serve to characterize the aerodynamics of the wind turbine. The iso-characteristic curves can also be referred to synonymously as characteristic curves with the same Cp value or as characteristic curves with the same power coefficient.

[0020] According to one step, the wind turbine is operated at a normal operating point when there is no demand for throttling. Such a normal operating point is characterized by the fact that the wind turbine is not operated at a throttled rate. The normal case is therefore that the wind turbine is operated at a normal operating point. However, depending on the wind turbine, it can of course also be the case that a constant demand for throttling is present. But even in this case, a normal operating point is generally provided at which the wind turbine would operate even if there is no demand for throttling.

[0021] In addition, the wind turbine is operated at a throttle operating point, in which the wind turbine, in response to a throttle request, operates at a reduced output compared to the normal operating point. At the throttle operating point, the wind turbine is throttled compared to the normal operating point, namely in its output power. The turbine therefore delivers less power or generates less power from the wind than at the normal operating point. Nevertheless, the wind turbine must still be operated; therefore, when it is stopped, it is also throttled to a certain extent, but the wind turbine is no longer operating in this sense.

[0022] Furthermore, the operation of the wind turbine is intended to switch from the throttle operating point to a reserve operating point. In the reserve operating point, the wind turbine is operated with increased output power compared to the throttle operating point. The switch from the throttle operating point to the reserve operating point occurs in response to a power increase request. The wind turbine therefore delivers more power in the reserve operating point, i.e., is operated with a higher output power, than in the throttle operating point. The reserve operating point is thus illustratively located between the throttle operating point and the normal operating point. However, it is also fundamentally conceivable that the same amount of power is delivered in the reserve operating point as in the normal operating point. In this case, the normal operating point and the reserve operating point can still be different, for example, have a different tip speed ratio.Preferably, however, less power is delivered at the reserve operating point than at the normal operating point.

[0023] It is now proposed that the throttle operating point be selected such that it has a higher tip speed ratio than the reserve operating point. It is generally assumed that the operating points being compared—i.e., the normal operating point, the throttle operating point, and the reserve operating point—are operated at the same wind speed. Of course, the wind speed can change, and especially in a sustained throttle situation, where the wind turbine is operated at the throttle operating point for an extended period, it can be assumed that the wind speed will change at least slightly.For the comparative description, it must nevertheless be assumed that the underlying wind speed is the same in each case. If the wind speed increases and the throttle operating point changes as a result, this increased wind speed also corresponds to a changed normal operating point and a changed reserve operating point, whether they are activated at that moment or not. Thus, the comparisons always refer to a situation in which the three operating points mentioned are based on the same wind speed.

[0024] Apart from that, a change, especially from the throttle operating point to the reserve operating point, will be carried out so quickly that it can actually be assumed that the wind speed remains approximately unchanged during this changeover process.

[0025] When selecting the throttle operating point, it is therefore suggested that it has a higher tip speed ratio than the reserve operating point. In addition, when selecting the throttle operating point, it is suggested that it lies on an associated iso-characteristic curve in the λθ diagram, and that the associated iso-characteristic curve at the throttle operating point has a negative characteristic gradient, with the tip speed ratio decreasing with increasing blade angle. The associated iso-characteristic curve is therefore the one that also includes the throttle operating point. As already explained, such iso-characteristic curves are closed characteristic curves, which is in the nature of things, as long as they do not leave the display area of ​​the diagram. These characteristic curves, or the iso-characteristic curve in question, therefore have sections with a negative gradient and others with a positive gradient. The gradient is also zero at two points.In particular, every iso-characteristic curve generally has two regions with a negative slope. One of these regions is likely to have a lower tip speed ratio than the normal operating point, and this region is not suitable for the proposed throttle operating point. The other region, which is proposed here, is essentially located "top right" in the λθ diagram with respect to the optimal operating point, which corresponds to the normal operating point. Therefore, in the proposed region where the throttle operating point is to be selected, the tip speed ratio and the blade angle are predominantly higher than the tip speed ratio and the blade angle of the normal operating point.

[0026] However, the proposed range refers to the reserve operating point and compared to this, the tip speed ratio must be selected to be larger and the iso characteristic curve must have a negative characteristic curve slope.

[0027] Here, it was particularly recognized that the proposed selection of the throttle operating point results in a higher speed compared to the reserve operating point. If the output power is to be increased in response to the power increase request, particularly if it is to be increased quickly, the proposed selection of the throttle operating point is advantageous. To change, the speed of the throttle operating point must be reduced in order to reach the reserve operating point. This requires more power to be delivered, and this power is therefore immediately available as soon as this speed reduction is implemented. In particular, the speed reduction can even be triggered by increasing the output power.

[0028] It was also recognized that when operating a wind turbine in the partial load range there is often still potential to increase the speed without reaching technical limits. In particular, it was recognized that the nominal speed has not yet been reached and that an increase in speed is therefore possible without endangering the wind turbine. However, it was also recognized that, depending on the prevailing wind speed, the speed may be only slightly below the nominal speed, but that there is still a small potential to increase the speed. By changing the operating point in the range in which the iso-characteristic curves have a negative gradient, especially if this gradient is at least 1 / 1°, a significant reduction in the power coefficient can be achieved.This allows for reaching an operating point with less power output while increasing the speed and absorbing power. However, the resulting increase in speed can be comparatively small, while at the same time, the change in blade angle is also comparatively small. The throttle operating point can thus be easily reached, and in particular, a quick transition back to the reserve operating point is possible.

[0029] At the same time, however, it was recognized that simply selecting a higher speed does not necessarily lead to a beneficial change, or at least not to an optimal change. In particular, it was recognized that the throttle operating point must also have a worse power coefficient, i.e. a lower Cp value, than the reserve operating point, because this is a prerequisite for less power output. This can be achieved by adjusting the rotor blades. In order to return from the throttle operating point to the reserve operating point during the aforementioned change, the rotor blades must also be rotated back again. It was recognized that adjusting the rotor blades requires a certain minimum time, due to the dynamics of the adjustment drives. It was also recognized that in the areas where the iso-characteristic curves have a negative gradient, the iso-characteristic curves are comparatively close to one another.This means that a comparatively strong change in the Cp value can be achieved with a comparatively small adjustment of the rotor blades.

[0030] Thus, it is proposed that the throttle operating point be selected relative to the reserve operating point so that it can be transitioned to the reserve operating point with as little blade adjustment as possible, while simultaneously reducing the rotor speed during this transition, thus allowing kinetic energy to be converted into power. The transition thus occurs as quickly as possible and simultaneously allows the increased output power, namely according to the desired reserve operating point, to be delivered and fed in right at the beginning of the transition.

[0031] It is preferably proposed that the characteristic curve slope at the throttle operating point decreases by a value that is at least 0.5 / 1° in magnitude. It has been recognized that a significantly negative slope thus belongs to particularly closely spaced iso-characteristic curves and that the Cp value can therefore be changed particularly quickly there, i.e. a large change in the Cp value with the smallest possible change in the blade angle. This effect occurs particularly at tip speed ratios that are greater than the tip speed ratio of the normal operating point. Therefore, it is also proposed as an additional condition that the throttle operating point has a tip speed ratio that is greater than the tip speed ratio of the normal operating point.

