Adjusting a rotor blade pitch angle

Abstract

A method of adjusting a pitch angle of a rotor blade of a rotor of a wind turbine is provided. The method comprises varying the pitch angle of the rotor blade during wind operation of the wind turbine from a starting angle to reach a limit angle (1, 2) at which the rotor blade and/or an operational value (P) of the wind turbine reaches a predefined threshold limit (P1/2; 1, 2), deriving a safe angle (3, 4) dependent on the limit angle (1, 2), operating the rotor blade at the safe angle (3, 4) or an angle from the safe angle (3, 4) away from the limit angle (1, 2). A pitch angle adjustment system is also provided for the same purpose.

Claims

1. A method of adjusting a pitch angle () of a rotor blade of a rotor of a wind turbine, comprising: a) varying the pitch angle of the rotor blade during power-generating operation of the wind turbine from a starting angle to reach a limit angle applicable to current boundary conditions, the limit angle comprising a stall angle (1, 2), wherein the varying of the pitch angle is effective to ensure the rotor blade starts to stall, b) deriving a safe angle (3, 4) dependent on the stall angle (1, 2), c) operating the rotor blade at the safe angle (3, 4) or an angle lower than the safe angle (3, 4) during power-generating operation and for a period of time, d) repeating steps a, b, and c together to ensure the respective limit angle reflects respective current boundary conditions at different points in time during power-generating operation, and reducing a rotational speed of the rotor to below a speed limit before varying the pitch angle.

2. The method according to claim 1, wherein the stall angle (1, 2) comprises an angle at which a decrease of lift is measured after reaching a maximum lift value.

3. The method according to claim 1, wherein the repeating of steps a, b, and c occurs at given time intervals.

4. The method according to claim 1, wherein the repeating of steps a, b, and c occurs after an appearance of a specific operational condition of the wind turbine.

5. The method according to claim 4, wherein the specific operational condition comprises a standstill and/or an appearance of a specific weather condition.

6. The method according to claim 1, wherein a status parameter value (SPV) representing a status of the rotor blade is derived from the stall angle (1, 2).

7. The method according to claim 6, wherein the status parameter value (SPV) is derived such that it comprises a material condition parameter value representing an amount of wear of the rotor blade and/or such that it comprises a soiling parameter value representing a soiling state of the rotor blade.

8. The method according to claim 7, wherein upon the status parameter value (SPV) being beyond a certain predefined threshold value, an alarm signal (AS) is generated.

9. The method according to claim 1, wherein in step b) the safe angle (3, 4) is derived from the stall angle (1, 2) by subtracting from the stall angle (1, 2) a safety margin angle ().

10. The method according to claim 9, wherein the safety margin angle () is adjusted in dependence of an age value and/or a status parameter value (SPV) of the rotor blade.

11. The method according to claim 3 wherein step a) is repeated at given time intervals comprising at least twice a day.

12. The method according to claim 5 wherein the specific operational condition comprises the appearance of the specific weather condition, and the specific weather condition comprises precipitation.

13. The method according to claim 8 wherein the alarm signal (AS) is used to trigger a cleaning process (S) of the rotor blade.

14. The method according to claim 8 wherein the alarm signal (AS) is used to trigger an automatic cleaning process of the rotor blade.

15. A method of adjusting a pitch angle () of a rotor blade of a rotor of a wind turbine, comprising: a) operating the rotor blade at a predetermined maximum permissible angle during power-generating operation of the wind turbine, b) varying the pitch angle of the rotor blade during wind operation of the wind turbine from the predetermined maximum permissible angle in order to reach a limit angle at which a power output reaches a maximum power output or a limit angle at which the rotor blade starts to stall, c) deriving a safe angle (3, 4) dependent on the limit angle (1, 2), d) operating the rotor blade at the safe angle (3, 4) or an angle lower than the safe angle (3, 4), and reducing a rotational speed of the rotor to below a speed limit before varying the pitch angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a front view of a wind turbine according to the prior art,

(2) FIG. 2 shows a front view of one of the rotor blades of the wind turbine of FIG. 1,

