METHOD FOR CONTROLLING A WIND TURBINE AND CORRESPONDING WIND TURBINE

Abstract

A method for controlling a wind turbine, the wind turbine having a generator with controllable generator torque and an aerodynamic rotor with rotor blades with adjustable pitch angle, the aerodynamic rotor driving the generator with variable rotor speed depending on a wind speed, comprising the steps operating the wind turbine in a subrated mode when the wind speed is below a predetermined rated wind speed, operating the wind turbine in a rated mode when the wind speed is at or above the predetermined rated wind speed, estimating the wind speed and operating the wind turbine in subrated mode or in rated mode in dependence on the estimated wind speed.

Claims

1. A method for controlling a wind turbine, the wind turbine having a generator with controllable generator torque and an aerodynamic rotor with a plurality of rotor blades with adjustable pitch angles, the aerodynamic rotor driving the generator with variable rotor speed depending on a wind speed, the method comprising: operating the wind turbine in a subrated mode when the wind speed is less than a predetermined rated wind speed, operating the wind turbine in a rated mode when the wind speed is greater than the predetermined rated wind speed, estimating the wind speed, and operating the wind turbine in the subrated mode or in the rated mode in dependence on the estimated wind speed.

2. The method according to claim 1, further comprising: operating the wind turbine in a transient mode, when the operating mode changes from the subrated mode to the rated mode or vice versa, and wherein the operating the wind turbine, comprises operating the wind turbine in subrated mode, in the transient mode, or in the rated mode depending on the estimated wind speed.

3. The method according to claim 1, further comprising: measuring the rotor speed, in the subrated mode, determining a set value for the pitch angles depending on the measured rotor speed, and determining a generator torque setpoint based on the measured rotor speed, adjusting the pitch angles of the plurality of rotor blades depending on the set value, and adjusting the generator torque depending on the determined generator torque setpoint.

4. The method according to claim 3, wherein at least in the subrated mode, the method comprises determining the set value for the pitch angle based on a predetermined relationship between rotor speed values and pitch angle values.

5. The method according to claim 1, wherein the generator is a direct driven synchronous generator having a generator rotor with permanent magnets.

6. The method according to claim 1 wherein at least in the subrated mode and in the rated mode: a rotor thrust of the aerodynamic rotor is controlled based on the estimated wind speed to limit a maximum rotor thrust or a maximum rotor thrust change.

7. The method according to claim 1, wherein: in at least one mode chosen from the rated mode and the transient mode, when the operating mode changes over from the subrated mode to the rated mode, the method comprises: controlling the rotor speed based on a rotor speed setpoint, and determining the rotor speed setpoint depending on an estimated wind speed.

8. The method according to claim 1, wherein: in at least at least one mode chosen from the rated mode and the transient mode, when the operating mode changes over from the subrated mode to the rated mode, the method comprises: controlling the rotor power based on a rotor power setpoint, and determining the rotor power setpoint depending on an estimated wind speed.

9. The method according to claim 1, further comprising: starting the wind turbine, wherein starting the wind turbine includes determining a set value for the pitch angle based on a predetermined relationship between rotor speed values and pitch angle values, wherein determining the set value comprises using a look-up-table, wherein the pitch angle is reduced with increasing rotor speed, and wherein the starting comprises starting with an idling pitch angle of more than 20° until an operational pitch angle of less than 10° has been reached.

10. The method according to claim 1, further comprising at least one step chosen from: determining a pitch angle pitch rate setpoint based on a pitch angle setpoint for controlling the pitch angle in the subrated mode determining a rotor speed pitch rate setpoint based on a rotor speed setpoint for controlling the rotor speed at least in the rated mode, determining a rotor power pitch rate setpoint based on a rotor power setpoint for controlling a rotor power at least in the rated mode, determining a rotor thrust pitch rate setpoint based on a rotor thrust setpoint for controlling a rotor thrust.

11. The method according to claim 1, wherein_the pitch angle of each rotor blade is adjusted based on at least one input variable chose from: a measured pitch angle, a pitch angle pitch rate setpoint, a rotor speed pitch rate setpoint, a rotor power pitch rate setpoint, and a rotor thrust pitch rate setpoint, wherein the rotor thrust pitch rate setpoint has a minimum limit having an absolute value.