[0032] According to one embodiment, it is proposed that the reserve operating point be operated with a lower output power than the normal operating point. In the reserve operating point, the wind turbine therefore delivers less power than in the normal operating point. However, the output power of the throttle operating point is also lower than in the reserve operating point. The reserve operating point thus lies between the throttle operating point and the normal operating point in terms of the output power level. When the wind turbine switches from the throttle operating point to the reserve operating point, the output power is increased, but not to the level of the normal operating point.

[0033] Additionally, or alternatively, it is proposed that the reserve operating point have a lower tip speed ratio than the throttle operating point. Thus, in the reserve operating point, the wind turbine rotates more slowly than in the throttle operating point. This makes it possible to initially generate power by braking the wind turbine to increase the output power from the throttle operating point to the reserve operating point.

[0034] Optionally, the reserve operating point can have a higher tip speed ratio than the normal operating point. In this case, the tip speed ratio of the reserve operating point is lower than that of the throttle operating point, but higher than that of the normal operating point. Accordingly, its speed is also lower than that of the throttle operating point and higher than that of the normal operating point.

[0035] Here, too, it was recognized that the return from the throttle operating point to the reserve operating point could be facilitated by the increased initial speed. Furthermore, it was recognized that, in this sense, a further transition from the reserve operating point to the normal operating point would also be possible with the aid of kinetic energy.

[0036] According to a further embodiment, it is proposed that the throttle operating point and / or the reserve operating point is selected as a function of a distance between two iso-characteristic curves at the throttle operating point or at the reserve operating point or as a function of a gradient of the power coefficient of the throttle operating point or the reserve operating point.

[0037] Each iso-characteristic curve represents a power coefficient. The closer two iso-characteristic curves are to each other, the greater the change in the power coefficient for an otherwise identical change in an operating point in the λθ diagram. A small change in the blade angle value and / or the tip speed ratio value of an operating point in the λθ diagram thus leads to a rapid change in the power coefficient if two iso-characteristic curves are close together in this area. Therefore, it was recognized that the throttle operating point and the reserve operating point can be particularly close to each other where the distance between two iso-characteristic curves is also small.

[0038] Accordingly, it is proposed to select the throttle operating point and the reserve operating point based on the distance between two iso-characteristic curves. However, it may also be sufficient to select only one of these two operating points based on the distance between two iso-characteristic curves.

[0039] Alternatively, one of the two operating points or both operating points can be selected depending on a gradient of the power coefficient of the throttle operating point or the reserve operating point. The higher the gradient of the power coefficient in terms of magnitude, i.e. the greater the slope of the power coefficient of the respective operating point, the closer the throttle operating point and the reserve operating point can be to each other. In this respect, the gradient of the power coefficient of the throttle operating point can be considered to select the throttle operating point, or the gradient of the power coefficient of the reserve operating point can be considered to select the reserve operating point. In particular, it is proposed to select the throttle operating point and / or the reserve operating point where the gradient of the power coefficient is particularly large in terms of magnitude.

[0040] As a power coefficient gradient, a derivative of the power coefficient based solely on the blade angle can be selected. It has been particularly recognized that a large derivative of the power coefficient based on the blade angle indicates a range in which large changes in the power coefficient can be achieved with a small change in the blade angle, thus resulting in large differences in the corresponding power outputs at the respective operating points.

[0041] However, a gradient can also be chosen that is derived from both the blade angle and the tip speed ratio. To ensure a comparable dimension between the blade angle and the tip speed ratio, it may be useful to normalize the blade angle to a whole degree. In the λθ diagram, the length of one degree of blade angle adjustment would then correspond to the length of a change in the tip speed ratio by the value 1.

[0042] According to the invention, the throttle operating point and / or the reserve operating point are selected such that a derivative of the power coefficient of the throttle operating point or the reserve operating point with respect to the blade angle exceeds a predetermined minimum derivative value in terms of absolute value. Thus, the derivative of the power coefficient with respect to the blade angle is considered here. The larger the absolute value of this derivative, the more the power coefficient changes with a change in the blade angle. As a result, a large change in the power coefficient can be achieved with a small change in the blade angle if this derivative is selected to be correspondingly large. Therefore, a predetermined minimum derivative value is used as a basis for the absolute value of the derivative. This also allows a quantitative measure to be specified.

[0043] It is particularly likely that the power coefficient will decrease with increasing blade angle. This is due in particular to the fact that a blade angle of 0° or a few degrees is often assumed to be the optimal operating point for performance. However, reduced power production is planned for the throttle operating point and possibly also for the reserve operating point. Thus, the output power is lower while the blade angle is increased. Therefore, the absolute value of the derivative is considered, as its absolute value is likely to be negative.

[0044] Here, too, the underlying idea is that, with a large derivative of the power coefficient, a large change in power can be achieved with only a small change in the angle of attack. This can be advantageous for both the throttle operating point and the reserve operating point. According to one embodiment, it is particularly proposed for both operating points, and this ensures that the transition from the throttle operating point to the reserve operating point can be carried out comparatively quickly. The duration of the change between these two operating points, i.e., in particular, the duration of the transition from the throttle operating point to the reserve operating point, can be determined in particular by the time required for the necessary blade angle adjustment.

[0045] Preferably, the minimum derivative value for the throttle operating point is at least 0.05 / 1°. Here, it was particularly recognized that the power coefficient can logically be reduced from a physical maximum of just over 0.45 to a maximum of zero. A change in the power coefficient by a value of 0.3 therefore represents a significant reduction in power or a significant reduction in the power coefficient. To this end, it is proposed that the rotor blade be adjusted by a maximum of 6° to achieve such a change in the power coefficient. This results in the proposed minimum derivative value, namely by the calculation: 0.3 / 6° = 0.05 / 1°. This means that the rotor blade can be adjusted by a maximum of 6° to achieve a change in the power coefficient by the value 0.3.

[0046] Additionally or alternatively, it is proposed that the minimum derivative value for the reserve operating point be at least 0.1 / 6°. Here it was recognized that although a large derivative of the power coefficient with respect to the blade angle can be useful for the reserve operating point, its magnitude can nevertheless be somewhat lower at the reserve operating point than at the throttle operating point. This is based in particular on the recognition that the reserve operating point has a higher power coefficient than the throttle operating point. The reserve operating point is therefore closer to the optimum operating point or normal operating point in terms of the power coefficient. It was also recognized that it can be particularly important to be able to change the power quickly from the throttle operating point in response to a request for an increase in power. This means that the change at the throttle operating point must preferably be more dynamic than at the reserve operating point.

[0047] According to one embodiment, the method is characterized in that the wind turbine can be characterized by a characteristic curve field of iso-characteristic curves in the λθ diagram. Thus, there are many iso-characteristic curves, each of which is assigned to a power coefficient, i.e., a Cp value. In this respect, the wind turbine is described in the usual way by such a characteristic curve field in the λθ diagram, at least in the manner already explained above.

[0048] It is further proposed that the characteristic curve family be defined as a standardized characteristic curve family in which the blade angle is standardized to whole degrees (°). This allows a gradient to be defined in the characteristic curve family that is independent of the choice of physical unit. Thus, for example, a change in the tip speed ratio by 1 is equated in magnitude to a change in the blade angle by 1°, which was recognized as appropriate.

[0049] In the standardized characteristic curve field, a first sub-region is formed in which the blade angles are greater than the blade angle at the normal operating point, and the iso-characteristic curves have a negative characteristic gradient, where the tip speed ratio decreases with increasing blade angle. Thus, only this region is considered, which, to put it bluntly, lies in the upper right of the characteristic curve field, where the blade angles are plotted with increasing values ​​along the abscissa and the tip speed ratios are plotted with increasing values ​​along the ordinate.

