Method and computing module for determining pitch angle adjustment signals of a wind turbine based on the maximum rotational speed

Abstract

A method and corresponding arrangement are provided for determining pitch angle adjustment signals for adjusting a pitch angle of a rotor blade connected to a rotation shaft of a wind turbine. The method includes obtaining a first maximal speed signal indicative of a first desired maximal rotational speed of the rotation shaft. The method also includes deriving a first pitch angle adjustment signal based on the first maximal speed signal. The method further includes obtaining a second maximal speed signal indicative of a second desired maximal rotational speed of the rotation shaft different from the first desired maximal rotational speed of the rotation shaft. Further, the method includes deriving a second pitch angle adjustment signal based on the second maximal speed signal. The second pitch angle adjustment signal is different from the first pitch angle adjustment signal.

Claims

1. A method for determining pitch angle adjustment signals for adjusting a pitch angle of a rotor blade connected to a rotation shaft of a wind turbine, the method comprising: first, obtaining a power signal expressing power output of the wind turbine; second, obtaining a first maximal speed signal indicative of a first desired maximal rotational speed of the rotation shaft for a first operational mode; third, deriving a first pitch angle adjustment signal based on the first maximal speed signal and the power signal; fourth, obtaining a second maximal speed signal indicative of a second desired maximal rotational speed of the rotation shaft different from the first desired maximal rotational speed of the rotation shaft for a second operational mode; and fifth, deriving a second pitch angle adjustment signal based on the second maximal speed signal and the power signal, wherein the second pitch angle adjustment signal is different from the first pitch angle adjustment signal; sixth, adjusting the pitch angle of the rotor blade to a first pitch angle based on the first pitch angle adjustment signal; and seventh, adjusting the pitch angle of the rotor blade to a second pitch angle based on the second pitch angle adjustment signal.

2. The method according to claim 1, wherein the power signal expresses that the power output of the wind turbine is smaller than a nominal power output of the turbine.

3. The method according to claim 1, wherein the second desired maximal rotational speed of the rotation shaft is smaller than the first desired maximal rotational speed of the rotation shaft.

4. The method according to claim 1, wherein, when the second desired maximal rotational speed is desired as the rotational speed of the rotation shaft, the adjusting the of pitch angle to the second pitch angle results in a higher power output of the wind turbine than adjusting the pitch angle to the first pitch angle.

5. The method according to claim 1, wherein the first pitch angle and/or the second pitch angle increases or decreases or is constant for increasing power output of the wind turbine and/or for increasing wind speed.

6. The method according to claim 1, wherein the second desired maximal rotational speed is smaller than the first desired maximal rotational speed, wherein the second pitch angle is greater than the first pitch angle.

7. The method according to claim 6, wherein, for a given power output and/or wind speed, the second pitch angle is all the more greater than the first pitch angle the smaller the second desired maximal rotational speed is compared to the first desired maximal rotational speed.

8. The method according to claim 6, wherein the second pitch angle increases more strongly for increasing power output and/or increasing wind speed than the first pitch angle.

9. The method according to claim 1, wherein the second desired maximal rotational speed is between 50% and 90% of the first desired maximal rotational speed.

10. The method according to claim 1, wherein the first pitch angle and/or the second pitch angle is constant for power output below a threshold.

11. The method according to claim 1, wherein the first pitch angle is constant for a rotational speed below the first desired maximal rotational speed and/or wherein the second pitch angle is constant for a rotational speed below the second desired maximal rotational speed.

12. The method according to claim 1, wherein, when the second desired maximal rotational speed is desired as the rotational speed of the rotation shaft, the adjusting the pitch angle to the second pitch angle results in a higher power output of the wind turbine than adjusting the pitch angle to the first pitch angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention are now described with reference to the accompanying drawings. The invention is not limited to the illustrated or described embodiments. In the drawings like reference numerals may denote like elements in structure and/or function. Thereby, reference numerals of similar elements may differ only in the first digit.

