Method for controlling a wind turbine when the power setpoint value is changed

Abstract

A method is disclosed for controlling a wind turbine, where the wind turbine includes with a tower and a rotor and comprises having at least one rotor blade with an adjustable blade pitch angle, and where a change in a power value takes place in a time interval (TE) and by the control of one or more operating parameters which determine power to be fed in by the wind turbine. The method comprises determining a parameterized time function of a tower deflection for the time interval (TE). A series of boundary conditions are defined for the parameterized time function of the tower deflection and a thrust of the rotor of the wind turbine is determined for the parameterized time function of the tower deflection. A function is then calculated for controlling the one or more operating parameters from the thrust of the rotor.

Claims

1. A method for controlling a wind turbine, the wind turbine including a tower and a rotor, and comprising at least one rotor blade including an adjustable blade pitch angle, wherein a change in a power value takes place in a time interval (TE) and by control of one or more operating parameters which determine power to be fed in by the wind turbine, the method comprising: determining a parameterized time function of a tower deflection for the time interval (TE), wherein a series of boundary conditions are defined for the parameterized time function of the tower deflection; determining a thrust of the rotor of the wind turbine for the parameterized time function of the tower deflection; calculating a function for controlling the one or more operating parameters from the thrust of the rotor; and controlling the rotor based on the calculated function, wherein the parameterized time function of the tower deflection comprises a linearly increasing portion and an oscillatory increasing portion.

2. The method according to claim 1, wherein the change in the power value comprises a change in a power setpoint value.

3. The method according to claim 2, wherein the parameterized time function of the tower deflection exhibits a maximum tower deflection which does not exceed beyond an end position of the tower deflection.

4. The method according to claim 3, wherein the parameterized time function of the tower deflection does not exceed a maximum change in the power setpoint value in a predetermined time interval.

5. The method according to claim 1, wherein the function for controlling the one or more operating parameters comprises adjustment of the blade pitch angle of the at least one rotor blade.

6. The method according to claim 1, wherein the parameterized time function of the tower deflection comprises one or more tower-specific constants (ω.sub.T).

7. The method according to claim 1, wherein the parameterized time function of the tower deflection complies with boundary conditions, and wherein first and second derivatives of the parameterized time function of the tower deflection are cleared when a new power setpoint value is reached.

8. A method for controlling a wind turbine, the wind turbine including a tower and a rotor, and comprising at least one rotor blade including an adjustable blade pitch angle, wherein a change in a power value takes place in a time interval (TE) and by control of one or more operating parameters which determine power to be fed in by the wind turbine, the method comprising: determining a parameterized time function of a tower deflection for the time interval (TE), wherein a series of boundary conditions are defined for the parameterized time function of the tower deflection; determining a thrust of the rotor of the wind turbine for the parameterized time function of the tower deflection; calculating a function for controlling the one or more operating parameters from the thrust of the rotor; and controlling the rotor based on the calculated function, wherein the parameters of the parameterized time function are determined by boundary conditions so that the parameterized time function describes an unambiguous course of movement for the tower.

9. The method according to claim 8, wherein the parameterized time function of the tower deflection comprises a linearly increasing portion and an oscillatory increasing portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method according to the invention is described in more detail below. The figures show:

(2) FIG. 1 illustrates a graphical representation of an example of the time-dependent course of the tower deflection with different applied force curves; and

(3) FIG. 2 illustrates a further graphical representation of an example for the course of the tower deflection over time with a non-linear force progression.

DETAILED DESCRIPTION OF THE INVENTION

(4) The central idea of the method according to the invention is to define the parameterized time function L(t), which determines a change in the operating parameters. For the change of the operating parameters, the thrust acting on the tower of the wind turbine over time is calculated. From the force progression over time and knowing the oscillation equation for the tower of the wind turbine, the tower oscillation can then be determined using initial conditions. The oscillation curve of the tower determines how strongly the tower is mechanically loaded.