[0050] In the standardized characteristic curve family, each operating point is characterized by a gradient value that quantifies the maximum gradient of the power coefficient at that operating point in the standardized characteristic curve family. A common definition of a gradient is used here. The gradient thus describes the direction in which the power coefficient changes most significantly. This can be in the direction of the blade angle, in the direction of the tip speed ratio, but especially in a direction that is a combination of these two directions. The direction is thus defined by the maximum gradient. The corresponding value of the gradient is the gradient value. The gradient direction is not used further here and is only required to clearly define the maximum gradient value.

[0051] In this first sub-range of the standardized characteristic curve field, there is an operating point with a maximum gradient for each iso-characteristic curve. All of these operating points, one of which can be found for each iso-characteristic curve, can be connected with a characteristic curve, and this characteristic curve is referred to here as the gradient characteristic curve. Thus, a gradient characteristic curve can be represented that connects all of these operating points with a maximum gradient value in the positive sub-range under consideration.

[0052] It is now proposed that the throttle operating point and / or the reserve operating point be selected such that they each lie on this gradient characteristic curve. Here, too, the idea is that the throttle operating point and / or the reserve operating point lie in the range of a derivative of the power coefficient that is as large as possible in terms of absolute value, in order to achieve a rapid and significant change in power with the smallest possible change in blade angle. It was also recognized that this maximum gradient with regard to the blade angle and the tip speed ratio in the first sub-range considered particularly characterizes the range in which a small change in the blade angle leads to a large change in the power coefficient. The change in power can therefore be achieved with a small blade adjustment and can therefore be achieved quickly.

[0053] It is therefore proposed to define this gradient characteristic, because this is where the derivative of the power coefficient with respect to the blade angle is greatest. By defining or plotting this gradient characteristic, this area can be clearly illustrated.

[0054] If both the throttle operating point and the reserve operating point are selected so that they lie on this gradient characteristic, the transition from the throttle operating point to the reserve operating point can preferably be made along this gradient characteristic. At least with regard to blade pitch, this is an optimized solution for the fastest possible transition between the throttle operating point and the reserve operating point.

[0055] If the gradient characteristic is not precisely selected for the selection of the throttle operating point and / or the reserve operating point, the solution found may no longer be optimal, but it may nevertheless be a good solution. Therefore, at least according to one embodiment, it is proposed to select the throttle operating point and / or the reserve operating point at least within a gradient band that includes the gradient characteristic. This also makes it possible to consider additional boundary conditions, because selecting an operating point within the band rather than on a characteristic creates a degree of freedom for changes.

[0056] For example, considerations regarding wear and tear on the wind turbine can be considered as a boundary condition. Adaptations can also be implemented this way. In particular, the characteristic curve field may be subject to slight changes, for example, due to contamination on the rotor blades. By making an appropriate selection within the gradient band, such a boundary condition and / or such a variation can be easily accounted for, while at the same time maintaining the idea of ​​selecting the throttle operating point and / or the reserve operating point at least close to the gradient characteristic curve in order to achieve a large change in the power coefficient there through a small change in the blade angle.

[0057] Such a gradient band has an upper band limit that lies above the gradient characteristic curve by a tip speed difference. Furthermore, the gradient band has a lower band limit that lies below the gradient characteristic curve by a lower tip speed difference. It is thus proposed to form the gradient band by shifting the gradient characteristic curve upwards by one value in the direction of the tip speed number to form the upper band limit, and by shifting it downwards by an equal or different value with respect to the tip speed number to form the lower band limit.

[0058] Preferably, the upper and lower tip speed ratio differences each have a maximum value of 2. Thus, if the gradient characteristic curve has a tip speed ratio of 9 at one point, the gradient band can range between the tip speed ratio values ​​of 7 and 11. However, the upper and lower tip speed ratio differences can also have different values.

[0059] Regarding the suggestion to select a maximum value of 2 in each case, and in particular a maximum value of 1, it was recognized that a change in the tip speed ratio around the value 2 in the range of the gradient characteristic curve leads to only moderate changes in the derivative of the power coefficient with respect to the blade angle. Thus, a change of a maximum of +2 or -2 in the tip speed ratio can still preserve the character of the characteristic curve field, so that the derivative of the power coefficient with respect to the blade angle can still be considered high. For values ​​even greater than 2, changes in the respective operating point in the characteristic curve field can lead to significant changes in the character of the characteristic curve field, so that for correspondingly larger values ​​of the change in the tip speed ratio, a range outside the gradient band is assumed.

[0060] Alternatively, it is proposed that the upper tip speed difference have a maximum value of 1 and the lower tip speed difference a maximum value of 4. In this case, the band limits are therefore selected differently. It was recognized that excessively high speed and thus excessively high tip speed ratios should be avoided in order to prevent an unstable operating point, so the distance for the upper band limit is selected to be narrow. This problem does not exist for lower speeds, at least only for much larger speed deviations, so a band of 4 was recognized as appropriate here.

[0061] According to one embodiment, it is proposed that the wind turbine has a first output power, a first rotational energy, and a first blade angle at the throttle operating point, and that it has a second output power, a second rotational energy, and a second blade angle at the reserve operating point. The first output power is less than the second output power, and a difference between the second output power and the first output power forms a differential output power of the operating points. Furthermore, the first rotational energy is greater than the second rotational energy, and a difference between the first and second rotational energy forms a differential rotational energy. These properties of the operating points or the properties of the wind turbine at these operating points form the basis of further consideration, particularly for the selection of at least one of the operating points.

[0062] Furthermore, it is taken into account that a blade pitch time is required to adjust the rotor blades from the first blade pitch to the second blade pitch. This blade pitch time depends on the pitch speed of the wind turbine's rotor blades and also on the difference between the blade angles between the throttle operating point and the reserve operating point, i.e., the amount by which the blades must be rotated.

[0063] The product of the differential power output and the blade pitch time multiplied by 50% forms the characteristic differential energy. Half of the product of the differential power output and the blade pitch time is thus this characteristic differential energy. This characteristic differential energy, which can also be referred to simply as differential energy here, is initially a definition of the term. However, it also has the following physical meaning.

[0064] If the wind turbine is adjusted from the throttle operating point to the reserve operating point, based on the assumptions and definitions mentioned above, the output power increases. Ideally, it increases linearly over time, especially when the rotor blades are adjusted uniformly and the output power is also increased proportionally to the adjustment of the rotor blades. Relative to the starting value, i.e. the output power of the throttle operating point, the output power in the transition range is a difference value higher than the output power of the throttle operating point, and in the ideal case mentioned, this difference value increases linearly over time until the output power of the reserve operating point is reached. If this difference value of the output power is integrated over this time, the characteristic difference energy is obtained.If a linear increase is assumed, the difference energy is the integration of the difference in output power over the time in which the engine changes from the throttle operating point to the reserve operating point.

[0065] Based on this finding, it is now proposed that the throttle operating point and / or the reserve operating point be selected such that the differential rotational energy corresponds to the characteristic differential energy.

[0066] The throttle operating point and / or the reserve operating point are selected so that they have different speeds, and these are selected in relation to each other in such a way that a differential rotational energy is established. This differential rotational energy is based on the characteristic differential energy, i.e., the energy that characterizes the transition from the throttle operating point to the reserve operating point.