(2) FIG. 1 and FIG. 2 illustrate graphs considered during a method according to an embodiment of the present invention;

(3) FIG. 3 and FIG. 4 illustrate graphs to explain operational modes of a wind turbine considered in a method according to an embodiment of the present invention;

(4) FIG. 5 illustrates a graph from which pitch angles or signals may be derived and which may be considered in a method according to an embodiment of the present invention;

(5) FIG. 6 illustrates schematically an arrangement for adjusting a pitch angle according to an embodiment of the present invention comprising a computing module for determining pitch angle adjustment signals according to an embodiment of the present invention; and

(6) FIG. 7 schematically illustrates a cross-sectional view of a rotor blade for defining in particular a pitch angle of the rotor blade and other properties of the rotor blade related to methods according to embodiments of the present invention.

DETAILED DESCRIPTION OF INVENTION

(7) The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit.

(8) FIG. 1 illustrates a graph 100 showing on an ordinate the optimal pitch angle β and on an abscissa a normalized maximal rotational speed (as a ratio with respect to a design maximum rotational speed), wherein a curve 103 indicates the optimal pitch angle β in dependence of the normalized maximum rotational speed.

(9) A point 105 at coordinates (1, −2°) may correspond to a situation, where the wind turbine is operated at the design rotational speed of the rotation shaft. According to regulations or a demand by a wind farm controller the maximum rotational speed may be set to be above or to be below the maximum design rotational speed.

(10) The graph 100 illustrated in FIG. 1 relates to an external condition, wherein the wind speed amounts to 9 m/s. As can be taken from the graph 100 in FIG. 1 the optimal pitch angle β varies in a range between around 13° and −2°. In particular, at a normalized maximum rotational speed of 0.3 the optimal pitch angle amounts to around 13° and then decreases up to the normalized maximum rotational speed of 0.9 to a value of approximately −2°. From the normalized maximum rotational speed of 0.9 the optimal pitch angle β (curve 103) increases from around −2° to about 0° at the normalized maximum rotational speed of 1.2. In particular, at a wind speed of 9 m/s the wind turbine may output electric power below the rated electric power or the nominal power output.

(11) The graph 101 illustrated in FIG. 2 shows on its abscissa the normalized maximum rotational speed and on its ordinate the power increase for rotational speed corrected pitch reference, i.e. the increase of power output, when the pitch angle is set according to curve 103 in graph 100 illustrated in FIG. 1. In particular, the curve 107 in the graph 101 of FIG. 2 illustrates the power increase in percent relative to the case where the pitch angle is set to −2.0° corresponding to the value of the pitch angle at the maximum design rotational speed (point 105 in graph 100 of FIG. 1). As can be taken from the graph 101 of FIG. 2, the power output increase amounts to about 100% at the normalized maximum rotational speed of 0.3 and then decreases for increasing normalized maximum rotational speed to about 0% at the normalized maximum rotational speed of around 0.65. Thus, setting the pitch angle according to the curve 103 illustrated in graph 100 of FIG. 1 will in particular improve the power output in the region below 0.65 (in particular between 0.3 and 0.65) of the normalized maximum rotational speed, in particular, when lowering the maximum rotational speed of the rotation shaft below the design rotational speed (corresponding to the abscissa value 1.0 in the graphs 100 and 101).

(12) In particular, FIG. 3 and FIG. 4 show what may be called the “nominal operating trajectory of rotational speed and electrical power output”, respectively. In average the rotational speed and power may be as shown in FIG. 3 and FIG. 4 as function of the wind speed.

(13) FIG. 3 illustrates a graph 209 illustrating on its abscissa the wind speed in m/s and on its ordinate the generator speed or rotational speed of the rotation shaft in rpm. In particular, a curve 211 illustrates the dependency of the rotational speed on the wind speed when the maximal rotational speed is set to a first desired maximal rotational speed ms1 which corresponds to approximately 1500 rpm.

