METHOD FOR CONTROLLING THE ROTOR SPEED OF A WIND TURBINE

Abstract

Method for controlling a rotor speed of a rotor of a wind turbine at rated or curtailed operation conditions the rotor being an aerodynamic rotor having one or a plurality of rotor blades, and the wind turbine further having a tower and a generator wherein a pitch control provides a pitch angle set value depending on an actual rotor speed for setting a pitch angle of the rotor blades, a main control provides a main power or torque set value for controlling the power or torque of the generator, and an additional control provides an additional power or torque set value depending on the actual rotor speed , wherein the additional power or torque set value is provided as an offset value and is added to the main power or torque set value respectively, wherein the additional power or torque set value is calculated depending on a control deviation of the rotor speed, and optionally, in combination with the additional control, or instead of it, a maximum power control provides a maximum power value as a varying value for limiting an output power of the generator and the maximum power value is calculated depending on a predetermined power limit value, and depending on a predetermined reference duration, in order to provide for the reference duration an average power reaching or at least not exceeding the predetermined power limit value.

Claims

1. A method for controlling a rotor speed of a rotor of a wind turbine at rated or curtailed operation conditions the rotor being an aerodynamic rotor having one or a plurality of rotor blades, the wind turbine comprising a tower and a generator, the method comprising: using a pitch control to provide a pitch angle set value depending on an actual rotor speed for setting a pitch angle of the rotor blades, using a main control to provide a main power or torque set value for controlling the power or torque of the generator, and using an additional control to provide an additional power or torque set value depending on the actual rotor speed, wherein the additional power or torque set value is provided as an offset value and is added to the main power or torque set value, and wherein: the additional power or torque set value is calculated depending on a control deviation of the rotor speed, or a maximum power control provides a maximum power value as a varying value for limiting an output power of the generator, and the maximum power value is calculated depending on: a predetermined power limit value, and a predetermined reference duration, to provide for the reference duration an average power reaching or at least not exceeding the predetermined power limit value.

2. The method according to claim 1, wherein the additional power or torque set value is calculated depending on the control deviation of the rotor speed, and the maximum power control provides the maximum power value as the varying value for limiting the output power of the generator, and the maximum power value is calculated depending on: the predetermined power limit value, and the predetermined reference duration, to provide for the predetermined reference duration the average power reaching or at least not exceeding the predetermined power limit value.

3. The method according to claim 1, comprising: calculating the additional power or torque set value depending on the control deviation of the rotor speed using a nonlinear and/or a time variant control algorithm, and/or wherein the main control provides the main power or torque set value depending on the rotor speed.

4. The method according to claim 1, comprising: calculating the power or torque set value to counteract generator torque fluctuations caused by fluctuations of the rotor speed due to fluctuations in the wind speed, wherein the main control is designed to keep the power constant or reduce fluctuations of the power in case of fluctuating rotor speed, resulting in control related fluctuations of a generator torque, and wherein the additional power or torque set value is calculated to counteract such control related fluctuations of the generator torque.

5. The method according to claim 1, wherein the additional power or torque set value is calculated such that: a longitudinal tower load in a direction of an axis of rotation of the generator is reduced by applying the additional power or torque set value, and a lateral tower load perpendicular to the axis of rotation of the generator is reduced by applying the additional power or torque set value.

6. The method according to claim 1, comprising calculating the additional power or torque set value in dependence on at least one of the maximum power value or the rotor speed set value.

7. The method according to claim 1, wherein the additional power or torque set value is calculated by multiplying: a signal representative of the control deviation of the rotor speed, and a variable gain signal, wherein the variable gain signal is a signal representative of a scaled reference torque and/or calculated depending on: the maximum power value, and the rotor speed set value, and a gain factor, and/or a gain limiter, and/or a gain change rate limiter.

8. The method according to claim 7, comprising: calculating the variable gain signal by: calculating a reference torque value by dividing the maximum power value by the rotor speed set value, and multiplying the reference torque value with the gain factor.

9. The method according to claim 7, wherein: the gain signal is limited by the gain limiter, and/or a change rate of the gain signal is limited by a gain change rate limiter.