[0067] The idea behind this choice is that, ideally, in response to the power increase request, this power increase should be provided immediately. Ideally, the fully increased power should not be available until the reserve operating point is reached. Accordingly, it is proposed that these operating points be chosen so that the increased power can be provided immediately from the difference in rotational energy for this transition time. In particular, the entire power difference can be provided from the rotational energy at the beginning of the transition. During the transition from the throttle operating point to the reserve operating point, however, the power generated from the wind also gradually increases, and thus the proportion of rotational energy required to provide the difference in power gradually approaches zero until the reserve operating point is reached.

[0068] To achieve this, the throttle operating point can be adjusted so that the differential rotational energy is set relative to a given reserve operating point, or the reserve operating point can be adjusted so that the differential rotational energy is set relative to the given throttle operating point. However, both operating points can also be adjusted together or selected so that the differential rotational energy is set.

[0069] It is also considered that the two operating points, or rather the one of the two that is adjusted, are found using an iterative process. In particular, it was recognized that not only must the operating points be selected so that the differential rotational energy reaches a certain value, but also that the characteristic differential energy relevant for this depends on the blade pitch time and thus also on the selected operating points.

[0070] However, it was also recognized that the transition from the throttle operating point to the reserve operating point may not be ideally linear. Accordingly, it is proposed that the throttle operating point and / or the reserve operating point be selected such that the differential rotational energy lies above or below the characteristic differential energy by a maximum of a predetermined percentage deviation value. It is preferably proposed that the percentage deviation value be a maximum of 60%, preferably a maximum of 40%, in particular a maximum of 20%. This allows for the fact that the progression is not ideally linear to be taken into account. Furthermore, in the case of an iterative process, the process can also be terminated earlier if, for example, a deviation of 20% is permitted.

[0071] However, it is proposed that the maximum percentage deviation should be 60%. This ensures that the selection of the operating points, or at least one of the operating points, is based on the differential rotational energy and also on the characteristic differential energy, so that the described effect can be exploited without excessive deviation leading to an operating point with an unnecessarily high or unnecessarily low speed.

[0072] Preferably, a deviation of 40% may be sufficient for this. This is particularly recommended when the boundary conditions are relatively well known, especially the power changes in the transition range from the throttle operating point to the reserve operating point. With particularly good knowledge of these transitions and thus also particularly good knowledge of the characteristic curve field, and also when linear or nearly linear behavior can be assumed in the transition range, 20% is advantageous as the maximum deviation.

[0073] According to one embodiment, it is proposed that in order to set the throttle operating point, in particular starting from the normal operating point, an intermediate operating point is first set in a first adjustment step, wherein the intermediate operating point has the same Cp value as the throttle operating point, or at least has a Cp value changed by less than 20%. Thus, starting from the normal operating point or another starting point from which the throttle request is to be implemented, the throttle operating point is not controlled directly, i.e., via the most direct route. Instead, an intermediate operating point is first selected which already has the same Cp value as the throttle operating point to be ultimately controlled. From there, while maintaining the Cp value, the operating point is then changed from the intermediate operating point to the throttle operating point.

[0074] In particular, it was recognized here that power can be reduced by changing the operating point to an intermediate operating point. For example, an operating point with a low speed can be found without adjusting the blade angle. Such an operating point can also lead to less wear due to the lower speed. Such an operating point can be set, for example, by initially increasing the power output slightly from the normal operating point, e.g. by a few percent, or just by one percent. This power is then slightly higher than the maximum power that can be generated from the wind at that moment. This leads to a drop in speed. The operating point therefore changes and leaves the optimal range. This worsens the power coefficient and the operating point moves to the intermediate operating point.

[0075] However, the intermediate operating point is not particularly well-suited for quickly increasing power in response to a demand for increased power. Especially when the speed at the intermediate operating point is low, little or no momentum is available to temporarily increase power in the short term.

[0076] However, it was recognized that such a power increase request is not to be expected immediately after a change to the intermediate operating point. In particular, it is proposed that the intermediate operating point is only activated if it is known or expected that a power increase request will not occur. This is particularly known or expected if the grid operator operates the electrical supply grid in a mode in which a power increase request is not provided for. In particular, the power increase request can be designed in such a way that the wind turbine must react to a drop in frequency. This can also be referred to as frequency control mode or mode with P(f) control. However, such a mode can be excluded or temporarily deactivated by the grid operator.

[0077] The wind turbine can then switch to the intermediate operating point and, if necessary, operate in this intermediate operating point for a longer period, namely as long as the frequency control mode is not active.

[0078] At the same time, however, it is intended that the intermediate operating point has the same Cp value as the throttle operating point. It has been recognized that, with regard to other conditions, especially boundary conditions, the initially controlled operating point with reduced output power may not be optimal. Especially if the operating point is to be able to respond to a power increase request, for example, because the frequency control mode is active or will soon be activated, it may be better to select the throttle operating point.

[0079] Therefore, it is proposed that the intermediate operating point be controlled first, and from there, while maintaining the Cp value, the throttle operating point is controlled or switched to the throttle operating point. The intermediate operating point can also be maintained for a longer period of time.

[0080] In particular, it is proposed that, in a second adjustment step, the system switches from the intermediate operating point to the throttle operating point, and that the change in the λθ diagram is essentially represented as a change along an iso-characteristic curve. When switching from the intermediate operating point to the throttle operating point, the Cp value is therefore left unchanged, or at most, modified with a maximum deviation of 20% of the Cp value of the iso-characteristic curve of the throttle operating point. In this respect, small changes in the Cp value, which are limited to 20% here, may be acceptable and can be used, in particular, to compensate for small deviations that could arise, for example, from changes in the aerodynamics of the rotor blades due to contamination.

[0081] According to one embodiment, it is proposed that the throttle request for requesting operation of the wind turbine at the throttle operating point be embodied as an external specification received via a data interface, and / or that the power increase request for requesting operation of the wind turbine at the reserve operating point be embodied as a rule for responding to a state of the electrical supply grid detected by the wind turbine. In particular, the power increase request is embodied as a rule for implementing a power increase starting from the throttle operating point depending on a detected grid frequency of the electrical supply grid.

[0082] Here it was particularly recognized that two completely different requirements can exist and the two operating points are adapted to this.

[0083] The throttling operating point is thus selected when an external specification is present, which can also be received via a data interface. This already implies that the throttling request is relatively long-term. For example, such a throttling request could be one in which the grid operator requests a reduction in output power for a period of 15 minutes or longer. The throttling operating point is then controlled, and the wind turbine operates at this point for an extended period.

[0084] In this respect, the power increase request is more of a quick request. Here, there is a regulation to react to a condition of the electrical supply grid. A drop in frequency, in particular, can therefore lead to the power increase request. For example, a drop in the grid frequency below a first lower frequency limit can result in the wind turbine having to increase its power within less than 40 ms. Such a short-term request often does not allow the transmission and corresponding processing of a corresponding request signal. It should be noted that if one wanted to transmit such a signal externally, this frequency event would also have to be recorded at the corresponding external location.

[0085] Instead, the wind turbine is prepared to react to such a frequency event as quickly as possible. This could involve a disconnection in the electrical grid, resulting in a power cut. This could also mean a power plant supplying the grid being disconnected. This would then spontaneously cause a power deficit in the electrical grid, and the grid would immediately react with a drop in frequency. The wind turbine, in turn, can detect this drop in frequency and must also react immediately and increase its power.