(3) FIG. 3 shows a cross-sectional view of the rotor blade of FIG. 2,

(4) FIG. 4 shows a diagramme showing the lift coefficient behaviour of two wind turbine blades according to the prior art,

(5) FIG. 5 shows in a schematic block diagramme two embodiments of pitch angle adjustment systems according to aspects of the invention,

(6) FIG. 6 shows a diagramme of an adjustment of the angle of attack of a rotor blade using a method according to aspects of the invention,

(7) FIG. 7 shows two power output curves, one of a wind turbine according to the state of the art and one of a wind turbine according to an embodiment of the invention,

(8) FIG. 8 shows a schematic block diagramme of steps of an embodiment of the method according to aspects of the invention,

DETAILED DESCRIPTION OF INVENTION

(9) FIGS. 1 to 4 have already been explained above with reference to the prior art.

(10) FIG. 5 shows a block diagramme of two embodiments of pitch angle adjustment systems 33, 33 according to aspects of the invention:

(11) A rotor blade 3 of a wind turbine 1 such as the ones depicted in FIGS. 1 to 3 is equipped with such pitch angle adjustment systems 33, 33. The first pitch angle adjustment system 33 comprises a central unit 23, a pitch variator 25, a measurement sensor 27 and an analyzation and derivement unit 29. The pitch variator 25, the measurement sensor 27 and the analyzation and derivement unit 29 are all connected to a pitch adjustment actuator 31 via one common or via several interfaces (not shown). The second pitch angle adjustment system 33 comprises all the components of the first pitch angle adjustment system 33 plus the pitch adjustment actuator 31.

(12) The pitch adjustment actuator 31 adjusts the pitch angle. It is driven by the pitch variator 25 which gives pitch orders PO to it. Thereby, the pitch variator 25 varies the pitch angle of the rotor blade during wind operation of the wind turbine 1 from a starting angle to reach a limit angle, in this case a stall angle at which the rotor blade starts to stall.

(13) In return, the pitch adjustment actuator 31 generates measurement data MD which are handed over to the measurement sensor 27. These measurement data MD represent the lift force, represented by the lift coefficient C1 of the rotor blade 3 as described with reference to FIGS. 3 and 4. From those measurement data MD the measurement sensor 27 generates a limit measurement signal SMShere a stall measurement signal SMSonce the limit angle has been reached. From this stall measurement signal SMS, i.e. from the information that the stall angle has been reached at a certain pitch angle, the analyzation and derivement unit 29 generates a safe angle SPA which it hands over to the pitch adjustment actuator 31.

(14) The pitch adjustment actuator 31 is ordered not to exceed the safe angle SPA during normal operation. Thus, it operates the rotor blade at the safe angle SPA or at an angle from the safe angle SPA away from the limit angle, i.e. an angle lower than the safe angle SPA.

(15) FIG. 6 shows in a diagramme the adjustment behaviour of the pitch angle adjustment systems 33, 33 in more detail. Over the time t there is depicted the angle of attack of the rotor blade 3 which is (amongst others) dependent on the pitch angle of the rotor blade 3.

(16) Initially, at a first point of time t.sub.0 the rotor blade 3 is pitched such that the rotor blade 3 has a starting angle of attack .sub.0. It is then pitched towards a (first) stall angle .sub.1 at which the rotor blade 3 starts to stall. This occurs at a second point of time t.sub.1. The pitch angle adjustment systems 33, 33 then reduces the angle of attack by a safety margin angle .sub. to a (first) safe angle .sub.3 which is the stall angle .sub.1 minus the safety margin angle .sub.. The rotor blade 3 is pitched at this safe angle .sub.3 or below until step a) of the method according to aspects of the invention is repeated at a third point of time t.sub.2 after a preferably predefined time span .sub.t, which is for instance half a day. Then, the angle of attack is again at the point of time t.sub.2 increased to a (second) stall angle .sub.2, which is in this case lower than the previous (first) stall angle .sub.1. This new (second) stall angle .sub.2 is reached at a fourth point of time t.sub.3. This reduction in amount of the stall angles .sub.1, .sub.2 is for instance due to soiling of the rotor blade 3. Accordingly, from the fourth point of time t.sub.3, the angle of attack is again reduced by the safety margin angle .sub. to a (second) safe angle .sub.4 which is the new (second) stall angle .sub.2 minus the same safety margin angle .sub.. The rotor blade 3 is then pitched at this second safe angle .sub.4 or below until step a) of the method according to aspects of the invention is repeated yet again.

