Controlling motions of floating wind turbines

Abstract

A motion controller for a floating wind turbine with a plurality of rotor blades, is arranged to control a motion of the floating wind turbine in a yaw direction. The controller adjusts the blade pitch of each rotor blade so as to create a net force to control the motions. The controller includes a control action which is proportional to a yaw offset angle and/or a control action which is proportional to an integral of the yaw offset angle.

Claims

1. A motion controller for a floating wind turbine with a plurality of rotor blades, wherein the controller is arranged to adjust a blade pitch of each of the rotor blades so as to create a net force to control a motion of the floating wind turbine in a yaw direction, wherein the controller includes a control action which is proportional to a yaw offset angle and/or a control action which is proportional to an integral of the yaw offset angle, wherein the controller is adapted to calculate a dynamic and collective pitch for all of the plurality of rotor blades to counteract an axial motion of the floating wind turbine to obtain a first adjustment, calculate a dynamic and individual blade pitch for each of the plurality of blades to counteract a yaw motion of the floating wind turbine to obtain a second adjustment calculate a dynamic and individual blade pitch for each of the plurality of blades and/or a variation in rotor torque to counteract an in-plane motion of the floating wind turbine to obtain a third adjustment; and combine the first, second and third adjustments to cause simultaneous control of the axial motions, yaw motions and in-plane motions of the floating wind turbine.

2. The motion controller as claimed in claim 1, wherein the controller does not include a control action which is proportional to the derivative of the yaw off-set angle.

3. The motion controller as claimed in claim 1, wherein the controller is arranged to operate only when the yaw off-set is above a certain threshold angle.

4. The motion controller as claimed in claim 1, wherein the controller is also arranged to control a motion in the in-plane direction.

5. The motion controller as claimed in claim 1, wherein the controller is also arranged to control a motion in the axial direction.

6. The motion controller as claimed in claim 1, wherein the controller is arranged to control the motion of the floating wind turbine in a plurality of directions simultaneously.

7. The motion controller as claimed in claim 4, wherein the controller is arranged to adjust the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion velocity so as to provide a damping force for at least one of the motions.

8. The motion controller as claimed in claim 1, wherein the controller is arranged to adjust the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion displacement so as to provide a restoring force.

9. The motion controller as claimed in claim 1, wherein the controller is further arranged to control the torque of the load presented to the rotor to control a motion in the in-plane direction.

10. The motion controller as claimed in claim 1, wherein the input to the controller is based on a measurement of the velocity of the motions of the floating wind turbine.

11. The motion controller as claimed in claim 9, wherein the controller uses a low pass filter on the velocity input.

12. A method of controlling a floating wind turbine structure comprising a plurality of rotor blades, the method comprising: adjusting a blade pitch of each of the rotor blades so as to create a net force to control a yaw motion of the floating wind turbine, wherein a blade pitch adjustment is calculated using a controller which includes a control action which is proportional to a yaw offset angle and/or a control action which is proportional to an integral of the yaw offset angle, wherein the controller is adapted to calculate a dynamic and collective pitch for all of the plurality of rotor blades to counteract an axial motion of the floating wind turbine to obtain a first adjustment, calculate a dynamic and individual blade pitch for each of the plurality of blades to counteract a yaw motion of the floating wind turbine to obtain a second adjustment; calculate a dynamic and individual blade pitch for each of the plurality of blades and/or a variation in rotor torque to counteract an in-plane motion of the floating wind turbine to obtain a third adjustment; and combine the first, second and third adjustments to cause simultaneous control of the axial motions, yaw motions and in-plane motions of the floating wind turbine.

13. The method as claimed in claim 12, wherein the blade pitch adjustment is calculated using a controller which does not includes a control action which is proportional to the derivative of the yaw off-set angle.

14. The method as claimed in claim 12, wherein the blade pitch is only adjusted when the yaw off-set is above a certain threshold angle.

15. The method as claimed in claim 12, wherein the method comprises adjusting a blade pitch of each rotor blade so as to create a net force to also control an in-plane motion.

16. The method as claimed in claim 12, wherein the method comprises adjusting the blade pitch of the rotor blades to also control a motion in the axial direction.

17. The method as claimed in claim 12, the method comprising controlling the motion of the floating wind turbine in a plurality of directions simultaneously.

18. The method as claimed in claim 15, the method comprising adjusting the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion velocity so as to provide a damping force for at least one of the motions.

19. The method as claimed in claim 12, the method comprising adjusting the blade pitch of each rotor blade with a phase relative to the floating wind turbine motion displacement so as to provide a restoring force.

20. The method as claimed in claim 12, the method comprising controlling the torque of the load presented to the rotor to control a motion in the in-plane direction.

21. The method as claimed in claim 12, the method comprising inputting to the controller a measurement of the velocity of the motions of the floating wind turbine.