[0086] It was particularly recognized here that the throttle operating point and the reserve operating point should be advantageously coordinated with these two different types of requirements. In particular, the throttle operating point is designed as a longer-term operating point. The reserve operating point can be designed as a short-term operating point. In particular, the throttle operating point and / or the reserve operating point should be selected so that a transition from the throttle operating point to the reserve operating point can occur quickly, in particular so that the power is increased to this power increase requirement as quickly as possible.

[0087] In addition, appropriate monitoring criteria must also be established, namely, for the throttle operating point, the evaluation of corresponding signals received externally, and for the reserve operating point, a correspondingly continuous monitoring of the grid frequency, or possibly another state variable of the electrical supply grid. Such a state variable of the electrical supply grid can, in particular, be a phase shift and / or a change in the voltage amplitude of the grid voltage, in addition to the aforementioned frequency change.

[0088] According to one embodiment, it is proposed that the wind turbine be operated at the throttle operating point for a longer period than at the reserve operating point. In particular, it is provided that it be operated at the throttle operating point for at least 20 times as long as at the reserve operating point.

[0089] Here, it was recognized that the throttle operating point is intended for a longer power reduction. It is particularly adapted to a case where a throttle request requires a power reduction for at least a quarter of an hour, but especially for a significantly longer period. The throttle operating point must therefore be set and designed for the longer term. This does not rule out the possibility, for example, that a power increase request might suddenly occur after just one minute; however, this would lead to a very short-term power increase. After that, the operating point can return to the throttle operating point.

[0090] The reserve operating point, on the other hand, is intended for short-term power support. Especially in response to a drop in frequency, this should lead to a power increase as quickly as possible. It is therefore primarily a response to an incident in the electrical supply grid, which can also be referred to as a fault. Such faults can often be rectified within a few seconds, and the reserve operating point is intended to provide support during this time. However, should this fault persist for a somewhat longer period, the reserve operating point can of course be maintained for a correspondingly longer period.

[0091] According to the invention, a wind energy plant according to claim 10 is also proposed. This has a rotor with rotor blades adjustable in their blade angle (θ), wherein the rotor is operable at a variable speed, and wherein the wind turbine is operable in a partial load range in which there is insufficient wind to operate the wind turbine at rated power, the wind turbine is operable at a variably predeterminable operating point, and the operating point is characterized by the blade angle (θ) and a tip speed ratio (λ), and wherein the operating point can be represented in a λθ diagram, wherein in the λθ diagram the tip speed ratio (λ) is plotted against the blade angle (θ), and the operating point can be represented in the λθ diagram as a pair of values ​​from its blade angle (θ) and its tip speed ratio (λ), wherein each operating point is assigned a power coefficient (Cp value), and several operating points with the same Cp value can be represented as an iso-characteristic curve in the λθ diagram,and the wind turbine has a control device, prepared to operate the wind turbine in the partial load range, and prepared to operate the wind turbine at a normal operating point, at which the wind turbine is not throttled if there is no request for throttling, to operate the wind turbine at a throttle operating point, at which the wind turbine is operated in response to a throttle request with a throttled output power compared to the normal operating point, and to change the operation of the wind turbine from the throttle operating point to a reserve operating point, at which the wind turbine is operated with a higher output power compared to the throttle operating point, in response to a power increase request, wherein the operation of the wind turbine is implemented in such a way,that the throttle operating point has a higher tip speed ratio than the reserve operating point and the throttle operating point lies on an associated iso-characteristic curve in the λθ diagram and the associated iso-characteristic curve in the throttle operating point has a negative characteristic gradient, in which the tip speed ratio decreases with increasing blade angle, wherein the throttle operating point and / or the reserve operating point are selected such that in each case a derivative of the power coefficient of the throttle operating point or of the reserve operating point with respect to the blade angle exceeds a predetermined minimum derivative value, in particular that for the throttle operating point the minimum derivative value is at least 0.3 / 6° (= 0.05 / 1°) and / or for the reserve operating point the minimum derivative value is at least 0.1 / 6°.

[0092] The wind turbine thus has a control system and is prepared to operate the wind turbine in the partial load range and to operate it in such a way that the wind turbine operates at a normal operating point, at a throttled operating point, or at a reserve operating point, depending on requirements. This also includes switching the wind turbine's operation from the throttled operating point to the reserve operating point when a corresponding power increase is required. All of this is implemented in the control system. In particular, a corresponding program can be stored and implemented for this purpose. The specific specifications for the respective operating points can also be stored through corresponding functional relationships.In addition, or alternatively, different operating points can be stored for different wind speeds and selected depending on the wind speed and requirements. Intermediate points, i.e., operating points for wind speeds for which no operating point is stored, can be determined by interpolation from neighboring operating points.

[0093] In particular, the operation of the wind turbine is implemented in such a way that the throttle operating point has a higher tip speed ratio than the reserve operating point, and the throttle operating point lies on a corresponding iso-characteristic curve in the λθ diagram, and the corresponding iso-characteristic curve has a negative characteristic gradient at the throttle operating point, with the tip speed ratio decreasing with increasing blade angle. All of these relationships can be implemented functionally and / or by storing wind speed-dependent operating points that meet these criteria.

[0094] In particular, it is proposed that the wind turbine, in particular the control device, be prepared to execute a method according to one of the embodiments described above. In particular, the corresponding control method is implemented in the control device.

[0095] According to one embodiment, it is proposed that a data interface be provided for receiving and evaluating an external signal as an external specification of the throttling request. The throttling request can thus be transmitted as an external signal and thus as an external specification from an external source, for example, from a network operator. Such a signal is received and evaluated via the data interface. At the very least, the evaluation is carried out in such a way that the received signal is recognized as a throttling request and can then be further processed in the control device.

[0096] It is also proposed that a detection device be provided for detecting and evaluating a state of the electrical supply grid. Such a detection device can be provided in particular as a fast voltage detection device, with which, for example, the grid voltage and thus in particular also its frequency is detected at corresponding connection terminals of the wind turbine. A state detected in this way is also evaluated to check whether a power increase request exists as a regulation for responding to the detected state. For example, the detected voltage is evaluated with regard to its frequency and a check is carried out to determine whether this frequency has such a value, in particular such a deviation from a normal state, that this can be interpreted as indicating a power increase request. Such a power increase request is then responded to accordingly.

[0097] According to one embodiment, it is proposed that a control memory be provided, and that control instructions be stored in the control memory for operating the wind turbine according to at least one method according to an embodiment described above. Corresponding control instructions can thus be stored in the control memory.

[0098] Additionally or alternatively, it is proposed that operating points be stored in the control memory, i.e. stored, which depend on the wind speed. In particular, this is proposed for several wind speeds, and for each of these several wind speeds, a normal operating point, a throttle operating point, and a reserve operating point are stored. These three operating points are selected such that they fulfill the relationships between these three operating points described above. Accordingly, these operating points can be determined for different wind speeds before commissioning a wind turbine and then implemented. For example, these operating points can be checked in simulations to determine whether the criteria are met.It is also possible to check the operating points offline during ongoing operation of the wind turbine, for example, to take into account any changes in the turbine's properties. Such newly determined operating points can then be implemented in the wind turbine.

[0099] The invention will now be explained in more detail below using exemplary embodiments with reference to the accompanying figures. Figure 1 shows a perspective view of a wind turbine. Figure 2 shows a power-speed diagram for different wind speeds. Figure 3 shows a λθ diagram. Figure 4 shows a timing diagram illustrating a transition from a throttle operating point to a reserve operating point.

