A METHOD FOR ESTIMATING A WIND SPEED INCLUDING CALCULATING A PITCH ANGLE ADJUSTED FOR BLADE TORSION

Abstract

A method for estimating a wind speed at a wind turbine is disclosed, said wind turbine comprising a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle. A blade torsion contribution, representing torsion introduced in the wind turbine blades, is derived, based on an obtained rotational speed, ω, of the rotor, and an obtaining a pitch angle, θ, of the wind turbine blades. An adjusted pitch angle, θ′, is calculated as a sum of the obtained pitch angle, θ, and the derived blade torsion contribution, and a wind speed, v.sub.est, is estimated, based on the obtained rotational speed, ω, and the calculated adjusted pitch angle, θ′. An accurate and reliable estimate for the wind speed is thereby obtained. The wind turbine may be controlled in accordance with the estimated wind speed, v.sub.est.

Claims

1. A method for estimating a wind speed at a wind turbine, said wind turbine comprising a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle, the method comprising: obtaining a rotational speed, ω, of the rotor, obtaining a pitch angle, θ, of the wind turbine blades, deriving a blade torsion contribution, based on the obtained rotational speed, ω, and the obtained pitch angle, θ, calculating an adjusted pitch angle, θ′, as a sum of the obtained pitch angle, θ, and the derived blade torsion contribution, and estimating a wind speed, v.sub.est, based on the obtained rotational speed, ω, and the calculated adjusted pitch angle, θ′.

2. A method according to claim 1, further comprising controlling the wind turbine in accordance with the estimated wind speed, v.sub.est.

3. A method according to claim 1, wherein deriving a blade torsion contribution comprises using a blade torsion look-up table comprising interrelated values of rotational speed, ω, pitch angle, θ, and blade torsion contribution.

4. A method according to claim 3, further comprising generating the blade torsion look-up table by means of a model based simulation.

5. A method according to claim 1, wherein estimating a wind speed, v.sub.est, comprises using a c.sub.P look-up table comprising interrelated values of rotational speed, ω, wind speed, v, adjusted pitch angle, θ′, and power coefficient, c.sub.P.

6. A method according to claim 1, wherein estimating a wind speed, v.sub.est, is performed as an iterative process.

7. A method according to claim 1, wherein estimating a wind speed, v.sub.est, comprises estimating a power output, P.sub.est, of the wind turbine, and comparing the estimated power output, P.sub.est, to a measured power output, P.sub.meas, of the wind turbine.

8. A wind turbine comprising a rotor carrying a set of wind turbine blades, each wind turbine blade having a variable pitch angle; and a controller configured to implement an operation, comprising: obtaining a rotational speed, ω, of the rotor, obtaining a pitch angle, θ, of the wind turbine blades, deriving a blade torsion contribution, based on the obtained rotational speed, ω, and the obtained pitch angle, θ, calculating an adjusted pitch angle, θ′, as a sum of the obtained pitch angle, θ, and the derived blade torsion contribution, and estimating a wind speed, v.sub.est, based on the obtained rotational speed, ω, and the calculated adjusted pitch angle, θ′.

9. A computer program product comprising computer readable executable code which, when executing on one or more processors, implements an operation, comprising: obtaining a rotational speed, ω, of the rotor, obtaining a pitch angle, θ, of the wind turbine blades, deriving a blade torsion contribution, based on the obtained rotational speed, ω, and the obtained pitch angle, θ, calculating an adjusted pitch angle, θ′, as a sum of the obtained pitch angle, θ, and the derived blade torsion contribution, and estimating a wind speed, v.sub.est based on the obtained rotational speed, ω, and the calculated adjusted pitch angle, θ′.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The invention will now be described in further detail with reference to the accompanying drawings in which

[0044] FIG. 1 is a block diagram illustrating a method according to an embodiment of the invention,

[0045] FIG. 2 is a graph illustrating estimated wind speed as a function of free wind speed for a prior art method and a method according to an embodiment the invention, and

[0046] FIG. 3 is a flow chart illustrating a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a block diagram illustrating a method according to an embodiment of the invention, in the form of a closed loop approximation process. A rotational speed, ω, of the rotor of a wind turbine, and a pitch angle, θ, of the wind turbine blades of the wind turbine are measured and supplied to an estimating block 1. Furthermore, a previously estimated wind speed, v.sub.est, is also supplied to the estimating block 1.