10. The method according to claim 7, comprising reducing loads on the tower of the wind turbine by: setting the gain factor in a range of 10% to 200%, and/or setting the gain factor in a range of 105% to 200%, and/or setting the gain factor in a range of 110% to 150%, and/or setting the gain factor to a value above 100% in order to increase a generator torque with increasing rotor speed, and/or calculating the gain factor depending on an overload capability of the generator, describing a capability of the generator to exceed a rated power value and/or a rated generator torque

11. The method according to claim 1, wherein the maximum power controller operates such that in a repeating manner for each current time: the average power is calculated for a time period having a length of the predetermined reference duration and ending at the current time, and the maximum power value is calculated depending on the calculated average power and a previously calculated maximum power value.

12. The method according to claim 1, wherein: the maximum power value is calculated to rise and/or to take values above the predetermined power limit, when the calculated average power is below the predetermined power limit, and the maximum power value is calculated to fall and/or to take values below the predetermined power limit, when the calculated average power is above the predetermined power limit.

13. The method according to claim 1, wherein: the maximum power value is used to amend the main power or torque set value, and/or the maximum power value is used to limit an overall power or torque set value defined as a sum of the main power or torque set value and the additional power or toque set value, and/or the maximum power value is used to amend the additional power or torque set value.

14. The method according to claim 1, wherein the additional power or torque set value, and/or the maximum power value is calculated such that: a load reduction is achieved without reducing an annual energy production, and/or an annual energy production is increased while not increasing the load when compared with the same control but without the additional control and/or without the maximum power control, respectively.

15. The method according to claim 1, wherein the predetermined reference duration is in a range of 5 to 30 minutes.

16. A wind turbine comprising: an aerodynamic rotor having one or more rotor blades, the wind turbine being adapted for adapted for controlling a rotor speed of an aerodynamic rotor at rated or curtailed operation conditions, a tower, a generator, a pitch control configured to provide a pitch angle set value depending on an actual rotor speed for setting a pitch angle of the rotor blades, a main control configured to provide a main power or torque set value for controlling the power or torque of the generator, and an additional control configured to provide an additional power or torque set value depending on the actual rotor speed, wherein the additional control is adapted such that the additional power or torque set value is provided as an offset value and is added to the main power or torque set value respectively, wherein the power or torque set value is calculated depending on a control deviation of the rotor speed.

17. The wind turbine according to claim 16, comprising a maximum power control for providing a maximum power value as a varying value for limiting an output power of the generator and the maximum power control is adapted such that the maximum power value is calculated depending on: a predetermined power limit value, and a predetermined reference duration, to provide for the predetermined reference duration an average power reaching or at least not exceeding the predetermined power limit value.

18. A windfarm comprising a plurality of wind turbines according to claim 16.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0093] Below the invention is explained by a way of examples using embodiments based on the attached figures.

[0094] FIG. 1 shows a wind turbine in a perspective view.

[0095] FIG. 2 shows a wind farm in a schematically view.

[0096] FIG. 3 shows a power to rotor speed curve explaining different aspects.

[0097] FIG. 4 shows a generator torque to rotor speed curve explaining different aspects.

[0098] FIG. 5 shows a control structure.

[0099] FIG. 6 shows two diagrams explaining the operation of a maximum power control.

DETAILED DESCRIPTION

[0100] FIG. 1 shows a schematic illustration of a wind power installation according to the invention. The wind power installation 100 comprises a tower 102 and a nacelle 104 on the tower 102. An aerodynamic rotor 106 comprising three rotor blades 108 and a spinner 110 is provided on the nacelle 104. The aerodynamic rotor 106 is caused to effect a rotational movement by the wind during operation of the wind power installation and thereby also rotates an electrodynamic rotor of a generator, which is coupled to the aerodynamic rotor 106 directly or indirectly. The electrical generator is arranged in the nacelle 104 and generates electrical energy. The pitch angles of the rotor blades 108 can be varied by pitch motors on the rotor blade roots 109 of the respective rotor blades 108.