(14) Another curve 213 illustrates the dependency of the rotational speed of the rotation shaft when the maximal rotational speed is set to a second desired maximal rotational speed ms2 which corresponds approximately to 1100 rpm. According to an embodiment, the maximum rotational speed ms1 may be considered as a first desired maximal rotational speed and ms2 may be considered as a second desired maximal rotational speed. According to an embodiment, the first desired maximal rotational speed may be the design rotational maximal speed. When the desired maximal rotational speed is set to the value ms2 the rotational speed increases in a region 215 from around 500 rpm (reference sign 217) to the value ms2 and the rotational speed remains constant in a region 219 and beyond the region 219. When the desired maximal rotational speed is set to the value ms1 the rotational speed increases in a region 221 from around 500 rpm (reference sign 217) to the value ms1 and the rotational speed remains constant in a region 223 and beyond the region 223.

(15) In particular, the desired maximum rotational speed ms2 is reached for lower wind speed than the desired maximum rotational speed ms1.

(16) When the desired maximal rotational speed is set to the value ms1 the rotational speed increases from the value 500 rpm (reference sign 217) in a region (wind speed region) 221 in a linear fashion to the first desired maximal rotational speed ms1. In a region 223 and beyond the region 223 the rotational speed is maintained at the value ms1. According to embodiments of the present invention, adjusting the pitch angle of a rotor blade is performed in the regions 215, 219 (or only in region 219), when the desired maximal rotational speed is set to the second desired maximal speed ms2. Further, according to an embodiment, the pitch angle is adjusted in the regions 221, 223 (or only in region 223), when the first desired maximal rotational speed ms1 is set as the desired maximal rotational speed.

(17) Plot 225 in FIG. 4 illustrates on its abscissa the wind speed in m/s and on its abscissa the power output P of the wind turbine in kW. Thereby, a curve 227 depicts the power output P of the wind turbine in dependence of the wind speed. As can be seen in the union of the regions 215 and 219 or the union of regions 221 and 223, respectively, the power output increases from a value slightly above 0 kW to a value at about 2200 kW. Beyond the wind speed of around 12 m/s (reference sign 229) the power output remains at the nominal power output np. Thus the regions 215, 219 or 221, 223, respectively, correspond to running conditions of the wind turbine, where the wind turbine is operated below the nominal power np or rated power output of the wind turbine.

(18) In particular when using the reduced maximal speed ms2 it may be necessary to reduce the power as well, if the same generator torque should be maintained (i.e. if the nominal generator torque should not be increased). Note that power=speed*torque, so when the maximal speed is reduced it may be necessary to lower the power as well in order not to overload mechanical components, e.g. the drive train. Similarly, this may also be necessary in order not to overload the power electronics or electrical components, e.g. caused by too high currents.

(19) In particular, the adjustment of the optimal pitch angle according to curve 103 in plot 100 of FIG. 1 is especially performed in regions 215, 219 or 221 and 223, respectively. In particular, the adjustment of the optimal pitch angle according to curve 103 in plot 100 of FIG. 1 may be applied in situations, where the wind turbine is operated below the rated or nominal power, but where the wind speed is large enough such that the rotational speed of the rotation shaft may exceed the desired maximal rotational speed.

(20) According to an embodiment, the pitch angle reference may be based on the maximum rotational speed. The power captured by a wind turbine may be expressed as:
P=0.5.Math.ρA.Math.C.sub.p.Math.ν.sup.3

(21) where P is the power captured by the wind turbine [W], ρ is the air density [kg/m.sup.3], A is the rotor swept area [m.sup.2], C.sub.p is the power coefficient of the turbine, ν is the rotor effective wind speed [m/s].

(22) Thereby, the power coefficient C.sub.p may be a function of the pitch angle β and the tip speed ratio (the ratio between the blade tip speed and the wind speed). This means that the turbine efficiency may be maximized for a certain pitch angle β and for a given tip speed ratio.

(23) If the pitch reference is not compensated according to the curve 103 depicted in graph 100 of FIG. 1, a sub-optimal operation point may be obtained. However, if the pitch angle β is set based on the current maximum rotational speed, i.e. the first desired maximum speed ms1 or the second desired maximal rotational speed ms2, respectively, it may be possible to maximize power production P of the wind turbine.

