FLOATING WIND TURBINE CONTROL BELOW RATED WIND SPEED

Abstract

A motion controller for a floating wind turbine including a number of rotor blades is provided. The motion controller is arranged to adjust the blade pitch of each rotor blade when the floating wind turbine is operating in winds below the rated wind speed so as to create a net force that damps a surge motion of the floating wind turbine. Also provided is a method of damping the motion of a floating wind turbine and a wind turbine having such a motion controller.

Claims

1. A motion controller for a floating wind turbine comprising a plurality of rotor blades, wherein the motion controller is arranged to adjust the blade pitch of each rotor blade when the floating wind turbine is operating in winds below the rated wind speed so as to create a net force that damps a surge motion of the floating wind turbine.

2. A motion controller as claimed in claim 1, wherein the controller is arranged to calculate the blade pitch adjustment of one or more of the rotor blades based on an input of the surge motion.

3. A motion controller as claimed in claim 2, wherein the input is based on a measurement and/or estimation of the surge velocity of the floating wind turbine.

4. A motion controller as claimed in claim 3, wherein the blade pitch adjustment is proportional to the surge velocity.

5. A motion controller as claimed in claim 3, wherein the controller is arranged to adjust the blade pitch of each rotor blade with a phase relative to the floating wind turbine surge velocity so as to provide the damping force.

6. A motion controller as claimed in claim 2, wherein the input of the surge motion is measured and/or estimated using the output from one or more sensors.

7. A motion controller as claimed in claim 2, wherein the controller is configured to use a low pass filter on the input.

8. A motion controller as claimed in claim 7, wherein the low pass filter comprises a transfer function.

9. A motion controller as claimed in claim 7, wherein the low pass filter is arranged to filter out frequencies above 0.017 Hz.

10. A floating wind turbine comprising a rotor with a plurality of rotor blades, and the motion controller of claim 1.

11. A method of controlling a floating wind turbine comprising a plurality of rotor blades, the method comprising: adjusting the blade pitch of each rotor blade when the wind turbine is operating in wind speeds below the rated wind speed so as to create a net force that damps a surge motion of the floating wind turbine.

12. A method as claimed in claim 11, the method comprising: inputting to the controller an input of the surge motion of the floating wind turbine; and adjusting the blade pitch of each rotor blade based on the input of the surge motion.

13. A method as claimed in claim 12, wherein the input is based on a measurement and/or estimation of the surge velocity of the floating wind turbine

14. A method as claimed in claim 13, wherein the blade pitch adjustment is proportional to the surge velocity.

15. A method as claimed in claim 13, the method comprising adjusting the blade pitch of each rotor blade with a phase relative to the floating wind turbine surge velocity so as to provide the damping force.

16. A method as claimed in claim 12, wherein the input of the motion is measured and/or estimated using the output from one or more sensors.

17. A method as claimed in claim 16, wherein the one or more sensors comprise a motion sensor and/or a global positioning sensor.

18. A method as claimed in claim 12, the method comprising using a low pass filter on the input.

19. A method as claimed in claim 18, wherein the low pass filter comprises a transfer function.

20. A method as claimed in claim 18, wherein the low pass filter filters out frequencies above 0.017 Hz.

21. A method as claimed in claim 11, wherein the method is performed using the controller of claim 1.

22. A computer program product comprising instructions that, when executed on processing circuitry for a floating wind turbine, will configure the processing circuitry to perform the method of claim 11.

Description

[0082] Certain embodiments will now be described by way of example only and with reference to the accompanying drawings in which:

[0083] FIG. 1 shows a floating wind turbine;

[0084] FIG. 2 is a block diagram of a conventional controller for a floating wind turbine;

[0085] FIG. 3 is a block diagram of a controller for a floating wind turbine with active surge damping control according to an embodiment of the invention; and

[0086] FIGS. 4 and 5 are graphs showing results from a simulation comparing a floating wind turbine having a controller without active surge damping with a floating wind turbine having a controller with active surge damping control.

[0087] Turning to FIG. 1, there is illustrated a floating wind turbine assembly 1. It comprises a turbine rotor 2 mounted to a nacelle 3. The nacelle is in turn mounted to the top of a structure which comprises a tower 4 secured to the top of a floating body 5, which in the example shown is a spar-buoy like structure. The disclosed principles of controlling surge motion are applicable to all floating structures for floating wind turbines. The floating body is secured to the sea bed by one or more anchor lines 7 (only one is shown), these could be taut or catenery mooring lines. The nacelle contains an electrical generator that is connected to the turbine rotor by any known means such as a reduction gearbox, by direct connection to the electrical generator or hydraulic transmission etc. (these items are not shown). The nacelle also contains a control unit.

[0088] The floating wind turbine is subject to incoming wind U.sub.W forces and wave 9 forces. (The waves 9 on the water's surface are shown schematically.) These forces will cause the floating wind turbine assembly 1 to move about in the water. Movement in the axial direction (i.e. axial relative to axis A of the rotor) is referred to as surge motion.