12. The method according to claim 1 comprising: controlling a rotor power in a feed-forward control based on a rotor power setpoint and on at least one input variable chosen from: a measured rotor speed, a measured pitch angle, and the estimated wind speed.

13. The method according to claim 2, wherein: in the subrated mode, the generator torque is adjusted based on a measured rotor speed, in the transient mode, the rotor power is increased with increasing wind speed based on the estimated wind speed, and in the rated mode, the rotor power is controlled to a constant value based on an estimated wind speed in a feed-forward control and/or a rotor thrust setpoint is decreased with increasing wind speed and is set based on the estimated wind speed in a feed-forward control.

14. A wind turbine comprising: a generator with controllable generator torque, an aerodynamic rotor with a plurality of rotor blades with adjustable pitch angles, wherein_the aerodynamic rotor is configured to drive the generator with variable rotor speed depending on a wind speed, a wind speed estimator configured to estimate the wind speed; and a turbine controller configured to: operate the wind turbine in a subrated mode when the estimated wind speed is less than a predetermined rated wind speed, and operate the wind turbine in a rated mode when the estimated wind speed is greater than the predetermined rated wind speed.

15. The wind turbine according to claim 14, wherein the turbine controller is configured to: operate the wind turbine in a subrated mode when the wind speed is below a predetermined rated wind speed, operate the wind turbine in a rated mode when the wind speed is at or above the predetermined rated wind speed, estimate the wind speed, and operate the wind turbine in the subrated mode or in the rated mode in dependence on the estimated wind speed.

16. The wind turbine according to claim 14, wherein the generator is designed as a direct driven synchronous generator.

17. The wind turbine according to claim 16, wherein the direct driven synchronous generator has a generator rotor with permanent magnets.

18. The method according to claim 4, wherein determining the set value comprises using a look-up-table for the predetermined relationship between rotor speed values and pitch angle values.

19. The method according to claim 1 comprising: controlling a rotor thrust in a feed-forward control based on a rotor thrust setpoint and on at least one input variable chosen from: a measured rotor speed, a measured pitch angle, and the estimated wind speed.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0089] The i.e., may now be explained by way of example based on an embodiment taking the enclosed Figures into account.

[0090] FIG. 1 shows a wind power installation in a perspective view.

[0091] FIG. 2 is a structural diagram of a wind turbine controller according to one aspect.

[0092] FIG. 3 is a structural diagram of a wind turbine controller according to FIG. 2 in more detail.

[0093] FIG. 4 is a structural diagram of a wind turbine controller according to FIG. 3 in more detail.

[0094] FIG. 5 is a structural diagram of a wind turbine controller according to FIG. 4 in more detail.

DETAILED DESCRIPTION

[0095] FIG. 1 shows a wind power installation 100 comprising a tower 102 and a nacelle 104. A rotor 106 having three rotor blades 108 and a spinner 110 is arranged on the nacelle 104. The rotor 106 is set in rotation motion by the wind during operation and drives a generator in the nacelle 104 as a result.

[0096] FIG. 2 illustrates the control concept according to one aspect in a fairly schematic overview. Accordingly, there is the turbine block 200 basically representing the wind turbine. For controlling the wind turbine according to wind turbine block 200 there is shown a wind turbine controller 202 receiving the pitch angles of all three blades θ1, θ2 and θ3, and the measured rotor speed Ω.sub.rot. The wind turbine is according to one aspect a direct drive wind turbine.

[0097] Based on the measured blade angles and the measured rotor speed the wind turbine controller 202 outputs a pitch rate setpoint for each of the three rotor blades {dot over (θ)}.sub.Ω1.sup.sp, {dot over (θ)}.sub.Ω2.sup.sp, {dot over (θ)}.sub.Ω3.sup.sp and the generator torque setpoint T.sub.gen.sup.sp. These setpoints are inserted in the turbine block 200 and will there be considered accordingly. I.e., the pitch actuators will operate according to the pitch rate setpoint and the generator torque will also be set according to its setpoint. Preferably the generator is a synchronous generator with permanent magnets. In this case the generator torque is adjusted by means of adjusting at least one stator current.

[0098] FIG. 3 shows some more details of FIG. 2 and the wind turbine controller 202 of FIG. 2 is illustrated in FIG. 3 as a rectangular with a dashed line.