22. The method as claimed in claim 21, the method comprising using a low pass filter on the velocity input.

23. A motion controller for a floating wind turbine with a plurality of rotor blades, wherein the controller is arranged to adjust a blade pitch of each of the rotor blades so as to create a net force to control a motion of the floating wind turbine in a direction other than the axial direction, wherein the controller is arranged to only control a motion of the floating wind turbine when that motion exceeds a certain threshold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying figures in which:

(2) FIG. 1 shows a simulation snapshot plot with and without active in-plane damping with use of individual blade pitch control;

(3) FIG. 2 shows a simulation snapshot plot with and without active in-plane damping with use of generator torque control below the rated wind speed;

(4) FIG. 3 shows the rotational angle of the blades as seen in the positive x-direction (wind direction);

(5) FIG. 4 shows simulation snapshot plot with and without active yaw motion control with use of individual blade pitch control; and

(6) FIG. 5 shows a wind turbine incorporating a controller according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) Turning first to FIG. 5, 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 motions 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 catenary mooring lines. The nacelle contains an electrical generator which 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.

(8) 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.

(9) The control in the nacelle is arranged to determine a blade pitch adjustment necessary to control the motions of the floating wind turbine. The controller is further arranged to adjust the blade pitch of each rotor blade independently so as to create a net force to control a motion of the floating wind turbine in an axial direction, an in-plane direction and a yaw direction.

(10) If the axial motion is to be damped in a frequency range close to the resonance frequency ?.sub.x, a force must be created that opposes the axial velocity of the rotor motion. If a dynamic and collective blade pitch motion is performed at frequency ?.sub.x and amplitude ??.sub.0A an axial force (thrust) on the rotor opposing the axial motion may be obtained. For a harmonic axial motion, the blade pitch angle should be
?=?.sub.0+??.sub.0A cos(?.sub.xt+?.sub.0) [1]

(11) Here ?.sub.0 is the pitch angle that is set out by the conventional controller aiming) for constant power production. ??.sub.0A cos(?.sub.xt+?.sub.0) is the additional blade pitch angle to achieve damping. ?.sub.0 is a phase angle between the axial velocity and the maximum pitch angle and t is time.

(12) If the in-plane motion is to be controlled in a frequency range close to the resonance frequency for in-plane motion ?.sub.y, a force must be created that opposes the in-plane velocity and/or the in-plane excursion of the rotor motion. If a dynamic blade pitch motion is performed giving an additional angle ?? when passing the top position and a corresponding ??? when passing the lower position, a net in-plane force is obtained due to the changed lift forces on the blade. If this in-plane force then is varied with a frequency equal to the natural frequency of the in-plane motion, a net damping effect may be obtained. For a harmonic in-plane motion with frequency ?.sub.y and a rotor frequency ? the blade pitch of each of the blades should have an individual pitch of the form:
??.sub.yj=??.sub.yA cos(?.sub.yt+?.sub.y)cos(?t+?.sub.j0+?.sub.y) [2]

(13) In this example cosine harmonic functions are used to provide a smooth pitch angle variation and thus smooth force variation. However, any smooth periodic function with the prescribed frequency could be applied.

(14) Here j is the blade number (j=1, 2 or 3 for a three bladed rotor), ??.sub.yA is the amplitude of the blade pitch angle, ?.sub.y, ?.sub.y are phases of the blade pitch motion relative to the in-plane velocity and the top point position of the blade respectively, ?.sub.j0 is the phase corresponding to the initial position of each blade on the rotor, i.e. for a three bladed rotor ?.sub.j0=2?/3(j?1). ?.sub.y can be varied so that the force is either in phase with the in-plane velocity (damping) or in phase with the in-plane excursion (restoring). For a harmonic oscillation these components have always a phase difference of ?/2. The control may be tuned to both provide a restoring and a damping effect.

(15) To generate a dynamic yaw moment, a similar principle as for the in-plane motion may be used. The yaw natural frequency replaces the in-plane natural frequency in [2], and the phases are changed to generate a yaw force rather than an in-plane force. Alternatively, the yaw frequency does not need to correspond to the yaw natural frequency and may also change over time. In the special case of harmonic motion, this may be represented as
??.sub.6j(t)=??.sub.6A cos(?.sub.6t+?.sub.6)cos(?t+?.sub.j0+?.sub.6) [3]

(16) ?.sub.6 is typically ?.sub.y??/2. As with the in-plane motion, the phase angle ?.sub.6 may be tuned to obtain an optimum balance between damping and restoring forces.