[0048] In the estimating block 1, an estimated power output, P.sub.est, is calculated, based on the supplied parameters, ω, θ and v.sub.est. This includes deriving a power coefficient, c.sub.P, e.g. by means of a c.sub.P look-up table, based on an estimated tip speed ratio, λ.sub.est, and the pitch angle, θ.

[0049] However, before deriving the power coefficient, c.sub.P, an adjusted pitch angle, θ′, is calculated as the sum of the measured pitch angle, θ, and a blade torsion contribution representing torsion introduced in the wind turbine blade, resulting in the wind turbine blade being twisted about a longitudinal axis of the wind turbine blade. The blade torsion contribution is derived on the basis of the measured rotational speed, ω, and the measured pitch angle, θ, e.g. using a blade torsion look-up table. This may, e.g., be performed in the manner described above. The adjusted pitch angle, θ′, is used as an input for deriving the power coefficient, c.sub.P. Thereby the power coefficient, c.sub.P, is derived on the basis of the actual pitch angle, θ′, which the wind experiences when meeting the wind turbine blade, instead of on the basis of the set pitch angle, θ, which is not representative for the angular position of the wind turbine blade, due to the torsion introduced in the wind turbine blade. Accordingly, the derived power coefficient, c.sub.P, is accurate and reliable, and thereby the estimated power output, P.sub.est, is also accurate and reliable.

[0050] The estimated power output, P.sub.est, is compared to a measured power output, P.sub.meas, at a comparator 2. This results in an error signal, P.sub.err. If the error signal, P.sub.err, is zero, then P.sub.est=P.sub.meas, indicating that the estimated wind speed, v.sub.est, supplied to the estimator block 1 is equal to or close to the actual wind speed prevailing at the wind turbine.

[0051] If the error signal, P.sub.err, is positive, then the measured power output, P.sub.meas, is larger than the estimated power output, P.sub.est, indicating that the expected power output at the estimated wind speed, v.sub.est, is lower than the actual power output. This indicates that the estimated wind speed, v.sub.est, is lower than the actual wind speed prevailing at the wind turbine, and the estimated wind speed, v.sub.est, should therefore be increased.

[0052] Similarly, if the error signal, P.sub.err, is negative, then the measured power output, P.sub.meas, is smaller than the estimated power output, P.sub.est, indicating that the expected power output at the estimated wind speed, v.sub.est, is higher than the actual power output. This indicates that the estimated wind speed, v.sub.est, is higher than the actual wind speed prevailing at the wind turbine, and the estimated wind speed, v.sub.est, should therefore be decreased.

[0053] The adjustments to the estimated wind speed, v.sub.est, described above, are performed in the following manner. The error signal, P.sub.err, is multiplied by a gain factor, k.sub.est, at multiplier 3, and the resulting signal is supplied to an integrator 4. At the integrator 4 the signal received from the multiplier 3 is integrated, resulting in a new estimated wind speed, v.sub.est, which is supplied to the estimating block 1 for the next iteration.

[0054] FIG. 2 is a graph illustrating estimated wind speed as a function of free wind speed for a prior art method and a method according to an embodiment the invention. The free wind speed is the wind speed prevailing immediately in front of the rotor of the wind turbine. It is attempted to estimate this wind speed, e.g. in the manner described above with reference to FIG. 1. If the wind speed is estimated accurately, a diagonal graph will result, i.e. the estimated wind speed is equal to the free wind speed at all wind speeds.