(24) Typically, the optimal pitch angle may be fixed for operation below the rated rotational speed (also known as the nominal speed). The curve 103 illustrated in plot 100 of FIG. 1 illustrates in which way the optimal pitch angle β depends on the maximal rotational speed. From plot 101 of FIG. 2 it may be concluded that a significant power gain can be achieved by setting the pitch angle β according to the curve 103 illustrated in the plot 100 of FIG. 1.

(25) The optimal pitch angle (as illustrated with curve 103 in plot 100 of FIG. 1) may change with different wind speeds. Typically, the wind speed may be mapped into a power value by using a power curve (such as a curve 227 as shown in plot 225 of FIG. 4) and the pitch reference may then be set using the current power or torque reference. The power curve 227 in plot 225 of FIG. 4 may specify the expected power production as a function of the wind speed. Alternatively, the pitch reference for below rated speed operation may be set as a function of the rotational speed or the wind speed. In particular, the rotational speed may also be expressed as a rotational frequency of the rotation shaft.

(26) FIG. 5 illustrates a plot showing on its abscissa the power p per unit (i.e. in units as a ratio of the nominal power output) and showing on its ordinate the optimal pitch reference or optimal pitch angle β in degrees. A curve 331 illustrates the dependency of the pitch angle β, when 100% of the design speed is set as the desired maximal rotational speed. As can be seen, the optimal pitch angle β according to curve 331 is constant (value β0) for increasing power output in a range of 0 to 1, i.e. 0 kW to the nominal power output, such as for example above 2000 kW. Alternatively, the optimal pitch angle β may also be non-constant for increasing power output depending on the aerodynamic properties of the rotor. In particular, the pitch angle may have to be changed (e.g. increased) typically in the part of region 219 in FIG. 3 and FIG. 4 where the rotational speed is constant.

(27) The curve 333 illustrates the case, when the desired maximal rotational speed is set to 70% of the design speed, wherein the design speed may in particular be the nominal maximal rotational speed as dictated by mechanical and/or electronic properties of the wind turbine taking into account where related configurations and/or load-related requirements or considerations. As can be seen the optimal pitch angle β according to curve 333 increases approximately linearly for power output above approximately 0.55 (threshold th3) up to a power output of 0.7. The end point of the solid portion of curve 333 may correspond to a nominal torque. The pitch angle may be adjusted according to the dashed portions of curves 333, 335, 337, 339, which may correspond to torque larger than a nominal torque, if the wind turbine is intended to (temporarily) be operated above rated or nominal torque.

(28) Lowering the maximum rotational speed often may result in an equal (or proportional) reduction of the maximum power output in order to avoid increasing the torque on the generator and particular the gearbox (if any). Also the electric currents in the power electronic (converter, generator) would increase if the torque increased. Therefore, the turbine would often have reached a region beyond regions 219, 223 in FIG. 3 and FIG. 4, where one may control the speed by pitching (apply a non-optimal pitch angle to reduce the cp value, being the power coefficient of the rotor).

(29) The curve 335 in FIG. 5 illustrates the case, when the desired maximal rotational speed is set at 60% of the design speed. As can be seen from FIG. 5, the optimal pitch angle according to the curve 335 is greater than the pitch angle for the curve 333 and greater than the pitch angle according to the curve 331. In particular, the pitch angle according to the situation, when the desired maximal rotational speed is set to 60% of the design speed (curve 335) increases approximately linearly for a power larger than about 0.33 (threshold th5) to a power of about 0.6 per unit.

(30) Curve 337 illustrates the optimal pitch angle β in the case where the desired maximal rotational speed is set to a first desired maximal rotational speed, in the illustrated example 50% of the design speed. Further, curve 339 illustrates the optimal pitch angle β in the case, when the desired maximal rotational speed is set at a second desired maximal rotational speed, in the illustrated example 40% of the design speed.