[0101] The wind power installation 100 comprises an electrical generator 101, indicated in the nacelle 104. Electrical power can be generated by means of the generator 101. An infeed unit 105, which can be configured as an inverter, in particular, is provided for feeding in electrical power. It is thus possible to generate a three-phase infeed current and/or a three-phase infeed voltage according to amplitude, frequency and phase, for infeed at a network connection point PCC. That can be effected directly or else jointly with further wind power installations in a wind farm. An installation controller (including a processor or microprocessor) 103 is provided for controlling the wind power installation 100 and also the infeed unit 105. The installation controller 103 can also acquire predefined values from an external source, in particular from a central farm computer.

[0102] FIG. 2 shows a wind farm 112 comprising for example three wind power installations 100, which can be identical or different. The three wind power installations 100 are thus representative of basically an arbitrary number of wind power installations of a wind farm 112. The wind power installations 100 provide their power, namely in particular the generated current, via an electrical farm network 114. In this case, the respectively generated currents or powers of the individual wind power installations 100 are added and a transformer 116 is usually provided, which steps up the voltage in the farm in order then to feed it into the supply network 120 at the infeed point 118, which is also generally referred to as PCC. FIG. 2 is merely a simplified illustration of a wind farm 112. Moreover, by way of example, the farm network 114 can be configured differently, with for example a transformer also being present at the output of each wind power installation 100, to mention just one different exemplary embodiment.

[0103] The wind farm 112 additionally comprises a central farm computer (such as a processor or microprocessor) 122. The central farm computer can be connected to the wind power installations 100 via data lines 124, or in a wireless manner, in order thereby to exchange data with the wind power installations and in particular to acquire measured values from the wind power installations 100 and to transmit control values to the wind power installations 100.

[0104] FIG. 3 shows a power to rotor speed curve 300. Said curve is basically shown for sub-rated conditions, i.e., below rated rotor speed n.sub.N and below rated power P.sub.N. However the diagram also shows the optimal rated operating point 302. This optimal rated operating point is reached and used for rated wind speeds and wind speeds above. Accordingly at this rated operating point 302 the output power P is at the level of rated power and the rotor speed n is at rated rotor speed n.sub.N. If the wind further rises this optimal rated operating point will not change. Instead the rotor blades will be pitched in order to extract less power from the wind than it could. This way the optimal rated operating point 302 can be maintained.

[0105] However in case of wind gusts quick changes of the wind speed occur and to simplify the explanations it is assumed, unless explained differently, that during such short deviations of the wind no pitching of the rotor blades will take place. Accordingly it is assumed for the following explanations that the pitch angle for all rotor blades is constant.

[0106] A typically used control concept is to keep the power constant during such wind gusts. This conventional control is represented by the constant power curve 304. In case of a positive wind gust the rotor speed n will increase and the wind turbine is controlled such that the power is kept constant. The same applies if there is a negative wind gust resulting in a drop of wind speed. In that case the rotor speed n will drop but the power is still kept constant by the control of the wind turbine. It is to be mentioned that even though the rotor speed falls during such negative wind gust to values below rated rotor speed n.sub.N, the wind speed is still above nominal wind speed and accordingly the power will not drop. In other words even though the rotor speed falls below the rated rotor speed n.sub.N the operation is not necessarily changing back to a sub-rated operation.

[0107] However if the rotor speed increases and the power is kept constant as shown by the constant power curve 304, the generator torque will fall, as shown in FIG. 4 and as explained below. If the rotor speed drops below rated rotor speed n.sub.N while the power is kept constant, the corresponding generator torque will raise.

[0108] Such dropping or raising of the generator torque causes loads to the wind turbine and is to be reduced and accordingly a constant torque power curve 306 is suggested. Accordingly in case of a positive wind gust resulting in an increasing rotor speed n, the power is controlled to raise as well having the effect that the resulting generator torque is kept constant. The same constant torque power curve 306 is thus used for falling rotor speeds in particular due to negative wind gusts. Accordingly at falling rotor speed n the power shall also fall according to the constant torque power curve 306. This also results in keeping the generator torque constant. This result is also shown in FIG. 4 and will be explained below.

