Wind turbine control arrangement

Abstract

A control arrangement for a variable-speed wind turbine includes a loading analysis module configured to analyse a number of environment values to establish whether the momentary wind turbine loading is lower than a loading threshold when the rotational speed of the aerodynamic rotor has reached its rated value; and a speed boost module configured to determine a speed increment for the rotational speed of the aerodynamic rotor if the wind turbine loading is lower than the loading threshold.

Claims

1. A control arrangement for a variable-speed wind turbine constructed to fulfil requirements of a wind class as defined by IEC 61400-1, the control arrangement comprising: a controller configured to: analyze a plurality of environment values to establish whether a momentary wind turbine loading is lower than a loading threshold when a rotational speed of an aerodynamic rotor has reached a rated value, wherein a rated speed is associated with a level of output power that is less than a rated output power for a class of wind turbine; and apply a speed increment to the rotational speed of the aerodynamic rotor if the wind turbine loading is lower than the loading threshold, and wherein a power/speed relationship follows an ideal trajectory for the class of wind turbine.

2. The control arrangement according to claim 1, wherein the plurality of environment values comprise any of: a blade load value, a turbulence intensity value, a tower acceleration value, and a yaw position value.

3. The control arrangement according to claim 1, wherein the controller is further configured to receive a pitch position value, an active power value, and a rotor speed value.

4. The control arrangement according to claim 3, wherein the controller is further configured to estimate a local turbulence intensity on a basis of the pitch position value, the active power value, and the rotor speed value.

5. The control arrangement according to claim 1, wherein the controller is further configured to process an environment value according to a relevant threshold.

6. The control arrangement according to claim 1, wherein the controller is further configured to compute a partial contribution of each environment value and to enable, on a basis of a sum of the partial contributions, an increase of rotational speed by the speed increment.

7. The control arrangement according to claim 1, wherein the controller is further configured to determine a wake exposure of the wind turbine and to enable, on a basis of the wake exposure, an increase of rotational speed by the speed increment.

8. The control arrangement according to claim 1, wherein the controller is further configured to determine a magnitude of a speed increment on a basis of an estimated turbulence level and/or an estimated load level.

9. A variable-speed wind turbine comprising the control arrangement according to claim 1, which is configured to augment a speed reference by the speed increment.

10. The variable-speed wind turbine according to claim 9, constructed to fulfil requirements of a turbulence class as defined by IEC 61400-1.

11. A method of operating a variable-speed wind turbine, the method comprising: establishing whether a momentary wind turbine loading is lower than a loading threshold when a rotational speed of an aerodynamic rotor has reached a rated speed, wherein the rated speed is associated with a level of output power that is less than a rated output power for a class of wind turbine; determining a speed increment for the rotational speed of the aerodynamic rotor on a basis of an ideal power/speed trajectory of the wind turbine if the wind turbine loading is lower than the loading threshold; and increasing a rotor speed by the speed increment.

12. The method according to claim 11, further comprising augmenting a speed reference by the speed increment.

13. The method according to claim 11, further comprising identifying a wake exposure of the wind turbine.

14. The method according to claim 13, wherein the rotational speed of the aerodynamic rotor is increased only if the wind turbine is not in a wake of another wind turbine.

Description

BRIEF DESCRIPTION

(1) It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 shows a block diagram of an embodiment of the inventive control arrangement;

(3) FIG. 2 shows a block diagram of an embodiment of the loading analysis module of FIG. 1;

(4) FIG. 3 shows a speed/power chart of a wind turbine controlled using the inventive method;

(5) FIG. 4 illustrates the incremental speed boost of the inventive method;

(6) FIG. 5 shows an exemplary wind park; and

(7) FIG. 6 shows a speed/power chart of a wind turbine controlled using a prior art approach.

DETAILED DESCRIPTION

(8) Objects in the diagrams are not necessarily drawn to scale. FIG. 1 shows a block diagram of an embodiment of the inventive control arrangement 2. The aerodynamic rotor and the generator are collectively regarded as a power producing unit 21, and the power output of the generator is largely determined by a pitch reference 21.sub.pitch (to the rotor blades) and a torque (or power reference) reference 21.sub.torque (to the generator).

(9) The inventive control arrangement 1 comprises a loading analysis module 10 or mild environmental condition analyser (MECA) 10 which receives various inputs that provide it with relevant data 10_1, . . . , 10_n such as a flap bending moment data 10_1 (e.g. from sensors of a blade load limiter module 22), turbulence intensity data (e.g. from a LIDAR unit), tower loading data, yaw position data, etc. From this information, the MECA 10 can assess the present environmental condition for the wind turbine, and can decide whether or not to allow a speed boost. This decision is output as a speed boost approval signal SB.sub.OK to a speed boost module 11. A boost level 11_in, i.e. the extent by which rotor speed can be increased, is provided to the speed boost module 11. The boost level 11_in may be a predetermined parameter or setting, and can place a limit on the maximum allowed boost. Within the constraint of this maximum allowed boost, the speed boost module 11 can then set one or more speed increments inc in order to approach this maximum 11_in. For example, successive speed increments inc can gradually approach the maximum 11_in in several small steps. The intended speed increment inc is then added to a speed reference 200 at summation module 20S. The speed reference 200 can originate from a park controller (not shown).