[0055] In the graph of FIG. 2 the solid line represents a wind speed estimated using a prior art method, and the dotted line represents a wind speed estimated using a method according to an embodiment of the invention.

[0056] In the solid graph, representing the prior art method, the measured pitch angle, θ, is used as an input for deriving the power coefficient, c.sub.P. It can be seen that this results in a graph which deviates from the diagonal. In particular, for high wind speeds the estimated wind speed is significantly lower than the free wind speed. Accordingly, an inaccurate estimated wind speed is obtained in this case, in particular at high wind speeds.

[0057] On the other hand, in the dotted graph, representing the method according to an embodiment of the invention, an adjusted pitch angle, θ′, is used as an input for deriving the power coefficient, c.sub.P. The pitch angle is adjusted by adding a blade torsion contribution to the measured pitch angle, θ. Thereby the input used for deriving the power coefficient, c.sub.P, reflects the actual conditions prevailing at the wind turbine blades, notably the effective pitch angle of the blade.

[0058] It can be seen that using the adjusted pitch angle, θ′, as an input for deriving the power coefficient, c.sub.P, results in a graph which substantially follows the diagonal. Thus, the estimated wind speed is equal to or very close to the free wind speed. Accordingly, a very accurate and reliable estimate for the wind speed is thereby obtained, when using a method according to an embodiment of the invention.

[0059] FIG. 3 is a flow chart illustrating a method according to an embodiment of the invention. The process is started at step 5. At step 6, an initial guess for an estimated wind speed, v.sub.est, is provided, in order to initiate an iterative process.

[0060] At step 7 a rotational speed, ω, of a rotor of a wind turbine, and a pitch angle, θ, of wind turbine blades of the wind turbine, are measured.

[0061] At step 8 a blade torsion contribution is derived, based on the obtained rotational speed, ω, and pitch angle, θ. The blade torsion contribution represents torsion introduced in the wind turbine blades, and resulting in an effective pitch angle of the wind turbine blades, which differs from the measured pitch angle, θ. The blade torsion contribution may, e.g., be derived using a blade torsion look-up table.

[0062] At step 9 an adjusted pitch angle, θ′, is calculated as the sum of the measured pitch angle, θ, and the derived torsion contribution. Thus, the adjusted pitch angle, θ′, represents the effective pitch angle, i.e. the pitch angle which is actually experienced by the wind meeting the wind turbine blade.

[0063] At step 10 a power coefficient, c.sub.P, is derived, based on the measured rotational speed, ω, the adjusted pitch angle, θ′ and the estimated wind speed, v.sub.est. Since the adjusted pitch angle, θ′, and not the measured pitch angle, θ, is used as an input for deriving the power coefficient, c.sub.P, it is ensured that the derived power coefficient, c.sub.P, is closer to an actual power coefficient of the wind turbine, because it more closely reflects the actually prevailing conditions.

[0064] At step 11 a new wind speed, v.sub.est, is estimated, based on the derived power coefficient, c.sub.P. Accordingly, the new estimated wind speed, v.sub.est, is also very accurate.

[0065] At step 12 the wind turbine is operated in accordance with the estimated wind speed, v.sub.est. Simultaneously, the new estimated wind speed, v.sub.est, is provided for the next iteration, at step 13. Accordingly, the process is returned to step 7, and the process described above is repeated. However, the new estimated wind speed, v.sub.est, is applied instead of the initial guess which was provided at step 6. Thus, the method illustrated in FIG. 3 is performed in an iterative manner, in which the estimated wind speed, v.sub.est, is improved for each iteration.

[0066] While embodiments of the invention have been shown and described, it will be understood that such embodiments are described by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the scope of the present invention as defined by the appended claims. Accordingly, it is intended that the following claims cover all such variations or equivalents as fall within the spirit and the scope of the invention.