[0109] According to one embodiment it is even suggested to use an increasing torque power curve 308. As can been seen in FIG. 3 this increasing torque power curve is steeper when compared to the constant torque power curve 306. Accordingly with increasing rotor speed n the power is even further increasing for this increasing torque power curve 308 when compared to the constant torque power curve 306. This will result in even increasing generator torque power curve 308 with increasing rotor speed, as is explained in FIG. 4 below. Such increasing generator torque is in particular suggested in order to reduce longitudinal loads or oscillations as this avoids swinging forward of the tower after a positive wind gust has pushed the tower head backwards.

[0110] In addition in FIG. 3 there is also shown a power limitation curve 310 showing an instantaneous overall power limitation such as a power limit of the inverter for feeding electrical power.

[0111] FIG. 4 shows a torque to rotor speed curve 400 corresponding to the power to rotor speed curve 300 of FIG. 3. Accordingly the torque to rotor speed curve 400 is also basically shown for sub-rated operation conditions as long as this curve is below-rated rotor speed n.sub.N and below-rated generator torque M.sub.N. However the description herein is concerned with the operation at rated operation conditions or alternatively curtailed operation conditions.

[0112] FIG. 4 also shows an optimal rated operating point 402 which corresponds to the optimal rated operating point 302 of FIG. 3. Here the wind speed is at or above rated wind speed and this optimal rated operating point 402 is basically controlled by means of pitching the rotor blades. However as explained with respect to FIG. 3 it is assumed that for the purpose of explaining aspects of the control the pitch angle of the rotor blades is constant.

[0113] FIG. 4 also shows a standard generator torque curve 404 which is related to the constant power curve 304 of FIG. 3. If the power is controlled to a constant value as shown and explained with respect to FIG. 3, the standard generator torque curve 404 is shown in FIG. 4. Accordingly with increasing rotor speed n the generator torque M decreases.

[0114] To improve that in order to reduce loads a controller according to the constant torque power curve 306 as explained and shown in FIG. 3 is suggested and the constant generator torque curve 406 results. Accordingly with increasing or decreasing rotor speed the generator torque according to the constant generator torque curve 306 remains constant. This lowers loads.

[0115] A more aggressive control aspect is to provide a control according to the increasing torque power curve 308 as shown and explained with respect to FIG. 3. The corresponding generator torque is shown by the increasing generator torque curve 408. Accordingly the power increase with increasing rotor speed is that strong, that the generator torque even increases with increasing rotor speed n. As explained above this may have advantages with respect to a longitudinal oscillation or longitudinal load.

[0116] In addition there is also shown a torque limitation curve 410 which also corresponds to the power limitation curve 310 according to FIG. 3.

[0117] FIG. 5 shows a control structure 500 for controlling the wind turbine 502. The control structure 500 may of course be part of the wind turbine 502 as it may be implemented in a controller of a wind turbine 502. However the turbine 502 is thus basically representing the physical part and behavior of the wind turbine. The output of the wind turbine is the actual rotor speed n.sub.i which may be a measured rotor speed or otherwise determined. Based on this actual rotor speed n.sub.i and a rotor speed set value n.sub.s a pitch control 504 provides a control signal for pitching the rotor blades. This control signal is provided as a derivative of a pitch angle set value {dot over (α)}.sub.s i.e., it provides a pitch rate. This pitch control 504 is basically provided for controlling the wind turbine and thus the rotor speed in an optimal rated operation point, such as the optimal operating points 302 or 402 according to FIGS. 3 and 4. The rotor speed maybe depicted by “n” or “ω”. It only differs in the used physical unity (typically 1/s or rad/s).

[0118] In addition and in particular for quick responses there is also a main control 506 providing a main power set value P.sub.s. According to a different embodiment the main control 506 may provide a main torque set value instead.

[0119] Such main control 506 may be a conventional main control. The main control 506 has the actual rotor speed n.sub.i as an input value, but the main control 506 may as well provide a constant value for the main power or torque set value, at least for high rotor speeds.