(31) For illustration and explanation the pitch angle β for the two cases will be compared at a normalized power output p of 0.2 for illustration. When the desired maximal rotational speed is set at the first desired maximal rotational speed (such as ms1 as illustrated in plot 209 of FIG. 3) the optimal pitch angle β amounts to β1. In contrast, when the second desired maximal rotational speed is set as the desired maximal rotational speed of the rotation shaft (i.e. ms2 as indicated in plot 209 in FIG. 3) the optimal pitch angle β amounts to β2 which is by an amount Δβ greater than the first pitch angle β1. This difference amounts to about 8° in the illustrated example. Further, the slopes of the curves 337 and 339 are different. In particular, the slope of the curve 337 at the point (p, β1) amounts to Δβ1/Δp which is smaller than the steepness of the curve 339 which amounts at the same abscissa value p to Δβ2/Δp.

(32) According to an embodiment a method for adjusting a blade pitch angle may comprise the following steps:

(33) 1. Determining the power reference (torque reference, wind speed, or rotational speed)

(34) 2. Determining the current maximum speed (i.e. defining the desired maximal rotational speed) which may be a reduction of the design speed (or nominal speed).

(35) 3. Adjusting the pitch reference accordingly (i.e. adjusting the pitch angle). This may be done by computing the optimal pitch reference or pitch angle or pitch adjustment signal as a function of the current maximal speed (the desired maximal speed) based on equations or look-up tables capturing the relations illustrated in FIGS. 1, 2 and/or 3. According to an embodiment the pitch angle is optimized regarding structural loads, acoustic noise emissions while at the same time optimizing energy efficiency.

(36) FIG. 6 schematically illustrates an arrangement 440 for adjusting a pitch angle of a rotor blade 441 according to an embodiment of the present invention comprising a computing module 443 according to an embodiment of the present invention. Via an input line 445 the computing module 443 receives a first maximal speed signal ms1 or a second maximal speed signal ms2 which indicate a first desired maximal rotational speed ms1 and a second desired maximal rotational speed ms2, respectively. Further, the computing module 443 receives via an input line 447 a signal indicative of a power output p (or P) of the wind turbine or torque or wind speed. Thereby p denotes a normalized power output (with respect to a nominal power output) and P denotes the absolute power output.

(37) Further, the computing module 443 has access to a storage device 449 which may store data or coefficients for a function representing at least one curve of the plots illustrated in FIGS. 1, 2 and/or 3. In particular, the curve 103 of plot 100 illustrated in FIG. 1 and the curves 331, 333, 335, 337, 339 illustrated in FIG. 5 may be represented in some data structure stored in the storage device 449 and being accessible by the computing module 443. In particular, the storage device 449 may provide calibration data or reference data to the computing module 443.

(38) Based on the maximal speed signal supplied via the line 445 and the power signal supplied by the line 447 the computing module 443 determines a pitch angle adjustment signal (based on a first maximal speed signal or a second maximal speed signal) and supplies the respective pitch angle adjustment signal s131, s132, respectively, via a signal line 451 to an actuator 453. The actuator 453, e.g. an electric motor or hydraulic system, is mechanically connected to the rotor blade 441 and mechanically rotates the rotor blade around its longitudinal axis 455 to adjust the pitch angle β according to derived pitch angle adjustment signal.

(39) FIG. 7 illustrates schematically a cross-sectional view (airfoil) of a rotor blade 541 as viewed along a longitudinal axis 555 of the rotor blade. The vertical axis 557 represents the rotor axis of the rotor 559 and the horizontal axis 561 lies within the plane of rotation in which the rotor blade 541 rotates.

(40) The rotor blade 541 comprises an upper surface 563 and a lower surface 565, wherein the lower surface 565 faces the wind propagating in a wind direction 567. A so-called chord line 569 is definable representing the straight line connecting the leading and trailing edges of the blade airfoil. The chord line 569 lies in a plane 571. An angle β between the rotation plane 561 and the chord plane 571 defines the blade pitch angle of the rotor blade 541. When the chord plane 571 coincides with the plane of rotation 561 the blade pitch angle is zero degree, when the chord plane 571 rotates clockwise, the blade pitch angle increases from zero to positive values. In particular, increasing pitch angle results in pitching towards feather, while decreasing pitch angle results in pitching towards stall.

(41) It should be noted that the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.