[0100] Figure 1shows a schematic representation 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. During operation of the wind turbine, the aerodynamic rotor 106 is set in rotation by the wind and thus also rotates an electrodynamic rotor or rotor of a generator, which is directly or indirectly coupled to the aerodynamic rotor 106. The electrical generator is arranged in the nacelle 104 and generates electrical energy. The pitch angles of the rotor blades 108 can be changed by pitch motors at the rotor blade roots 109 of the respective rotor blades 108.

[0101] The wind turbine 100 has an electrical generator 101, which is indicated in the nacelle 104. Electrical power can be generated by means of the generator 101. A feed-in unit 105, which can be designed particularly as an inverter, is provided for feeding in electrical power. This allows a three-phase feed-in current and / or a three-phase feed-in voltage to be generated according to amplitude, frequency, and phase for feeding into a grid connection point PCC. This can be done directly or jointly with other wind turbines in a wind farm. A system controller 103 is provided for controlling the wind turbine 100 and the feed-in unit 105. The system controller 103 can also receive default values ​​from external sources, in particular from a central farm computer.

[0102] Figure 2shows a diagram in which characteristic curves are drawn for different wind speeds which show the relationship between rotor power P and rotor speed n. Characteristic curves k 3 - k 13 are shown. The index refers in each case to the corresponding value of the wind speed, namely from 3 m / s for k 3 to 13 m / s for k 13 . It can be seen that for each of the characteristic curves k 3 - k 13 the power that can be generated P initially increases with increasing speed n until a maximum is reached. From the maximum onwards the power P then decreases again with increasing speed n. For optimum operation, i.e. for an optimum power coefficient and thus maximum power generation, the wind turbine is operated at the maximum of the characteristic curves.

[0103] This is the optimal operating characteristic 200 in Figure 2shown. However, the wind turbine cannot be operated at any desired speed, but should not exceed a nominal speed n N. This nominal speed n N is shown in the diagram. For wind speeds of 10 m / s, the maximum of the relevant characteristic curve, i.e. characteristic curve k 10 , is roughly in the range of the nominal speed. For characteristics with higher wind speeds, i.e. characteristic curves k 11 - k 13, the maxima are at higher speeds and accordingly the optimum operating characteristic curve bends when the maximum of the characteristic curve k 10 is reached and runs vertically from then on. From then on, the wind turbine is no longer operated at optimum mode, namely to protect it from overload.

[0104] The optimal operating characteristic curve 200 thus covers the partial load range up to this k-point in the characteristic curve k10, where the wind turbine cannot yet generate maximum power. The vertical range is therefore a regulated characteristic curve 220.

[0105] Thus, it was particularly recognized that for low wind speeds, both an increase in speed and a decrease in speed are possible options for power reduction. It was particularly recognized that this allows for a degree of freedom in the selection of a throttle operating point.

[0106] Figure 3shows a λθ diagram or a section thereof. In it, the tip speed ratio λ is shown on the ordinate and the blade angle θ on the abscissa with the unit degrees (°). The unitless tip speed ratio λ is thus plotted against the blade angle θ. Iso characteristics E 0.45 to E 0.00 are also entered in the λθ diagram. The index indicates the corresponding power coefficient of the respective characteristic. The power coefficient of the iso characteristic curve I 0.45 is therefore 0.45. The iso characteristics are plotted starting with a Cp value of 0.45 in equal 0.05 steps up to a Cp value of 0.05 and as the last value 0.00. The difference in the Cp value between two adjacent iso characteristics is therefore always 0.05.

[0107] The diagram also shows a turbine operating characteristic curve 300. This turbine operating characteristic curve 300 represents all of the wind turbine's operating points, which are set depending on the wind speed when there is no specific throttling requirement. An optimal operating point 302 lies essentially in the center of the ISO characteristic curve I 0.45 for a Cp value of 0.45. The Cp value of the optimal operating point 302 is likely to be slightly higher. Although the turbine operating characteristic curve 300 is a characteristic curve along which the operating points can change, it is still possible that the wind turbine will operate for a wider range of wind speeds at the optimal operating point 302. In other words, a large number of operating points for different wind speeds can essentially overlap at the optimal operating point 302.

[0108] However, this optimal operating point 302 can vary with wind speed. Due to boundary conditions, it is expected that at very low wind speeds, for example, below 5 m / s, a higher tip speed ratio is appropriate. Accordingly, a slight change in the blade angle θ may also be appropriate. To illustrate this, a suboptimal operating point 304 for low wind speeds is shown, or the branch of the turbine operating characteristic curve 300 near the suboptimal operating point 304 generally represents operation at low wind speeds.

[0109] At high wind speeds, the tip speed ratio will decrease with increasing wind speed toward the end of the partial load range and beyond. Upon leaving the partial load range, the blade angle θ is increased to derate the wind turbine. This is indicated by the derated characteristic curve branch 306.

[0110] There are many options for selecting a throttle operating point in the event of a throttle request, i.e., when the wind turbine is to permanently deliver less power in the partial load range than it could generate from the wind. First, ideally starting from the optimal operating point 302, an operating point should be found at which the power coefficient is correspondingly lower than at the optimal operating point 302. If, for example, the wind turbine is to generate approximately half as much power in the throttle operating point as in the optimal operating point, it is recommended to select the throttle operating point on the iso-characteristic curve I 0.25. Accordingly, the diagram shows the Figure 3 a throttle operating point 308 is shown as an illustrative example.

[0111] In addition, a reserve operating point 310 is provided which has a higher output power than the throttle operating point 308 and was therefore selected as an example on the ISO characteristic curve I 0.40 for a power coefficient of 0.4.

[0112] It is now proposed that the throttle operating point 308 has a higher tip speed ratio than the reserve operating point 310. Accordingly, these two operating points 308 and 310 are shown in the diagram of the Figure 3. If the system now switches from the throttle operating point 308 to the reserve operating point 310, namely in response to a power increase request, the speed is reduced slightly, because the tip speed ratio λ must decrease for this to occur. Assuming the same wind speed, the reduction in the tip speed ratio thus results in a reduction in the speed. This allows kinetic energy to be released, resulting in an immediate power increase that begins even before the reserve operating point 310 has been reached.

[0113] In the diagram of the Figure 3It can also be seen that the throttle operating point 308 lies on a corresponding iso-characteristic curve, i.e., the iso-characteristic curve I 0.25 , which has the desired Cp value of 0.25, and that this iso-characteristic curve I 0.25 has a negative characteristic gradient at this throttle operating point 308, with the tip speed ratio decreasing with increasing blade angle. Here, the gradient of the iso-characteristic curve I 0.25 is approximately -2.2 / 1°.

[0114] From the diagram of the Figure 3It can be seen that the throttle operating point 308, due to the proposed selection, namely including the selection that the characteristic curve slope at the throttle operating point is negative, is not in the range in which the turbine operating characteristic curve 308 lies or could lie at low wind speeds. It can also be seen that at these values, with a higher tip speed ratio than the reserve operating point, but with a negative characteristic curve slope, the characteristic curves are very close together. The distance shown in the Figure 3The difference between the throttle operating point 308 and the reserve operating point 310 is therefore comparatively small. To move from the iso-characteristic curve I 0.25 to the iso-characteristic curve I 0.4 within the selected range, only a blade pitch of less than 2° is required. This rapid change can be achieved through the proposed selection of the throttle operating point and the reserve operating point, in particular through the selection of the throttle operating point in relation to the reserve operating point.

