ROBUST CONTROL OF WIND TURBINES WITH SOFT-SOFT TOWER

Abstract

A controller for a wind turbine including a rotor and a nacelle arranged on a tower is provided, the tower having a fundamental frequency close to or below a rated rotational frequency of the rotor. The controller includes a rotor speed control module including a first linear time invariant control system adapted to generate a first pitch control signal based on a rotor speed error signal, a tower damping module including a second linear time invariant control system adapted to generate a second pitch control signal based on a nacelle acceleration signal, and an output module adapted to output a pitch control signal based on the first pitch control signal and the second pitch control signal. Furthermore, a wind turbine and a method of controlling a wind turbine is provided.

Claims

1. A controller for a wind turbine comprising a rotor and a nacelle arranged on a tower, the tower having a fundamental frequency close to or below a rated rotational frequency of the rotor, the controller comprising: a rotor speed control module comprising a first linear time invariant control system configured to generate a first pitch control signal based on a rotor speed error signal; a tower damping module comprising a second linear time invariant control system configured to generate a second pitch control signal based on a nacelle acceleration signal; and an output module configured to output a pitch control signal based on the first pitch control signal and the second pitch control signal, wherein the first linear time invariant control system comprises a plurality of first linear time invariant control units and a first interpolation unit, wherein the first interpolation unit is configured to generate the first pitch control signal based on an interpolation of respective outputs of the first linear time invariant control units.

2. The controller according to claim 1, wherein the second linear time invariant system comprises a plurality of second linear time invariant control units and a second interpolation unit, wherein the second interpolation unit is configured to generate the second pitch control signal based on an interpolation of respective outputs of the second linear time invariant control units.

3. The controller according to claim 1, wherein the first and/or second interpolation unit is configured to apply interpolation based on an operating point of the wind turbine, based on the pitch control signal and/or a wind speed signal.

4. The controller according to claim 1, wherein the plurality of first linear time invariant control units is a plurality of first state space control units, and/or wherein the plurality of second linear time invariant control units is a plurality of second state space control units.

5. The controller according to claim 1, wherein the nacelle acceleration signal is indicative of a fore-aft acceleration of the nacelle.

6. The controller according to claim 1, wherein the tower damping module is configured to dampen the 1.sup.st fore-aft eigen mode of the tower.

7. The controller according to claim 1, wherein the output module is configured to add the first and second pitch control signals to generate the pitch control signal.

8. The controller according to claim 1, wherein the tower damping module further comprising a moving notch filter configured to filter a selected multiple of the rotor rotational frequency from the second pitch control signal.

9. The controller according to claim 1, wherein the tower damping module further comprising a phase delay network configured to apply a gain over a frequency range to the second pitch control signal in dependency of an operating point of the wind turbine, in particular based on the pitch control signal and/or a filtered wind speed signal.

10. The controller according to claim 1, wherein the first linear time invariant control system is further configured to generate the first pitch control signal based on a nacelle acceleration signal.

11. The controller according to claim 1, wherein the first linear time invariant control system and the second linear time invariant control system are generated utilizing H-infinity methods and Quantitative Feedback Theory.

12. The controller according to claim 11, wherein the H-infinity and Quantitative Feedback Theory methods are utilized iteratively or exclusively to, for a plurality of selected operating points, define frequency domain tower load specifications and synthesize controllers based on wind turbine linear models.

13. A wind turbine comprising a rotor and a nacelle arranged on a tower, the tower having a fundamental frequency close to or below a rated rotational frequency of the rotor, the wind turbine further comprising a controller according to claim 1.

14. A method of controlling a wind turbine comprising a rotor and a nacelle arranged on a tower, the tower having a fundamental frequency close to or below a rated rotational frequency of the rotor, the method comprising: generating, in a rotor speed control module comprising a first linear time invariant control system, a first pitch control signal based on a rotor speed error signal; generating, in a tower damping module comprising a second linear time invariant control system, a second pitch control signal based on a nacelle acceleration signal; and outputting a pitch control signal based on the first pitch control signal and the second pitch control signal, wherein the first linear time invariant control system comprises a plurality of first linear time invariant control units and a first interpolation unit, wherein the first interpolation unit is adapted to generate the first pitch control signal based on an interpolation of respective outputs of the first linear time invariant control units.

Description

BRIEF DESCRIPTION

[0044] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0045] FIG. 1 shows a rotor speed control module of a wind turbine controller according to an embodiment of the present invention; and

[0046] FIG. 2 shows a tower damping module of a wind turbine controller according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0047] Embodiments of the present invention provide a controller for a wind turbine comprising a rotor and a nacelle arranged on a tower, the tower having a fundamental frequency close to or below a rated rotational frequency of the rotor. Such a tower is commonly referred to as a soft-soft tower. The controller comprises two control modules, namely a rotor speed control module 1 as shown in FIG. 1 and discussed below, and a tower damping module 2 as shown in FIG. 2 and discussed further below. The rotor speed control module 1 generates and outputs a first pitch control signal P1. The tower damping module 2 generates and provides a second pitch control signal P2. The first pitch control signal P1 and the second pitch control P2 are combined, in particular added, by an output module (not shown) and passed on to the pitch regulating system of the wind turbine, as an incremental signal indicating a signed amount of adjustment to be applied to the pitch.