[0120] However, the main control 506 used in this embodiment uses a look-up table for setting the main power set value P.sub.s. For rotor speeds at or above rated speed, in particular at or above a lower rotor speed limit value being below rated rotor speed, in particular being 0.5 to 1 rpm below rated rotor speed, the main power set value P.sub.s is set to high values above rated power. Accordingly the first limiter 512, that will further be explained below, may limit and thus reduce that value or a value based on that value. This way, the first limiter takes over the control for such high values of the main power set value P.sub.s.

[0121] There is also an additional control 508 that provides an additional power set value or an additional torque set value depending on which kind of control principle is applied, i.e., depending on whether the main control provides a main power set value as shown in FIG. 5 or a main torque set value according to a different embodiment not shown in FIG. 5. However any explanations given for the embodiment shown in FIG. 5 shall also apply for this second possible strategy using a main torque set value and using an additional torque set value.

[0122] The shown additional control 508 thus provides an additional power set value ΔP and this is added in the first summing point 510 to the main power set value P.sub.s. The result is the total power set value P.sub.sT. This value is limited by a first limiter 512 and the result is the limited power set value P.sub.sL. The first limiter 512 may be a safety element and most of the time the total power set value P.sub.sT may not reach the limit and in that case it is identical to the limited power set value P.sub.sL. Accordingly this limited power set value P.sub.sL is given as a corresponding power set value to the wind turbine 502 and accordingly the wind turbine is operated such that it provides an output power according to this inputted power set value.

[0123] However, if the main control 506 provides high main power set values P.sub.s the limited power set value P.sub.sL is also high and the first limiter 512 becomes more relevant. In order to avoid in that case that the additional power set value ΔP is cut off by the first limiter and thus deactivated, this additional power set value ΔP is also influencing the limit of the first limiter 512 via the 3.sup.rd summing point 542, also further explained below. Accordingly, while raising the total power set value P.sub.sT by the additional power set value ΔP, the limit of the first limiter 512 is synchronously raised and that way the additional power set value ΔP has full effect on the limited power set value P.sub.sL.

[0124] That may result in even higher values of the limited power set value P.sub.sL, i.e., above the maximum power value. However such too high values are acceptable as they appear not to be too big and there may also be a further limiter, in particular a limiter provided by an inverter for feeding the power into a supply grid.

[0125] Details of the additional control 508 are shown in the enlarged view at the bottom of FIG. 5. Input values for this additional control are the actual rotor speed n.sub.i, the rotor speed set value n.sub.s and a maximum power value P.sub.m. Using the second summing point 514 a rotor speed deviation Δn is calculated which is thus a control deviation of the rotor speed. Both terms are used synonymously. For this rotor speed deviation Δn a sign changer 515 achieves a reversal of the sign for this rotor speed deviation Δn which may further be limited by a first rate limiter 516 in order to avoid too quick changes of this rotor speed deviation. The reversal of the sign could also be implemented in other elements such as in the rate limiter 516. Instead, the inputs of the second summing point 514 could be changed.

[0126] In addition a reference torque M.sub.r is calculated by dividing the maximum power value P.sub.m by the rotor set value n.sub.s using a dividing element 518. This reference torque M.sub.r may also be limited by a second limiter 520. For adjusting or tuning the additional control 508 a gain factor 522 is provided. The reference torque, limited and multiplied by the gain factor may also be limited by a second-rate limiter 524 to avoid too quick changes of this value. The result is multiplied with the rate limited rotor speed deviation using the multiplying element 526. The result may also be limited by a third limiter 528. The result is the additional power set value ΔP.

[0127] If the additional control 508 shall according to the second embodiment provide an additional torque set value, the rotor speed deviation ΔM may be transformed into a relative rotor speed deviation Δn/n.sub.s. The remaining structure of the additional control 508 may remain unchanged. A possible amended part of the structure is indicated in the block 530 for this second embodiment. Accordingly a second dividing element 532 is placed between the second summing point 514 and the sign changer 515, or the first-rate limiter 516.