[0099] Accordingly, the wind turbine controller includes a generator torque controller 304 and a pitch rate controller 306. The generator torque controller 304 just receives the rotor speed Ω.sub.rot. Based on that the generator torque controller generates the generator torque setpoint T.sub.gen.sup.sp. The generator torque setpoint is inserted into the turbine block 200 but it is also used and thus inserted in the pitch rate controller 306.

[0100] The pitch rate controller 306 also receives the rotor speed Ω.sub.rot and the measured blade angles θ1, θ2 and θ3. Based on that the pitch rate controller 306 calculates the pitch rate setpoint for all three blades and inserts this into the turbine block 200. Again, the wind turbine is thus controlled accordingly.

[0101] In FIG. 4 further details of the pitch rate controller 306 according to FIG. 3 are shown. The turbine block 200 as well as the generator torque controller 304 are illustrated in the same manner as in FIG. 3. However, the pitch rate controller 306 according to FIG. 3 is split into a pitch constraints block 408 and several blocks each outputting in individual pitch rate setpoint. Accordingly, there is a pitch angle controller 410 outputting a pitch rate setpoint in order to control the pitch angle. This pitch rate setpoint is denoted by a pitch angle pitch rate setpoint. For controlling the rotor speed there is the rotor speed controller 412 outputting a pitch rate in order to control the rotor speed. The pitch rate setpoint outputted by the rotor speed controller 412 is denoted by a rotor speed pitch rate setpoint.

[0102] For controlling the rotor power there is a rotor power controller 414 outputting a pitch rate setpoint in order to control the rotor power. That pitch rate setpoint is denominated rotor power pitch rate setpoint.

[0103] For controlling the rotor thrust, in particular for limiting the rotor thrust, there is provided a rotor thrust controller 416. Again the rotor thrust controller 416 also controls the rotor thrust by outputting a pitch rate setpoint. That pitch rate setpoint is denominated as rotor thrust pitch rate setpoint.

[0104] All four pitch rate setpoints, i.e., the pitch angle pitch rate setpoint, the rotor speed pitch rate setpoint, the rotor power pitch rate setpoint and the rotor thrust pitch rate setpoint are inserted into the pitch constraints block 408. The pitch constraints block 408 basically generates one pitch rate setpoint for each blade to be submitted to the turbine block 200. Accordingly, the pitch rate setpoint outputted by the pitch constraints block 408 is generally the result of said four pitch rate setpoints inputted into the pitch constraints block 408.

[0105] However, the pitch angle controller 410, the rotor speed controller 412, the rotor power controller 414 and the rotor thrust controller 416 may not be operating all at the same time. The rotor thrust controller 416 provides a minimum value. However, the rotor thrust pitch rate setpoint may particularly only be a limit. Accordingly, the final pitch rate setpoint, outputted by the rotor thrust constraints block 518 of FIG. 5, should always be at or above the value of the rotor thrust pitch rate setpoint. In other words, if any pitch rate setpoint falls and reaches that limit of the rotor thrust controller 416, i.e., falls below the rotor thrust pitch rate setpoint, this limit overrules any other pitch rate setpoints and avoids the pitch rate setpoint to become even smaller.

[0106] The pitch angle controller 410 is preferably only active during subrated mode, whereas the rotor speed controller 412 and the rotor power controller 414 may only be active during transient mode and rated mode. Accordingly, the control concept may switch between the pitch angle controller 410 on the one hand and the rotor speed controller 412 and the rotor power controller 414 on the other hand.

[0107] One possibility to consider the rotor speed pitch rate setpoint and the rotor power pitch rate setpoint at the same time is to add both pitch rate setpoints together in a weighted function. Underlying this concept is the idea that rotor speed and rotor power will basically be at least partially coupled and thus both pitch rate setpoints may also be similar, at least they will usually have the same prefix, i.e., both be positive or both be negative at the same time.

[0108] The pitch constraints block 408 also considers the generator torque setpoint generated by the generator torque controller 304. The pitch constraints block 408 makes use of the wind speed estimator and the wind speed estimator makes use of the generator torque setpoint.