(17) Such procedure is carried out under a specific condition, in particular under a specific wind speed. It may be repeated at different wind speeds in order to receive a series of safe angles at different wind speeds. Alternatively and/or additionally the soiling or wear status parameter value of the rotor blade 3 can be determined from the stall angles .sub.1, .sub.2 (cf. below with reference to FIG. 8), from which also a series of safe angles for different wind speeds can be calculated.

(18) FIG. 7 shows two power curves C.sub.1, C.sub.2 of a wind turbine 1, the first power curve C.sub.1 representing the electric power output P (unscaled) over the wind speed v in m/s when the wind turbine 1 is operated according to the state of the art. The second power curve C.sub.2 represents the electric power output P (unscaled) over the wind speed v in m/s when the pitch angles of the rotor blades 3 of the wind turbine 1 have been adjusted according to an embodiment of the method according to aspects of the invention. It can be observed that due to the method according to aspects of the invention the rotor blades 3 are pitched such that the corresponding second power curve C.sub.2 rises steeper and thus reaches faster a maximum power output P.sub.1/2: In comparison with the first power curve C.sub.1, the maximum power output P.sub.1/2 is reached at a wind speed v which is 2 m/s lower, namely at a wind speed v.sub.2 of about 10 m/s in comparison with a wind speed v.sub.1 of about 12 m/s when regarding the first power curve C.sub.1. FIG. 7 may also be used as a reference for another embodiment of the method according to aspects of the invention. As described above, as the limit angle .sub.1, .sub.2 of the rotor blade 3 a stall angle .sub.1, .sub.2 can be used. However, it is also possible to use the angle at which the wind turbine 1 shows the maximum power output P.sub.1/2 as the limit angle.

(19) FIG. 8 shows a block diagramme of an embodiment of the method Z according to aspects of the invention, for which reference is made to FIG. 6, in particular to the process up to the first reduction of the pitch angle .

(20) In a first, optional step Y, the rotational speed of the rotor 9 of the wind turbine 1 is reduced to or below a speed limit SL. Then, in a second step X, the pitch angle, i.e. the angle of attack is varied during wind operation of the wind turbine 1 from the starting angle .sub.0 to reach the stall angle .sub.1 at which the rotor blade 3 starts to stall. In a third step W the safe angle .sub.2 is derived from the stall angle .sub.1, namely by subtracting the safety margin angle .sub. from the stall angle .sub.1. This safety margin angle .sub. may be predefined in an optional additional step R. In a fourth step V the rotor blade 3 is operated at the safe angle .sub.3 or an angle below that safe angle .sub.3. Further, in an optional additional step U a status parameter value SPV is derived from the stall angle .sub.1. The status parameter value SPV represents a status of the rotor blade 3, for instance its soiling state and/or its icing state and/or its wear. A further optional step T includes the generation of an alarm signal AS if the status parameter value SPV is beyond a certain predefined threshold. This alarm signal may in a further optional step S lead to an automatic cleaning process of the rotor blade 3, for instance if the status parameter value SPV represents a soiling state and/or an icing state which can be improved by cleaning.

(21) The dotted lines from step V and step U indicate that the method according to aspects of the invention is preferably carried out repetitively, as explained with reference to FIG. 6. In a second round, the same procedure as outlined above is done once again, therefrom resulting the second stall angle .sub.2 and the second safe angle .sub.4. It can also be observed that the status parameter values can be used for the definition of the safety margin angle .sub. in step R after the first repetition of the method Z.

(22) Although the present invention has been disclosed in the form of preferred 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.

(23) 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.