[0089] The controller in the nacelle is arranged to control a blade pitch of the rotor blades 2. In conventional controllers (e.g. the standard controller depicted in FIG. 2), when the wind turbine is operating in winds below the rated wind speed, the blade pitch is approximately constant at an angle that produces maximum power. In a controller according to the present invention, when the wind turbine is operating in winds below the rated wind speed the blade pitch is adjusted in order to damp the surge motion of the floating wind turbine.

[0090] FIG. 2 illustrates a conventional blade pitch controller 10 for a floating wind turbine. The standard blade pitch controller 10 is particularly for controlling the pitch of the blades of the floating wind turbine when the wind speed is below the rated wind speed.

[0091] When the wind speed is below the rated wind speed, the standard blade pitch controller 10 holds the pitch of the blades at an approximately constant angle according to an original blade pitch signal ?.sub.ref1. The pitch of the blades corresponding to the original blade pitch signal ?.sub.ref1 is typically selected to maximise the power extracted at wind speeds below the rated wind speed, and can be determined according to known methods. The standard blade pitch controller 10 does not account for any motions of the wind turbine structure itself, however.

[0092] FIG. 3 illustrates a motion controller 20 for a floating wind turbine according to an embodiment of the present invention. The motion controller 20 can account for surge motion that the floating wind turbine may undergo. The motion controller 20 comprises an active damping controller 22 for calculating an additional blade pitch adjustment signal ?.sub.ref2 for damping the surge motion. The active damping controller 22 is coupled to a standard blade pitch controller, e.g. the standard blade pitch controller 10 of FIG. 2.

[0093] The active damping controller 22 comprises an active damping control loop 24 for calculating the additional blade pitch adjustment signal ?.sub.ref2 for damping the rigid body surge motions of the wind turbine. The damping control loop 24 comprises a signal processing block 26, an active damping controller gain K.sub.s, and a controller transfer function h.sub.s(s).

[0094] Operation of the motion controller 20 occurs as follows. The standard blade pitch controller 10 receives or produces an original blade pitch signal ?.sub.ref1. In the present embodiment, the original blade pitch signal ?.sub.ref1 corresponds to an approximately constant angle or pitch of the rotor blades while the wind speed is below rated wind speed. In modified embodiments, the original blade pitch signal ?.sub.ref1 may not be constant and may include other adjustment signals.

[0095] In the active damping control loop 24, a measured or estimated surge velocity of the wind turbine vs (which may also be referred to as x.sub.s) is processed by the signal processing means 26 and is then operated on by the active damping controller gain K.sub.s and the controller transfer function h.sub.s(s). This produces the additional blade pitch adjustment signal ?.sub.ref2.

[0096] The additional blade pitch adjustment signal ?.sub.ref2 is then added to the original blade pitch signal ?.sub.ref1 to produce a total blade pitch adjustment signal ?.sub.ref which is used to control the blades of the wind turbine so as to damp the surge motion while maximising power extraction. This reduces the forces on the wind turbine structure and mooring system whilst maximising power output for the given wind speed.

[0097] The signal processing block 26 uses a sharp low pass filter with a filter frequency that is sufficiently below the wave frequency range (0.05 to 0.2 Hz) in order to avoid damping of wave induced motion, which would lead to bad performance with respect to key wind turbine parameters. The filter frequency may depend on the natural surge frequency of the floating wind turbine. It may be around 0.004 to 0.017 Hz.

[0098] The value of the active damping controller gain K.sub.s will be tailored depending on the frequencies of the surge motion being damped. The exact value that is used for this parameter may be found by conventional controller tuning.

[0099] FIGS. 4 and 5 show the results of a simulation to help illustrate the benefits of active wind turbine control below the rated wind speed that accounts for surge motion. FIG. 4 shows the surge motions for a floating wind turbine with and without active surge damping control for wind speeds below the rated wind speed. FIG. 5 shows the mooring line tension in the highest loaded mooring line from the same simulation. The simulation compares a scenario where the floating wind turbine uses a standard blade pitch controller with no active damping (e.g. the blade pitch controller of FIG. 2) with two scenarios where the floating wind turbine uses a motion controller with active damping for surge motion (e.g. the blade pitch controller of FIG. 3) with different active damping control values. It is seen from FIG. 4 that the surge motion response is reduced when active surge damping control is applied in accordance with the present invention. Further, as shown in FIG. 5, a corresponding reduction in tension variations in the main mooring line is seen when active surge damping control is applied.

[0100] In this simulation, a 12 MW floating wind turbine with an asymmetric mooring layout was modelled. FIGS. 4 and 5 depict a snapshot of the simulation between 0 and 600 seconds, where the total length of the simulation was 3700 seconds. Parameters of the simulation included that the mean wind speed was 8.5 ms-1, there was a turbulence class C, significant wave heights were set to 1.3 m, and the characteristic peak period was 6.3 s. The surge velocity measurement used by the controller was modelled as the average velocity between two DGPS measurements with 1 Hz resolution.

[0101] The fatigue damage in the main mooring lines and tower bottom was also calculated for the two active surge damping controller settings relative to the fatigue damage for the original control system for the case considered. For the extent of the 3700 second simulation for this particular set of parameter values, it was found that the relative fatigue damage in the highest loaded mooring line (3) is reduced to a factor 0.59-0.73 while the relative fatigue damage in the tower bottom is reduced to a factor 0.82-0.84, depending on active damping controller settings.