(5) In principle, such problems can be formulated mathematically in different ways. In order to gain a good understanding, the differential equation for tower deflection is used here. This is the differential equation of second order for a damped harmonic oscillation with an impressed excitation. Let L(t) be a time function describing the deflection of the tower, and let F(t) be a time function describing the force on the tower head. Starting from defined initial conditions for t=0, such as L(0)={dot over (L)}(0)=0, the behavior of the tower oscillation over time can be described by the following differential equations:

(6) $L+\frac{2D}{{\omega }_T}\stackrel{.}{L}+\frac{\stackrel{\mathrm{.Math.}}{L}}{{\omega }_T^2}=F.$

(7) For the sake of simplicity, the tower spring constant has been set to one, so that the oscillation is determined by the tower natural frequency ω.sub.T and the damping D of the tower oscillation.

(8) The boundary conditions indicate that the tower is at rest at the beginning of the observation (t=0) and has no initial speed. Of course, a starting position corresponding to the current tower position and speed could also be selected. The time function L(t), which describes the tower deflection, is substituted into the differential equation and thus gives the force function F(t).

(9) To emphasize the relation between the tower deflection in the time and force function, this is demonstrated with solving the differential equation. The following parameterized time function is assumed to be advantageous for the tower deflection:

(10) $L=\frac{L_E}{T_E}t-b*\sin \left(\frac{2\pi }{T_E}t\right),$
where L.sub.E is the end position of the tower deflection at time t=T.sub.E. T.sub.E describes here the duration of the predetermined time interval in which the power setpoint value is to be changed; b is a parameter which weights the oscillatory part of the time function. The first derivative of the time function L(t) is

(11) $\stackrel{.}{L}=\frac{L_E}{T_E}-b*\frac{2\pi }{T_E}\cos \left(\frac{2\pi }{T_E}t\right).$

(12) The second derivative of the time function L(t) is

(13) $\stackrel{\mathrm{.Math.}}{L}=b*{\left(\frac{2\pi }{T_E}\right)}^2\sin \left(\frac{2\pi }{T_E}t\right).$

(14) Taking into account the boundary condition L(T.sub.E)=L.sub.E, according to which the tower deflection has reached its end position L.sub.E at the end point in time T.sub.E, the following condition results for parameter b:

(15) $b=\frac{L_E}{2\pi }.$

(16) With this value of b, moreover, the first derivative at the start and end point in time is 0, so the tower head is at rest at the beginning (t=0) and at the end (t=T.sub.E) with a speed of zero.

(17) For the force curve, two cases can be distinguished. The first case is given when the time period T.sub.E is chosen as

(18) $\left(\frac{2\pi }{{\omega }_T}\right)=\frac{1}{f_T}.$
With the parameter value

(19) $T_E=\left(\frac{2\pi }{{\omega }_T}\right)=\frac{1}{f_T},$
substituted in the equation above, the following force curve F(t) results:

(20) $F\left(t\right)=\frac{L_E}{T_E}t+\frac{2D}{{\omega }_T}\frac{L_E}{T_E}\left(1-\cos \left(\frac{2\pi }{T_E}t\right)\right).$

(21) However, this equation is only true if

(22) $T_E=\left(\frac{2\pi }{{\omega }_T}\right)=\frac{1}{f_T}.$

(23) The second case is given if T.sub.E is specified independently of the tower natural frequency ω.sub.T, for example by grid requirements or other specifications. In this case the general rule applies:

(24) 0$F\left(t\right)=\frac{L_E}{T_E}t+\frac{L_E}{2\pi }\left(\frac{1}{{\omega }_T^2}\frac{{\left(2\pi \right)}^2}{T_E^2}-1\right)*\sin \left(\frac{2\pi }{T_E}\tau \right)+\frac{2D}{{\omega }_T}\frac{L_E}{T_E}\left(1-\cos \left(\frac{2\pi }{T_E}t\right)\right).$

(25) This force curve on the wind turbine leads to the tower deflection, which can be described with the time function L(t).