[0048] FIG. 1 shows a rotor speed control module 1 of a wind turbine controller according to an embodiment of the present invention. The rotor speed control module 1 comprises a (first) linear time invariant control system 10 coupled to receive a rotor speed error signal 11 as an input, and optionally a nacelle acceleration signal 17 as further input, as a basis for generating a (first) pitch control signal P1. The rotor speed error signal 11 is obtained as a difference between either a filtered rotor speed signal 12 or a rotor speed signal 13 and a rotor speed reference signal 16. Whether the filtered rotor speed signal 12 or the (unfiltered) rotor speed signal 13 is used, is determined by control signal 14 which controls the switch 15. The rotor speed reference signal 16 is the current set point for the controller. The linear time invariant control system 10 further receives a power loop pitch control signal 18 that is used to develop an anti-windup strategy in the control, and a filtered pitch control signal 19 that provides information about the operating point of the wind turbine.

[0049] In this exemplary embodiment, the (first) linear time invariant control system 10 is a state space control system comprising a number of state space control units, each state space control unit being developed for a particular operating point, such as low wind speeds, rated wind speeds, and high wind speeds. The state space control system 10 includes a scheduling control scenario which interpolates the different state space control units (or state-space represented controllers) according to predetermined interpolation control laws to generate a non-linear pitch control action. The below formulas show the state-space representation of one of these control units that are interpolated in the state space control system 10.

[00001]$X_{\mathrm{SControl}}\left(k+1\right)={\mathrm{Ad}}_{\mathrm{SControl}}{X}_{\mathrm{SControl}}\left(k\right)+{\mathrm{Bd}}_{\mathrm{SControl}}\left[\begin{array}{c}\mathrm{GenSpeedError}\left(k\right)\\ \mathrm{NacXAcc}\left(k\right)\end{array}\right]$${\mathrm{Pitch}}_{\mathrm{SControl}}\left(k+1\right)={\mathrm{Cd}}_{\mathrm{SControl}}{X}_{\mathrm{SControl}}\left(k\right)+{\mathrm{Dd}}_{\mathrm{SControl}}\left[\begin{array}{c}\mathrm{GenSpeedError}\left(k\right)\\ \mathrm{NacXAcc}\left(k\right)\end{array}\right]$

[0050] Here, k+1 is the present sample and k the last sample. Ad.sub.SControl, Bd.sub.SControl, Cd.sub.SControl and Dd.sub.SControl are the discretized state space matrices which represent the controller dynamics. X.sub.SControl(k+1) is the present vector of controller states and X.sub.SControl(k) is the vector of states of the last sample. Pitch.sub.SControl(k+1) is the output P1 from the control system 10. GenSpeedError(k) is the rotor speed error and NacXAcc(k) is the nacelle acceleration (in the fore-aft direction).

[0051] FIG. 2 shows a tower damping module 2 of a wind turbine controller according to an embodiment of the present invention. The tower damping module 2 comprises a (second) linear time invariant control system 20 coupled to receive the nacelle acceleration signal 17, a pitch control signal 21 (which is equal to P1 in FIG. 1), and an operational state signal 22 as inputs. Based on these inputs, the linear time invariant control system 20 determines an output signal 23 which is indicative of a pitch control correction amount to be applied to the first pitch control signal P1 determined by the rotor speed control module 1 (as described above) in order to reduce or eliminate undesirable impact from the oscillating tower movement. The output 23 is passed through a moving notch filer 30 to filter out a selected multiple of the rotor rotational frequency. The moving notch filter receives information on the particular frequency from multiplier 25 which multiplies a low-pass filtered rotor speed signal 26 and a factor 27, such as e.g., one or three, and inputs the resulting frequency to the filter 30. The filtered output signal 32 is passed on to phase delay network 35 which, dependent on a filtered wind speed signal 36 as an indication of the operating point of the wind turbine, applies a gain over a frequency range to the filtered output signal 32. The resulting output P2 from the phase delay network 35 is the (second) pitch control signal P2 that serves to reduce or eliminate the influence of tower oscillating movement on the pitch control of the wind turbine.

[0052] In this exemplary embodiment, the (second) linear time invariant control system 20 is a state space control system comprising a number of state space control units, each state space control unit being developed for a particular operating point, such as low wind speeds, rated wind speeds, and high wind speeds. The state space control system 20 includes a scheduling control scenario which interpolates the different state space control units (or state-space represented controllers) according to predetermined interpolation control laws to generate a non-linear pitch control action. The below formulas show the state-space representation of one of these control units that are interpolated in the state space control system 20.

X.sub.ATD(k+1)=Ad.sub.ATDX.sub.ATD(k)+Bd.sub.ATDNacXAcc(k)

Pitch.sub.ATD(k+1)=Cd.sub.ATDX.sub.ATD(k)+Dd.sub.ATDNacXAcc(k)

[0053] Here, k+1 is the present sample and k the last sample. Ad.sub.ATD, Bd.sub.ATD, Cd.sub.ATD and Dd.sub.ATD are the discretized state space matrices which represent the controller dynamics. X.sub.ATD(k+1) is the present vector of controller states and X.sub.ATD(k) is the vector of states of the last sample. PitchAm (k+1) is the output 23 from the control system 20. NacXAcc(k) is the nacelle acceleration (in the fore-aft direction).

[0054] 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.

[0055] 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.