[0109] FIG. 5 shows further details of the control structure. The turbine block 200 and the generator torque controller 304 are illustrated the same way as in the previous FIG. 4. FIG. 5 shows more details on how to generate the pitch angle pitch rate setpoint, the rotor speed pitch rate setpoint, the rotor power pitch rate setpoint and the rotor thrust pitch rate setpoint. The pitch constraints block 408 is also shown in more detail. However, said four pitch rate setpoints still form the input and are according to FIG. 5 inputted into a rotor thrust constraints block 518. The rotor thrust constraints block basically forms one single pitch rate setpoint and this is calculated in that way that the rotor thrust pitch rate setpoint is considered as a limit. The result is inputted into the pitch angle synchronization block 520, which also considers the measured pitch angles of all blades.

[0110] The pitch angle synchronization block 520 thus outputs a pitch rate setpoint also considering all measured pitch angles and inputs this result into a limitation block 522. The limitation block 522 provides a pitch angle limitation, a pitch rate limitation and a pitch acceleration limitation. Accordingly, if the pitch rate setpoint provided by the pitch angle synchronization block 520 is somehow resulting in exceeding any of said limits, the pitch rate setpoint will be amended accordingly. The limitation block 522 therefore considers the measured pitch angles and it also considers an estimated wind speed.

[0111] In FIG. 5 there is also as a further detail shown that the measured pitch angles are also filtered by a pitch angle filter 524. The measured rotor speed is also filtered by a rotor speed filter 526.

[0112] The estimated wind speed is provided by a wind speed estimator 528. The wind speed estimator receives as its input variables the measured rotor speed, the measured pitch angles and the generator torque setpoint. The wind speed estimator 528 basically works such that an equilibrium is found according to which the generator torque setpoint matches a rotor torque which depends on the measured rotor speed and the measured pitch angles.

[0113] The wind speed may be estimated depending on the rotor speed, the pitch angle and the generator torque setpoint. According to one aspect a look-up-table may be used having said three variable as input variables

[0114] For calculating the pitch angle pitch rate setpoint there is provided a pitch angle block 530 that transforms a pitch angle difference to a pitch rate. The pitch angle difference is received by substracting from a pitch angle setpoint which is calculated by means of a pitch angle setpoint block 532 a measured pitch angle. This is done in the angle substraction block 534. The pitch angle setpoint block 532 just calculates a pitch angle setpoint based on the measured rotor speed.

[0115] For calculating the rotor speed pitch rate setpoint a rotor speed block 536 is provided that receives a measured pitch angle as well as a speed difference and based on that it calculates the rotor speed pitch rate setpoint. The rotor speed difference is calculated by substracting from a rotor speed setpoint the measured rotor speed in the speed substraction block 538. The rotor speed setpoint is calculated by the rotor speed setpoint block 540 and the rotor speed setpoint block 540 receives as its input value the estimated wind speed. Accordingly, the rotor speed setpoint is calculated just based on the estimated wind speed.

[0116] For calculating the rotor power pitch rate setpoint there is provided a rotor power block 542. The rotor power block for calculating the rotor power pitch rate setpoint considers the measured rotor speed, the measured blade angle and the estimated wind speed. The estimated wind speed is considered directly and also indirectly by considering a rotor power setpoint which is calculated by means of the rotor power setpoint block 544 based on the estimated wind speed. As can be seen the rotor power pitch rate setpoint is calculated just by a feed-forward control. There is no comparison of a set value with a corresponding measured value, such as the angle subtraction block 534 or the speed substraction block 538. Thus, the rotor power pitch rate setpoint is determined by a feed-forward control.

[0117] The rotor thrust pitch rate setpoint is calculated by a rotor thrust block 546. The rotor thrust block also receives as input values the measured rotor speed, the measured pitch angle and the estimated wind speed. The rotor thrust block 546 receives the estimated wind speed also directly and indirectly by a rotor thrust setpoint determined by the rotor thrust setpoint block 548 based on the estimated wind speed. The rotor thrust pitch rate setpoint is also, as the rotor power pitch rate setpoint, determined by a feed-forward control and thus without any comparison of a set value with a measured value.

[0118] The i.e., or embodiments thereof can further be explained by the following Information.