[0128] However going back to the first embodiment the additional control 508 works as follows. For simplifying the explanations the 2.sup.nd limiter 520 and the 2.sup.nd rate limiter 524 shown in the addition control 508 may be neglected. Accordingly if a wind gust occurs a rotor speed deviation Δn will result at the second summing point 514.

[0129] Neglecting dynamical behavior of the maximum power P.sub.m and the rotor speed set value n.sub.s the reference torque M.sub.r corresponds to the generator torque if the generator produces the maximum power P.sub.m and rotates at the rotor speed set value n.sub.s. Multiplying such reference torque M.sub.r with the rotor speed deviation Δn thus results in a power value which is exactly the additional power the generator would generate when operating with the reference torque but with a higher rotor speed according to the rotor speed deviation Δn. Accordingly this additional power set value ΔP is outputted by the additional control 508 and added to the main power set value P.sub.s at the first summing point 510. Accordingly the output power of the wind turbine will rise with increasing rotor speed such that the generator torque is kept constant.

[0130] These explanations given above assume a gain factor 522 to be 1 or 100%. Accordingly if this gain factor 520 is set to a higher value than 100% the overall power, i.e., the power produced by the wind turbine will rise even further resulting in the generator torque also rising even though not as strong as the power. If the gain factor 522 on the other hand is set to a value below 1, i.e., to a value between 0 and 1, the power is not rising that strong with increasing rotor speed so that the generator torque is not constant but also dropping. However it is dropping with a smaller amount when compared to not adding any additional power.

[0131] The control structure 500 also shows a maximum power control 540. Such maximum power control may provide a varying maximum power value as will be described below with respect to FIG. 6. However this maximum power value P.sub.m provides an input to the additional control 508 as explained. In addition the maximum power value may be added to the additional power set value at the third summing point 542. The result is used to change the limit of the first limiter 512. This way the limited power set value P.sub.sL may vary in order to get the most possible annual energy production without violating any power limits.

[0132] At least for rotor speeds at or above rated rotor speed, or at or above the lower rotor speed limit explained above, the main power set value P.sub.s and also the total power set value P.sub.sT will most of the time be above the limit of the first limiter 512. Accordingly raising or lowering the limit will result in raising or lowering the limited power set value P.sub.sL. To ensure that the limited power set value P.sub.sL also depends on the additional power set value ΔP, this additional power set value ΔP is added at the 3.sup.rd summing point to the maximum power value P.sub.m and the result is used for setting or adjusting the limit value of the first limiter 512 and thus changing the limited power set value P.sub.sL accordingly.

[0133] Providing such corrected maximum power value is done by the maximum power control 540 as will be explained below with respect to FIG. 6.

[0134] FIG. 6 shows two diagrams, both showing a time series of a power value P for a time period from t.sub.0 to t.sub.1 having a length of a reference duration T. The time ti shows the actual time or current time. The time period from t.sub.0 to t.sub.1 also changes from one current time to the next current time and that is indicated by the arrows at t.sub.0 and t.sub.1 in both diagrams. Both diagrams also show a power limit value P.sub.L as a dashed line which shall not be exceeded by an average power P.sub.a which is shown by a dotted line.

[0135] The upper diagram A shows a situation which shall be improved and accordingly diagram B shows the improved situation and it shows the result of the operation implemented in the maximum power control 540 shown in FIG. 5.

[0136] According to diagram A the time series of the power value P is always below the power limit value P.sub.L. When for any reason the power value P drops, the control tries to raise the power value P again, but only up to the power limit value P.sub.L. Accordingly, the average power P.sub.a is not reaching the power limit value P.sub.L but is staying below it with a certain distance.

[0137] According to diagram B the time series of the power value P is not always below the power limit value P.sub.L. When for any reason the power value P drops, the control also tries to raise the power value P again. Therefor the left sides of both diagrams are similar. However, in diagram B, the power value is not only raised up to the power limit value P.sub.L but also above it.

[0138] This is done by increasing the power value P even above the power limit value P.sub.L when the average power P.sub.a is below the power limit value. The result is shown in diagram B and accordingly 20 the power value P was raised above the power limit value P.sub.L but was also reduced again to avoid that the average power P.sub.a would also rise above the power limit value P.sub.L.