(10) The delta or speed reference error 20.sub.err between target speed and actual speed 210 is input to a speed control module 20, which then computes a target pitch reference and a torque reference 21.sub.torque.

(11) In this exemplary embodiment, the target pitch reference is modified by a load limitation offset provided by a blade load limiter 22. The corrected pitch reference 21 pitch and the torque reference 21.sub.torque are then used to control the aerodynamic rotor and the generator in order to achieve the target rotational speed (speed reference 200 plus speed increment .sub.inc) and the target output power.

(12) FIG. 2 shows a block diagram of an embodiment of the MECA 10 module of FIG. 1. Here, each input signal 10_1, . . . , 10_n is processed in various stages SF, SO, ST, SG to determine its contribution to the ultimate boost speed decision SB.sub.OK. Depending on the type of input signal 10_1, . . . , 10_n, it can be processed by a filter stage SF and/or an operator stage SO and/or a threshold stage ST and/or a gain stage SG. Depending on the nature of the input signal 10_1, . . . , 10_n, the filter type of a filter stage SF can be any of low-pass, band-pass, high-pass etc.; the operator of an operator stage SO can be any of computation of standard deviation, absolute value, maximum value, etc.; the threshold stage ST can determine whether or not its input reaches a required threshold for it to be included in the final decision; the gain stage can determine the weighting of a partial contribution C1, . . . , C5. These considerations apply to input signals such as a LIDAR input 10_2, a blade load sensor input 10_1, a sensor input 10_3 such as a wind speed sensor, etc.

(13) The pitch position 10_5, active power 10_6 and actual rotor speed value 210 are fed to a look-up table 100 which returns a wind speed estimate which is then used by a turbulence estimator module 101 to obtain an estimate of the local turbulence intensity. The look-up table 100 can also deliver a thrust estimate, which is processed in conjunction with a tower acceleration input 10_4.

(14) The partial contributions C1, . . . , C5 resulting from the processing stages are summed to obtain a value for total loading 10.sub.total, which is then compared to a sum threshold SB.sub.thold. If the total loading 10.sub.total is less than the threshold SB.sub.thold, the rotational speed can in principle be boosted, and this possibility is indicated by the preliminary speed boost SB.sub.pre signal.

(15) In this exemplary embodiment, the decision to increase or boost the rotational speed also depends on the wake position of the wind turbine, i.e. whether or not the wind turbine is in the wake of another wind turbine, since the likelihood of excessive loading increases significantly when a wind turbine is in the wake of another wind turbine. To this end, the yaw attitude 10_7 of the wind turbine is fed to a wake module 102, which can avail of a park layout look-up table. The wake module 102 may also be informed of the yaw positions of other relevant wind turbines in the wind park. With this information, the wake module 102 can establish whether the wind turbine is in wake or out of wake. The wake state WS can be true (wind turbine is in wake) or false (wind turbine is out of wake), for example. A boost approval module 103 receives the wake state WS and the initial speed boost SB.sub.pre signal, and decides whether or not speed boost is approved. This boost approval module 103 prevents speed boost when the wind turbine is in wake, and enables speed boost as soon as the wind turbine is out of wake. Effectively, a positive speed boost SB.sub.OK signal to enable rotor speed boost is issued only when a wind turbine is out of wake, i.e. this wind turbine is not currently affected by the wake of an upstream wind turbine.

(16) FIG. 3 shows several speed/power curves of a wind turbine type, with power P (in Watts) along the Y-axis, and rotational speed (in radians per second) along the X-axis. It shall be understood that there is an infinite number of speed/power curves and the diagram only shows a few for the sake of clarity.

(17) Each of the curves shown in the diagram is associated with a specific integer wind speed and has a maximum power output value at a specific rotational speed. The curve C.sub.vmax corresponds to the wind speed vmax at which the wind turbine can reach its rated speed rated. The diagram also shows the ideal trajectory T.sub.ideal for that wind turbine type. Each point along the ideal trajectory T.sub.ideal is the maximum of speed/power curve. For the curve C.sub.vmax, the rated speed rated is associated with output power P0. The output power P0 which can be reached when the wind turbine is operating at its rated speed orated is less than the achievable rated output power P.sub.rated for that class of wind turbine.