[0119] In general, the following acronyms are used: [0120] NTM: normal turbulence model [0121] P: proportional (controller) [0122] PD: proportional-differential (controller) [0123] PDD: proportional-double differential (controller) [0124] SUMA: short unweighted moving average [0125] LUMA: long unweighted moving average

[0126] For power production the variable speed wind turbine having three blades with individual pitch is provided with a wind turbine controller. The wind turbine controller consists of a generator torque and pitch rate controller. The objective of the turbine controller is to optimize power production and to remain within the design loads and sound levels of the wind turbine. The optimum refers to a steady power production at rated conditions and a maximum power production at subrated conditions. Hence, the objective is divided into two control strategies: [0127] subrated operation: At subrated operation the production of power is maximized; and [0128] rated operation: For rated operation the production of rated power is maintained.

[0129] According to one aspect a direct drive wind turbine is suggested which is equipped with 3 pitch servo drives to actuate a desired pitch rate for the 3 blades and a power converter to actuate a desired torque for the generator.

[0130] As shown in FIG. 2, the wind turbine controller computes a pitch rate setpoint for the three pitch drives and a generator torque setpoint for the power converter. The controller algorithm for computing the setpoints requires the following input signals: [0131] the measured pitch angles of the 3 blades; and [0132] the measured angular velocity (speed) of the rotor.

[0133] Furthermore, the controller algorithm also requires: [0134] a generator power setpoint as the maximum actuated generator power; [0135] a rotor speed ratio setpoint to limit the rotor speed setpoint; [0136] a sound level reduction setpoint to set sound reduced operation; and [0137] a turbine state to communicate the turbine operation.

[0138] As illustrated in FIG. 2.2, the wind turbine controller consists of a generator torque and pitch rate controller.

Details of the Generator Torque Controller:

[0139] A generator torque setpoint is computed by means of a rotor speed measurement. The generator controller operates in the following 3 modes:

1. Subrated

[0140] For a measured rotor speed smaller than the rated generator speed the generator operates at subrated condition. A torque smaller or equal to the rated generator torque is selected from the generator speed-torque table such that the available rotor power for production is maximized.

2. Rated

[0141] For a measured rotor speed larger or equal to the rated generator speed the generator operates at rated condition. A torque smaller or equal to the rated generator torque is computed such that the rated generator power is maintained. During rated operation a transition to above-rated power is feasible if the above-rated wind condition is met. Then for a measured rotor speed larger or equal to the above-rated generator speed a torque smaller or equal to the above-rated generator torque is computed to yield above-rated generator power.

3. Transient

[0142] When the generator operates at rated condition and the measured rotor speed decreases below the rated generator speed the generator switches to a transient condition. In the transient mode the following substates are distinguished: [0143] (a) primary, preserve rated generator torque and return to rated operation; and [0144] (b) secondary, if (a) is unfeasible the generator returns to subrated operation.

[0145] The substates are being spanned by a set of rotor speed and generator torque points. The generator torque controller features bridging operation to avoid excitation of the 1st tower resonance frequency caused by tower passage of the blades. The bridging operation is setup by a smooth function which reduces the generator torque setpoint near the 1st tower resonance frequency matching rotor speed.

Details of the Pitch Rate Controller:

[0146] As shown in FIG. 4, the pitch rate controller from FIG. 3 can be divided into the following controllers: [0147] pitch angle control; [0148] rotor speed control; [0149] rotor power control; and [0150] rotor thrust control.

[0151] The pitch rate setpoints from the controllers are subjected to pitch constraints as shown in FIG. 4.

[0152] The controllers and pitch constraints from FIG. 5 can be explained as follows:

[0153] The pitch angle controller from FIG. 4 computes a pitch rate setpoint {dot over (θ)}.sub.θ.sup.sp using a pitch angle setpoint θ.sup.sp and measurement θ.sup.mv:

[00001]$\begin{array}{cc}{\stackrel{.}{\theta }}_{\theta }^{\mathrm{sp}}=H_{\theta }^c.Math.{\theta }^{\mathrm{sp}}& \left(1\right)\end{array}$

[0154] Let H.sub.θ.sup.c be a proper and stable closed loop transfer function related to the pitch angle-to-pitch rate block diagram 530 in FIG. 5. The pitch angle setpoint is obtained from a predefined rotor speed-pitch angle table between the minimum and maximum subrated generator speed. The pitch angle controller is a proportional (P) feedback controller which is designed for reference tracking to maximize power production.