(26) Assuming a tower damping D=3×10.sup.−3 and T.sub.E=2π/ω.sub.T, the oscillating term can be neglected and there is an approximately linear increase of the ramp force. FIG. 1 shows with line 10 the force curve over time, while the dotted line 12 shows the tower deflection over time. Line 14 shows the speed of the tower movement.

(27) FIG. 2 shows the behavior of the tower at a tower damping D=2×10-1 and T.sub.E<2π/ω.sub.T. This significantly stronger damping of the tower and the shorter time T.sub.E causes the cosine term and the sine term to come into play in term F(t), i.e. the tower oscillates. FIG. 2 shows the tower deflection indicated by the dashed line 18 that the force 20 acting on the tower does not grow linearly but fluctuates in an oscillatory manner. It can also be clearly seen that there is a different speed profile 22. It should be noted that in FIG. 2 the specified end point in time is to be reached earlier than in FIG. 1, i.e. the period TE is selected to be shorter.

(28) The relationship between the blade pitch angle as the operating parameter and the thrust of the rotor on the tower F.sub.s of the wind turbine is given by the following equation
F.sub.s=ct(pitch,TSR)*0.5*A*rho*v.sup.2,
where A is the rotor area, v is the wind speed, pitch is the blade pitch angle, TSR is the tip speed ratio and rho is the air density. It is the thrust coefficient which is a function of the blade pitch angle and the tip speed ratio TSR.

(29) From the equation above, the relationship between changes in operating parameters, such as blade pitch angle, and the thrust of the rotor on the tower Fs can now be determined. Of course, this relationship can also be used in the opposite direction, for example if there is a preferred time-related force curve for the tower. So, it is possible to reverse-engineer which blade pitch angle at which speed TSR is to be set at the wind turbine in order to achieve the desired time-related force curve.

(30) The above embodiment with the method according to the invention starts from a curve L(t) describing the deflection of the tower over time. Such a deflection-curve may occur in a situation in which a power setpoint value, in particular a setpoint of the actual power is changed. There are other situations where changes in the electrical setpoint value can cause oscillations of the tower which can be reduced or even avoided using the method according to the invention. Such situations encompass but are not limited to turbine-controlled stops during which the wind turbine is shut down. Another situation in which changes of the electrical setpoint value also occur is for example curtailment operation of the wind turbine. Curtailment operation can be used in different situations e.g. due to noise reduction, cast shadows in the proximity of the wind turbine, or due to turbulence either in a wind park or for a stand-alone wind turbine. The electrical power value is changed when the curtailment operation is initiated and when the curtailment operation ends. In both situations there is a change in the electrical setpoint value for the active power. Further rapid changes in the electrical power value can occur during a fault ride through (FRT) in which the wind turbine operation is controlled while grid voltage is low. During brake procedures for the wind turbine there can also occur situations which require a fast power ramp for the wind turbine.

(31) According to different grid-codes in different countries, there are different limitations known for the fastest power decrease which can be demanded, e.g. in Poland and Estonia a power decrease from 100% to 20% of the nominal power within 2 seconds can be demanded. Other grid-codes use different limitations such as the high voltage grid-code in Germany which can demand a power reduction from 100% to 46% nominal power within 2 seconds.

(32) The usual displacement of the nacelle of the wind turbine related to a vertical axis of the tower is about 2 m in nominal operation of the wind turbine. At fast power ramps with a quick change in the active power setpoint value of the wind turbine the nacelle can swing back and forth by up to 2.5 m. A typical period of a tower oscillation is 6 seconds for a 125 m tower.

LIST OF REFERENCE SIGNS

(33) 10 Linear force curve 12 Tower deflection 14 Speed 18 Tower deflection 20 Force curve 22 Speed