[0115] In Figure 3 In addition, a gradient characteristic curve 320 is shown, with an upper band limit 321 and a lower band limit 322. Both band limits are shown in dashed lines and different values ​​were chosen for the distance between the band limits, so that the gradient characteristic curve in the band lies further up towards the upper band limit.

[0116] Figure 4illustrates a preferred choice of the throttle operating point in relation to the reserve operating point, taking into account the kinetic energy of both operating points. Figure 4 Several time courses are shown, namely in four individual diagrams. Each diagram uses the same time axis. Figure 4 illustrates the change from a throttle operating point to a reserve operating point.

[0117] This change begins at time t 1 , at which the throttle operating point is left, and ends at time t 2 , when the reserve operating point is reached. The illustrated change may refer to a change from the throttle operating point 308 to the reserve operating point 310 according to Figure 3 relate.

[0118] To change the throttle operating point to the reserve operating point, the blade angle is adjusted approximately linearly from 6° to 4°. This is illustrated in the lower diagram I, which can be referred to as the blade angle curve diagram. It is assumed that the blade angle is adjusted at maximum pitch speed, and thus the blade pitch time T θ, i.e., the difference between t 2 and t 1 , is required for this.

[0119] At the same time, ideally also approximately linearly, the speed n decreases from the value n 1 at time t 1 , i.e., the speed of the throttle operating point, to the speed n 2 at time t 2 , i.e., the speed of the reserve operating point. This is shown in Diagram II, which thus forms a diagram illustrating the speed curve.

[0120] The throttle operating point thus has a lower output power with the value P 1 , whereas the reserve operating point has an increased output power with the value P 2 . Accordingly, the output power PA increases from the value P 1 to the value P 2 , namely from time t 1 to time t 2 .

[0121] The output power PA is understood to be the power that the wind turbine generates and delivers from the wind at the respective operating point. In the period between times t 1 and t 2 , i.e., in the transition period when the switch between the throttle operating point and the reserve operating point occurs, the output power PA is understood to be the power generated from the wind.

[0122] However, it is now proposed that the fed-in power be increased more rapidly initially in the transition range in order to reach the value P 2 as quickly as possible. Such power can be referred to as instantaneous power PI and is shown as a dashed line in the third diagram III. This third diagram III thus represents a diagram for the power curve.

[0123] Ideally, at time t 1 or immediately thereafter, enough power can be extracted from the kinetic energy of the wind turbine's rotor to immediately increase the power to the value P 2 . More power is then fed into the electrical grid than the available output power PA. However, this output power PA slowly approaches the instantaneous power PI and reaches it at time t 2 .

[0124] The idealized increase in the instantaneous power PI compared to the output power PA between t 1 and t 2 results in a roughly triangular region, which is shown hatched in this diagram III. Its area corresponds to an energy referred to as the characteristic difference energy ΔE c.

[0125] The fourth diagram (IV) also shows the curve of the kinetic energy E k of the wind turbine. Between times t 1 and t 2 , the curve of the kinetic energy E k corresponds approximately to the negative integral of the power difference range shown hatched in diagram III. Ideally, the difference between the initial kinetic energy E 1 and the kinetic energy E 2 reached at time t 2 corresponds to the characteristic difference energy ΔE c .

[0126] It should be noted that these Figure 4serves for illustrative purposes and is idealized. Of course, it is particularly important to note that with the very sharp increase in instantaneous power PI shortly after time t 1, with the correspondingly sharp drop in the kinetic energy value E k, the speed n would also decrease more sharply than shown. This is indicated in the second diagram II with a dotted line and denoted as n'.

[0127] A correspondingly faster decrease in speed could also affect the curve of the output power PA in diagram III. However, a faster decrease in speed n would not necessarily lead to a correspondingly faster increase in the output power PA at the same time, because Figure 3 It can be seen that a faster drop in speed, i.e. a faster drop in tip speed ratio, would not necessarily lead to a faster increase in the power coefficient.

[0128] Thus, it was recognized that the characteristic differential energy ΔE c can be calculated to a good approximation from the product of the differential output power, i.e., P 2 minus P 1 , and the blade pitch time T θ , additionally multiplied by 1 / 2. Accordingly, it is proposed to select the throttle operating point and / or the reserve operating point such that the differential rotational energy is approximately equal to the characteristic differential energy ΔE c .

Claims

1. A method for controlling a wind power installation (100), wherein - the wind power installation (100) has a rotor (106) with rotor blades (108) with an adjustable blade angle (θ), and the rotor (106) can be operated with a variable rotational speed (n), and wherein - the wind power installation (100) is operated in a partial load range in which sufficient wind is not present to operate the wind power installation (100) with rated power (PN), - the wind power installation (100) can be operated as an operating point (302, 308, 310) which can be specified in a changing fashion, and - the operating point (302, 308, 310) is characterized by the blade angle (θ) and a tip speed ratio (A), and wherein - the operating point (302, 308, 310) can be represented in a λθ diagram, wherein - in the λθ diagram the tip speed ratio (A) is plotted against the blade angle (θ), and - the operating point (302, 308, 310) can be represented in the λθ diagram as a value pair composed of a blade angle (θ) and tip speed ratio (λ), wherein - each operating point (302, 308, 310) is assigned a power coefficient (Cp value), and - a plurality of operating points (302, 308, 310) with the same Cp value can be represented as an iso-characteristic curve in the λθ diagram, comprising the steps: - operating the wind power installation at a normal operating point (302) at which the wind power installation (100) is not operated in a throttled fashion if there is no request for throttling, - operating the wind power installation (100) at a throttled operating point (308) at which, in response to a throttle request, the wind power installation (100) is operated with output power (PA) which is throttled in comparison with the normal operating point (302), and - changing over the operation of the wind power installation from the throttled operating point (308) to a reserve operating point (310) at which the wind power installation (100) is operated with higher output power in comparison with the throttled operating point (308), in reaction to a power increase request wherein - the throttled operating point (308) has an increased tip speed ratio in comparison with the reserve operating point (310), and - the throttled operating point (308) lies on an associated iso-characteristic curve in the λθ diagram, and the associated iso-characteristic curve has, at the throttled operating point (308), a negative characteristic curve gradient at which the tip speed ratio decreases as the blade angle increases, characterized in that - the throttled operating point (308) and / or the reserve operating point (310) are selected in such a way that in each case - a derivative of the power coefficient (CP) of the throttled operating point (308) or of the reserve operating point (310) according to the blade angle exceeds a predetermined minimum derivative value in terms of the absolute value, in particular in such a way that - for the throttled operating point (308) the minimum derivative value is at least 0.3 / 6° (= 0.05 / 1°) and / or - for the reserve operating point (310) the minimum derivative value is at least 0.1 / 6°.

2. The method as claimed in claim 1, characterized in that the characteristic curve gradient at the throttled operating point (308) decreases with a value which is at least 0.5 / 1° in terms of absolute value.

3. The method as claimed in claim 1 or 2, characterized in that - the reserve operating point (310) - is operated with lower output power in comparison with the normal operating point (302), and / or - has a reduced tip speed ratio in comparison with the throttled operating point (308), and optionally - has an increased tip speed ratio in comparison with the normal operating point (302).