[0155] During power production a wind speed .Math. is estimated using a rotor speed and pitch angle measurement Ω.sup.mv, θ.sup.mv and a generator torque setpoint T.sub.gen.sup.sp:

[00002]$\begin{array}{cc}\hat{U}=f\left({\Omega }^{\mathrm{mv}},{\theta }^{\mathrm{mv}},T_{\mathrm{gen}}^{\mathrm{sp}}\right)& \left(2\right)\end{array}$

[0156] Let f be the transfer function as a result of the estimation problem. The objective is to find an estimate such that the estimated rotor (aerodynamic) torque matches the reconstructed rotor (aerodynamic) torque.

[0157] By means of the estimated wind speed of eq: (2) a short unweighted moving average (SUMA) and long unweighted moving average (LUMA) wind speed is computed:

[00003]$\begin{array}{cc}{\hat{U}}_{\mathrm{suma}}\left(k\right)=\frac{1}{N_s}\underset{l=0}{\overset{N_s-1}{.Math.}}\hat{U}\left(k-l\right),{\hat{U}}_{\mathrm{luma}}\left(k\right)=\frac{1}{N_l}\underset{l=0}{\overset{N_l-1}{.Math.}}\hat{U}\left(k-l\right)& \left(3\right)\end{array}$

for:

N.sub.s=f.sub.s.Math.τ.sub.suma,N.sub.l=f.sub.s.Math.τ.sub.luma

[0158] Let f.sub.s the sample rate, τ.sub.suma and τ.sub.luma the short and long unweighted moving average time constant.

[0159] As a representation of the free upstream turbulence a normal turbulence model variance is computed by means of the LUMA wind speed. The normal turbulence model variance satisfies:

σ.sub.ntm.sup.2I=.sub.ref.sup.2.Math.(0.75.Math..Math..sub.luma+5.6).sup.2  (4)

with I.sub.ref the expected value of the turbulence intensity could be at 15 m/s. The rotor speed controller 412 from FIG. 4 computes a pitch rate setpoint {dot over (θ)}.sub.Ω.sup.sp by means of a rotor speed setpoint Ω.sub.rot.sup.sp, measurement Ω.sup.mv and pitch angle measurement θ.sub.mv:

{dot over (θ)}.sub.Ω.sup.sp=H.sub.Ω.sup.c(Ω.sup.mv, θ.sup.mv0.Math.Ω.sub.rot.sup.sp  (5)

[0160] Let H.sub.Ω.sup.c be a proper and stable closed loop transfer function related to the rotor speed-to-pitch rate block diagram 536 in FIG. 5. The rotor speed setpoint is obtained from a predefined wind speed-rotor speed table. For the wind speed the SUMA value of eq: 3 is used bounded by:

.Math..sub.luma−k.sub.σ.sub.−.sup.Ω.Math.σ.sub.ntm≤.Math..sub.suma≤.Math..sub.luma+k.sub.σ.sub.+.sup.Ω.Math.σ.sub.ntm  (6)

[0161] Let k.sub.σ.sub.−.sup.Ω and k.sub.σ.sub.+.sup.Ω be a gain to scale the normal turbulence model standard deviation below and above the LUMA wind speed.

[0162] The rotor speed controller is a proportional-differential (PD) feedback controller which is designed for disturbance rejection in order to retain the demanded rotor speed setpoint. The disturbance rejection is of main concern for rotor speed values above the demanded rotor speed setpoint. Hence, the nominal PD gain values are computed for pitch angle sensitivity functions of rotor (aerodynamic) torque above the demanded operating conditions. When operating at the demanded conditions the stability conditions are pursued by amplifying the nominal PD gain values depending on the pitch angle sensitivity functions of rotor torque. Scaling of the PD gain values is known as gain scheduling.

[0163] The rotor power controller from FIG. 4 computes a pitch rate setpoint {dot over (θ)}.sub.P.sup.sp using a rotor speed and pitch angle measurement Ω.sup.mv, θ.sup.mv and an estimated wind speed .Math. and a rotor power setpoint P.sub.rot.sup.sp:

{dot over (θ)}.sub.P.sup.sp=H.sub.P.sup.c(Ω.sup.mv,θ.sup.mv,.Math.,P.sub.rot.sup.sp)  (7)

[0164] Let k.sub.σ.sub.−.sup.P and k.sub.σ.sub.+.sup.P be a proper and stable transfer function related to the rotor power-to-pitch rate block diagram in FIG. 5 and where the rotor power setpoint is obtained from a predefined wind speed-rotor power table. For the wind speed the SUMA value of eq: (3) is used bounded by:

.Math..sub.luma−k.sub.σ.sub.−.sup.P.Math.σ.sub.ntm≤.Math..sub.suma≤.Math..sub.luma+k.sub.σ.sub.+.sup.P.Math.σ.sub.ntm  (8)

[0165] Let k.sub.σ.sub.−.sup.P and k.sub.σ.sub.+.sup.P be a gain to scale the normal turbulence model standard deviation below and above the LUMA wind speed.