(18) When the wind turbine is being operated at its rated speed .sub.rated, the loading analysis module continually monitors the loading to assess whether it is safe to increase the rotor speed. If a speed boost is approved, the rotor speed can be tentatively raised, allowing the wind turbine to adhere to the ideal trajectory T.sub.ideal. Starting from the maximum of curve C.sub.vmax, the output power can increase from the initial level P.sub.0 to its rated output power P.sub.rated while adhering to the ideal trajectory T.sub.ideal. In this way, the wind turbine can be controlled to extract the maximum possible amount of energy from the wind when this has increased (to within a safe level) beyond the rated wind speed for that wind turbine class. As a result, the AEP of the wind turbine can be increased significantly.

(19) With the inventive control approach, it is possible to maximise the power coefficient of a wind turbine type by identifying the mild environmental conditions that allow a careful increase in rotational speed. Instead of issuing references to maintain the rotational speed at the rated value .sub.rated even if the wind speed is higher than the rated wind speed v.sub.max, the rotational speed is allowed to gradually increase so that the power/speed relationship T.sub.SB can follow the ideal trajectory T.sub.ideal. Of course, as soon as the loading is deemed to be excessive, the rotational speed is reduced again towards its rated speed (or below the rated speed, as the case may be), again following the optimal power/speed trajectory T.sub.SB.

(20) FIG. 4 illustrates the incremental speed boost performed by the inventive method during operation of a wind turbine. The diagram shows cumulative wind loading L.sub.prior on a wind turbine controlled by a prior art control technique to operate at or near its rated speed. The cumulative wind loading L.sub.prior fluctuates according to collective changes in the environmental conditions, e.g. changes in turbulence, tower loading, wind speed, etc. A loading threshold L.sub.max for that wind turbine is indicated as a constant value.

(21) Embodiments of the invention are based on the premise that the loading on wind turbine is often less than a specified loading threshold L.sub.max. Embodiments of the invention aim to remedy the loss in efficiency arising from the gaps G.sub.prior between the loading threshold and the actual loading, since these gaps G.sub.prior indicate that the wind turbine is not extracting the maximum energy from the wind. In the inventive method, the total loading is estimated as explained in FIG. 2, and compared to a loading threshold (e.g. total load 10.sub.total is compared to threshold SB.sub.thold in FIG. 2). If the wind turbine is not in wake, the rotor speed can be increased to follow the ideal trajectory as explained above.

(22) The diagram shows an exemplary speed boost increments .sub.inc (in rad/s) that are added to the speed reference 200 as explained in FIG. 1. A speed boost increment .sub.inc is determined by the MECA 10 and the speed boost module 11 according to the observed loading on the wind turbine as explained above. As long as the actual loading does not exceed the loading threshold, the speed of the aerodynamic rotor may be carefully increased. The diagram shows the outcome of the inventive control method in the form of the more optimal cumulative loading L.sub.SB, indicating that the wind turbine is able to always extract the maximum energy from the wind.

(23) FIG. 5 shows an exemplary wind park, with a plurality of wind turbines 3 arranged in a suitable formation. A park controller 5 issues speed references 20_1, . . . , 20_n to the wind turbines 3 on the basis of input data 50_1, . . . , 50_n such as power demand input, turbine capacity, meteorological data, etc. An upstream wind turbine is not exposed to the wake of any other wind turbine and can be given a high speed reference for its aerodynamic rotor 30.

(24) As explained in the introduction, FIG. 6 shows several speed/power curves for of a wind turbine type, with power P (in Watts) along the Y-axis, and rotational speed (in radians per second) along the X-axis. It shall be understood that there is an infinite number of speed/power curves and the diagram only shows three for the sake of clarity. The curve C.sub.vmax corresponds to the wind speed v.sub.max at which the wind turbine can reach its rated speed rated. The diagram also shows the ideal trajectory T.sub.ideal for that wind turbine type. Each point along the ideal trajectory T.sub.ideal is the maximum of speed/power curve. For the curve C.sub.vmax, the rated speed .sub.rated is associated with output power P.sub.0.

(25) Using the prior art control techniques, the ideal trajectory T.sub.ideal can only be followed for a wind speed that does not exceed v.sub.max. If the wind speed increases beyond v.sub.max, the rotational speed is maintained at the rated value .sub.rated, and any increase in power output must follow the vertical trajectory T.sub.CSZ, and can only be achieved by equipping the rotor blades with physical add-ons such as serrations, vortex generators, etc. Starting from the maximum of curve C.sub.vmax, the output power can theoretically increase from the initial level P.sub.0 to its rated output power P.sub.rated. In this constant speed zone defined by bounds P.sub.0-P.sub.rated, the output power follows the sub-optimal vertical trajectory T.sub.CSZ. If the wind turbine is not constructed to achieve this power boost, the difference between actual and achievable output power results in an unnecessary reduction in AEP.

(26) Although the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(27) For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements. The mention of a unit or a module does not preclude the use of more than one unit or module.