4. The method as claimed in one of the preceding claims, characterized in that - the throttled operating point (308) and / or - the reserve operating point (310) is selected as a function of - a distance between two iso-characteristic curves at the throttled operating point (308) or at the reserve operating point (310) and / or - a gradient of the power coefficient of the throttled operating point (308) or of the reserve operating point (310).

5. The method as claimed in one of the preceding claims, characterized in that - the wind power installation (100) can be characterized by a characteristic curve diagram composed of iso-characteristic curves in the λθ diagram, and - the characteristic curve diagram can be defined as a standardized characteristic curve diagram in which the blade angle is standardized to entire degrees (°), - in the standardized characteristic curve diagram a first sub-region is formed in which - the blade angles are greater than the blade angles of the normal operating point and - the iso-characteristic curves have a negative characteristic curve gradient in which the tip speed ratio decreases as the blade angle increases, - in the standardized characteristic curve diagram each operating point is characterized by a gradient value which quantifies the maximum gradient of the power coefficient of the operating point in the standardized characteristic curve diagram, - wherein in the first sub-region of the standardized characteristic curve diagram there is an operating point with a maximum gradient value for each iso-characteristic curve, and a gradient characteristic curve can be represented which connects all these operating points with a maximum gradient value, wherein - the throttled operating point (308) and / or the reserve operating point (310) are selected such that they each lie on the gradient characteristic curve, and / or - lie in a gradient band including the gradient characteristic curve (320), wherein - the gradient band has - an upper band limit (321) which lies above the gradient characteristic curve by a tip speed ratio difference, and - a lower band limit (322) which lies below the gradient characteristic curve by a lower tip speed ratio difference, wherein - the upper and lower tip speed ratio differences preferably each have at maximum the value 2, in particular at maximum the value 1, or - the upper tip speed ratio difference has at maximum the value 1 and - the lower tip speed ratio difference has at maximum the value 4.

6. The method as claimed in one of the preceding claims, characterized in that the wind power installation (100) has - at the throttled operating point (308) - a first output power level, - a first rotational energy level and - a first blade angle, and - at the reserve operating point (310) - a second output power level, - a second rotational energy level and - a second blade angle, wherein - the first output power level is lower than the second output power level, and a difference between the second output power level and the first output power level forms a differential output power level of the operating points, - the first rotational energy level (E1) is higher than the second rotational energy level (E2), and a difference between the first and second rotational energy levels forms a difference rotational energy level, - a blade adjustment time is required to adjust the rotor blades from the first blade angle to the second blade angle, - a product of the differential output power level with the blade adjustment time multiplied by 50% forms a characteristic differential energy level (ΔEC), and - the throttled operating point (308) and / or the reserve operating point (310) are selected in such a way that the difference rotational energy level lies above or below the characteristic differential energy level by a maximum predetermined percentage deviation value, wherein the percentage deviation value is at maximum 60%, preferably at maximum 40% and in particular at maximum 20%.

7. The method as claimed in one of the preceding claims, characterized in that - in order to set the throttled operating point, in particular starting from the normal operating point, an intermediate operating point is firstly set in a first adjustment step, wherein - the intermediate operating point has the same Cp value as the throttled operating point, at least a Cp value which is changed by less than 20%, in particular in that - in a second adjustment step there is a changeover from the intermediate operating point to the throttled operating point, and the changeover can be represented in the λθ diagram essentially as a change along an iso-characteristic curve, at least with a maximum deviation of 20% of the Cp value of the iso-characteristic curve of the throttled operating point.

8. The method as claimed in one of the preceding claims, characterized in that - the throttle request to request the operation of the wind power installation at the throttled operating point is embodied as an external specification which is received by a data interface, and / or - the power increase request to request the operation of the wind power installation at the reserve operating point is embodied as a prescription for reaction to a state of the electric supply network which is sensed by the wind power installation, in particular as a prescription to carry out a power increase starting from the throttled operating point (308) as a function of a sensed network frequency of the electric supply network.

9. The method as claimed in one of the preceding claims, characterized in that - the wind power installation (100) is operated for a longer time period at the throttled operating point (308) than at the reserve operating point (310), in particular at least 20 times as long.

10. A wind power installation (100) with - a rotor (106) with rotor blades (108) with an adjustable blade angle (θ), wherein the rotor (106) can be operated with a variable rotational speed, and wherein - the wind power installation (100) can be operated in a partial load range in which sufficient wind is not present to operate the wind power installation with rated power (PN), - the wind power installation (100) can be operated as an operating point (302, 308, 310) which can be specified in a changing fashion, and - the operating point (302, 308, 310) is characterized by the blade angle (θ) and a tip speed ratio (A), and wherein - the operating point (302, 308, 310) can be represented in a λθ diagram, wherein - in the λθ diagram the tip speed ratio (A) is plotted against the blade angle (θ), and - the operating point (302, 308, 310) can be represented in the λθ diagram as a value pair composed of a blade angle (θ) and tip speed ratio (A), wherein - each operating point (302, 308, 310) is assigned a power coefficient (Cp value), and - a plurality of operating points (302, 308, 310) with the same Cp value can be represented as an iso characteristic curve in the λθ diagram, and the wind power installation (100) has a control device (103), prepared to operate the wind power installation (100) in the partial load range, and at the same time prepares - to operate the wind power installation at a normal operating point (302) at which the wind power installation (100) is not operated in a throttled fashion if there is no request for throttling, - to operate the wind power installation (100) at a throttled operating point (308) at which, in response to a throttle request, the wind power installation (100) is operated with output power which is throttled in comparison with the normal operating point (302), and - to changeover the operation of the wind power installation from the throttled operating point (308) to a reserve operating point (310) at which the wind power installation is operated with higher output power in comparison with the throttled operating point (308), in reaction to a power increase request, wherein the operation of the wind power installation (100) is implemented in such a way that - the throttled operating point (308) has an increased tip speed ratio in comparison with the reserve operating point (310), and - the throttled operating point (308) lies on an associated iso-characteristic curve in the λθ diagram, and the associated iso-characteristic curve has, at the throttled operating point (308), a negative characteristic curve gradient at which the tip speed ratio decreases as the blade angle increases, characterized in that - the throttled operating point (308) and / or the reserve operating point (310) are selected in such a way that in each case - a derivative of the power coefficient (CP) of the throttled operating point (308) or of the reserve operating point (310) according to the blade angle exceeds a predetermined minimum derivative value in terms of the absolute value, in particular in such a way that - for the throttled operating point (308) the minimum derivative value is at least 0.3 / 6° (= 0.05 / 1°) and / or - for the reserve operating point (310) the minimum derivative value is at least 0.1 / 6°.

11. The wind power installation as claimed in claim 10, characterized in that the wind power installation, in particular the control device (103), is prepared to carry out a method as claimed in one of claims 1 to 9.

12. The wind power installation (100) as claimed in claim 10 or 11, characterized in that - a data interface is provided for receiving and evaluating an external signal as an external specification of the throttling request, and - a sensing device for sensing and evaluating a state of the electric supply network for checking for a power increase request as a prescription for reaction to the sensed state.

13. The wind power installation (100) as claimed in one of claims 10 to 12, characterized in that a control memory is provided, and in the control memory, - control prescriptions are stored for operating the wind power installation according to at least one method as claimed in one of claims 1 to 9, and / or - operating points are stored operating points which are dependent on the wind speed, in particular in each case a normal operating point, a throttled operating point and a reserve operating point are stored for a plurality of wind speeds.