[0166] The rotor power controller is a proportional-double differential (PDD) feed forward controller which is designed to preserve the demanded rotor power setpoint.

[0167] The rotor thrust controller from FIG. 4 computes a pitch rate setpoint {dot over (θ)}.sub.F.sup.sp by means of a rotor speed and pitch angle measurement Ω.sup.mv, θ.sup.mv and an estimated wind speed .Math. and rotor thrust setpoint F.sup.sp:

{dot over (θ)}.sub.F.sup.sp=H.sub.F.sup.c(Ω.sup.mv,θ.sup.mv,.Math.,F.sup.sp)  (9)

[0168] Let H.sub.F.sup.c be a proper and stable transfer function related to the rotor thrust-to-pitch rate block diagram in FIG. 5. The rotor thrust setpoint is obtained from a predefined wind speed-rotor thrust table. For the wind speed the SUMA value of eq: (3) is used bounded by:

.Math..sub.luma−k.sub.σ.sub.−.sup.F.Math.σ.sub.ntm≤.Math..sub.suma≤.Math..sub.luma+k.sub.σ.sub.+.sup.F.Math.σ.sub.ntm  (10)

[0169] Let k.sub.σ.sub.−.sup.F and k.sub.σ.sub.+.sup.F be a gain to scale the normal turbulence model standard deviation below and above the LUMA wind speed. The rotor thrust controller is a PDD feed forward controller which is designed for load limitation by constraining the estimated rotor thrust to the maximum rotor thrust and reducing the rate of rotor thrust change.

[0170] According to FIG. 5, the pitch rate setpoints from the pitch angle, rotor speed, rotor power and rotor thrust control are subjected to the following set of constraints: [0171] rotor thrust constraint; [0172] pitch angle synchronize; and [0173] pitch angle, rate and acceleration limitation.

[0174] During power production the rotor thrust constraint selects the pitch rate setpoint which yields the smallest rate of rotor thrust change.

[0175] The pitch rate setpoints for the 3 blades are corrected through a P feedback controller such that the 3 pitch angles are synchronized to at least the threshold level for synchronization.

[0176] To determine the operation boundaries for power production limitations are introduced for pitch angle, rate and acceleration. The boundaries are defined by pitch-to-work and pitch-to-vane limits. Consider Table 2.1 for an overview of the limits.

TABLE-US-00001 TABLE 1 The pitch angle, rate and acceleration limitations. Pitch limitation Subrated Rated production production Pitch-to-vane Acceleration Constant Constant Rate Constant Constant Angle Constant Constant Pitch-to-work Acceleration Constant Constant Rate Constant Variable Angle Constant Variable

[0177] The pitch-to-work rate limit for rated production is obtained from a predefined rotor speed-pitch rate table. For the pitch-to-work angle limit a minimal tip speed ratio to pitch angle boundary is introduced. The minimal tip speed ratio is transformed in a tip speed ratio-minimal pitch angle table.

[0178] The measured pitch angle and rotor speed are subjected to filter operations to make them suitable for control actions. The filters are designed to suppress disturbances from structural and generator dynamics, respectively blade and tower bending modes and generator imbalance motions.

[0179] Accordingly, at least one aspect suggests a direct drive wind turbine being equipped with a control system to actuate a desired pitch rate for the 3 blades and a desired torque for the generator. The wind turbine controller of the control system is designed to optimize power production and to handle multiple operation objectives, with the structural integrity being guaranteed as far as possible.

[0180] For subrated power production the available rotor power for production is maximized using the generator torque, pitch angle and rotor thrust controller. Additionally, at rated power production one attempts to maintain rated generator power by using the generator torque, rotor speed, rotor power and rotor thrust controller. Finally, all demanded pitch rates are subjected to a set of constraints.