[0139] FIG. 6 shows for the average power P.sub.a only a snap-shot at time t.sub.1 illustrating the average power value P.sub.a as a constant value. However the average power P.sub.a is also be a dynamically changing value. That is illustrated by the chain line P.sub.adyn in diagram B.

[0140] As long as P.sub.adyn according to the chain line is below the power limit value P.sub.L the maximum power control calculates a maximum power P.sub.m that may be above the power limit value P.sub.L. However to provide a continuous curve the maximum power P.sub.m will not immediately be raised above the maximum power P.sub.m. In the diagram B that firstly happened at t.sub.x. But at t.sub.1 (the current time) P.sub.adyn according to the chain line reaches the power limit value P.sub.L and thus the maximum power value P.sub.m is lowered back to the power limit value P.sub.L and may be lowered further. Of course such lowering of the maximum power value P.sub.m may start before the average power reaches the power limit value P.sub.L.

[0141] In diagram B the right part of the power value is also depicted as in theory the maximum power value P.sub.m should be identical to the actual power value P. However, the curve shown on the left side of the diagram shows a drop in the power value which was not controlled by the maximum power value but might be a result of a negative wind gust.

[0142] The embodiments described herein are thus based on realizing that a known control rule aiming to basically keep the output power constant during rated operation at least partially leads to a significant drop in the generator torque. As a result the rotor additionally accelerates and initiates lateral oscillations. Such initiation and thus additional loads during operation shall be avoided or at least reduced.

[0143] The present disclosure may be part or an additional feature of the control software of the wind turbine, in particular that part for determining of actual power set values by the operational control. On the one hand the disclosure may reduce oscillations of the tower and thus reduce of loads during operation, and on the other hand it shall support the pitch-rotor speed-control.

[0144] In conventional operating control during rated operation a constant power is controlled, as was explained with respect to FIG. 3. It is to be noted that in over speed situation as well as in under speed situations, as far as possible, a constant power (usually the rated power) is controlled. In particular at under speed situations the power is kept constant due to assuming that a negative wind gust will last only for a short time. Due to P=M*ω the control of a constant power leads to a reduction of the generator torque with increasing rotor speed or it leads to an increase of the generator torque with dropping rotor speed as shown in FIG. 4. The generator torque acts in both cases in favor of the acceleration of the rotor or deceleration of the rotor caused by the wind. It is known that the acceleration or deceleration of the rotor respectively lead to stimulating of a lateral tower oscillation. Accordingly influencing the change speed of the torque can result in a reduction of tower loads. As an additional note since the rotor speed is driven aerodynamically the desire is to control the fast generator torque.

[0145] The rate of change is influenced by adding an offset to the actual power set value resulting in a levelling or even increasing of the generator torque. This power offset is determined using the mentioned relationship between power, torque and rotor speed. In FIGS. 3 and 4 there are in addition to the common power to rotor speed curve or torque to rotor speed curve the power or torque targets shown (dashed line) which result after applying the additional control and thus after applying the additional power set value (the power offset). In order to illustrate the different possibilities the version of the constant torque as well as the increased torque at over speed values were shown. In these illustrations there are in addition limits depending on the rotor speed shown which for example may be restrictions given by the power electronics. When applying such corresponding power set value these limits must be met.

[0146] FIG. 5 shows a block diagram for illustrating the dependencies and the ways for calculating. The calculation of the output value happen by means of the following steps: [0147] (1) Determining a reference torque for the actual operating condition with considering the actual rotor speed set value and the corresponding maximum power. [0148] (2) Determining the actual control deviation of the rotor speed based on the actual rotor speed and the rotor speed set value. As the rotor speed set value may change stepwise, the change rate in time of the control deviation of the rotor speed is limited in time, thereby avoiding power steps. [0149] (3) Determining a variable gain factor based on which in combination with the control deviation of the rotor speed the offset for the power set value is calculated as followed: [0150] (a) The variable gain factor is a product of the determined reference torque and a gain factor which is typically in the range of 100% to 150%. [0151] (b) The gain factor, in particular a static gain factor enables the parameterization which may besides the possibility of keeping the torque constant, also be used to deliberately raise the torque value for further load reduction and for supporting the pitch-rotor speed control. [0152] (c) As the reference values, rotor speed set value or maximum power value, by which the reference torque is defined may changes in a stepwise manner, the timely change rate of the variable gain factor is finally limited, in order to avoid stepwise changes in the power. [0153] (4) The offset of the power set value is a result of the product of the variable gain factor and of the control deviation of the rotor speed determined at step (2). [0154] (5) Finally the offset of the power set value is limited in positive as well as in negative direction in order to, e.g., consider limits of the hardware. [0155] (6) The suggested function is effectively finalized when this offset of the power set value is added to the actual power set value. Accordingly this describes the additional power set value to the main power set value.

[0156] During calculations, it was realized that with applying the above described method an increase of the average power results and accordingly there is effectively an increase of the annual energy production. Causes for the increase of the average power are: [0157] Nonlinearity of the Product of the Torque and the Rotor Speed [0158] The rotor speed control works against an instable control path, which at over speed accelerates quicker than it decelerates [0159] Stronger drops in wind power and thus under speed values of the rotor speed will already be reduced according to the power control and operational management due to reducing the generator torque. This already takes place without the suggested additional control.

[0160] The this way resulting additional power would in general lead to an increased tower load. The tower load reduction received by levelling the generator torque counteracts this, so that these effects together result in an increase of the annual energy production by keeping the loads approximately constant.

[0161] If the average increase of the power is for certain reasons not possible, e.g., due to limits of the electrical supply grid, there is a coupling of the above described method using the additional power or torque control with a temporary power boost (TPB) algorithm possible. The temporary power boost (TPB) algorithm corresponds to the described maximum power control. This way the TPB algorithm takes over the object to supervise the average power and in case of exceeding a limit value to reduce the power set value gradually or in case of falling below the limit value to gradually increase the power set value. This way a reference for the additional power or torque control is also gradually reduced or gradually increase respectively. According to FIG. 5 such relationship is indicated by corresponding arrows (cf. P.sub.m, P.sub.sL). In the meantime the generator torque is further as far as possible kept constant or purposely increased (depending on the parametrization) which is done by the additional control providing an additional power or torque set value. This combination thus leads to a reduction of loads by keeping the annual energy production basically constant. Keeping the annual energy production basically constant could be seen as the main result of the TPB algorithm, whereas the load reduction is mainly a result of the additional control.

[0162] Accordingly depending on the coupling of the additional control with or without said temporary power boost two different possibilities of use are suggested: [0163] (1) A load reduction by maintaining the annual energy production (this is in particular achieved with coupling with the temporary power boost algorithm). [0164] (2) An increase of the annual energy production with keeping the load basically constant (basically without coupling of the temporary power boost algorithm).

[0165] The disclosure is suggested to be implemented on a control of the wind turbine in real-time. In general the application is possible for new wind turbines, as well as on existing wind turbines. The possibility of the application may also depend on reserves of the electrical components as well as given boundary conditions due to official provisions.

[0166] In addition the disclosure may also lead to reducing loads of pitch components due to reduce travelling distances.

[0167] Accordingly as described above a solution was provided that avoids quick changes of the generator torque leading to a load reduction by keeping the annual energy production on its level, or an increase of the annual energy production by keeping loads on its level. In addition loads on pitch components are reduced by increasing the generator torque, and the rotor speed control of the pitch system is facilitated.

[0168] The disclosure is basically in addition to so far existing control concepts or control software of wind turbines. FIG. 5 shows how this additional control is integrated in an existing structure. This is shown in FIG. 5 as a block diagram. Accordingly the embodiments can be understood by a kind of additional module for power control, not needing a new development. Of course such power control is used for controlling the rotor speed. The input values for the suggested additional control are the actual rotor speed set value as well as a maximum power value, which must be met. The additional output power which may be positive as well as negative is added to the main power set value, at least according to one strategy.

[0169] The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.