(17) The total dynamic variation of the blade pitch angle will approximately be the sum of the three individual components of ?? above. The interaction effects between the forces depend upon the relation between the natural frequencies involved and the non-linearity in the lift and drag forces. A certain blade pitch angle will change the angle of attack close to the rotor axis more than close to the tip. For that reason, an individual tuning of the contributions should be performed to obtain the wanted motion reduction and avoid as far as possible negative impacts as reduced power production and increased blade loads. The above in-plane damper and yaw motion control principles do not need to be activated continuously. For example, one may have a motion monitoring system that detects if the motions (e.g. displacements) exceed certain limits, i.e. a certain threshold, and then activate the control system accordingly.

(18) If the in-plane (resonant) motion (roll) is excited by variation in the aerodynamic or generator torque, an alternative control option is available: That is to modify the generator torque controller to impose damping in the relevant frequency interval. This can be achieved by modifying the original generator torque control system to have an additional component in the generator torque reference signal that is proportional to the in-rotor plane tower velocity. For harmonic in-plane motion the generator torque reference can be formulated as
T.sub.ref=T.sub.ref 0(1+?T.sub.yA cos ?.sub.yt), [4]
where T.sub.ref 0 is the original generator torque reference signal and ?T.sub.yA is the relative amplitude of the additional torque control reference signal for active in-plane damping. This can be used in addition to the motion control which is achieved by pitching of the rotor blades or used on its own when it is desired to only control the in-plane motions.

(19) The principle of desired blade pitch angle for generating an in-rotor-plane harmonic force given in equation [2] can be applied to a control scheme with a generally non-harmonic behaviour based on a measurement of the in-rotor-plane velocity of a floating wind turbine.

(20) Consider a measurement of the in-rotor-plane horizontal velocity {dot over (y)}.sub.n, measured at the nacelle level. Then, an active in-plane damping control scheme can be formulated as
?.sub.ref,i=?.sub.c+?.sub.ir [5]

(21) where ?.sub.ref,i is the blade pitch angle reference signal for blade i, ?.sub.c is the collective blade pitch angle reference signal for all blades (including possibly active pitch motion damping control), while ?.sub.ir is the additional in-plane individual blade pitch angle reference for blade i which is controlled to give an in-rotor plane damping force by the equation:
?.sub.ir=K.sub.vr{dot over (y)}.sub.ncos(?.sub.i), [6]

(22) where K.sub.vr is the in-plane damping controller gain and ?.sub.i is the azimuth angle of blade i. It is often desired to provide additional damping at the natural frequency of the in-plane motion, and to consider higher frequencies like the wave frequencies as undesired disturbance. It can therefore be advantageous to use a low pass filter on the in-rotor-plane horizontal velocity {dot over (y)}.sub.n. In Laplace form a second order Butterworth filtering of {dot over (y)}.sub.n can be represented as

(23) $\begin{array}{cc}{\stackrel{.}{y}}_{\mathrm{nf}}\left(s\right)=\frac{{?}_c^2}{s^2+\sqrt{2}{?}_{c}s+{?}_c^2}{\stackrel{.}{y}}_n\left(s\right),& \left[7\right]\end{array}$

(24) where ?.sub.c is the cut-off frequency in the low pass filter and {dot over (y)}.sub.nf is the filtered nacelle velocity. An in-rotor-plane control scheme with low pass filtering can then be by formulated by combining equation [6] and equation [7]:
?.sub.ir=K.sub.vr{dot over (y)}.sub.nfcos(?.sub.i)[8]

(25) The active in-plane damping by use of individual blade pitch control can be applied both above and below the rated wind speed. The measured in-rotor-plane velocity could be measured directly or could be an estimate based on other measurements. A simulation snapshot plot with and without active in-plane damping with use of individual blade pitch control is shown in FIG. 1. FIG. 1 shows a time domain simulation snapshot plot of the in-plane motion for an environmental condition with significant wave height 5 m, peak period 10.7 s and mean wind speed 16.5 m/s. Conventional floating wind turbine collective blade pitch control system (ADC) and conventional system with active in-plane damping by use of individual blade pitch control (ADC+ARIC) are shown. The simulations are carried out with measurement of the nacelle sway velocity, K.sub.vr=?0.25 and

(26) ${?}_c=\frac{2?}{20}.$
The natural period in roll in this case is 30 seconds. A significant reduction in the in-plane motion is observed by applying ARIC, even if the parameter setting is not optimized.

(27) Due to the nonlinear nature of the aerodynamic forces on the rotor blades, it may be advantageous to apply gain scheduling techniques to schedule the in-plane damping controller gain with the operational condition based on measurements of e.g. rotor speed, blade pitch angle and/or wind speed.

(28) As mentioned above, an alternative method for damping of the in-rotor-plane motions of a floating wind turbine is to add an additional signal proportional to the in-rotor-plane horizontal tower velocity to the generator torque reference signal, T.sub.ref.

(29) Consider a measurement of the in-rotor-plane horizontal velocity {dot over (y)}.sub.n, measured at the nacelle level. Then, an active in-plane damping control scheme can be formulated as:
T.sub.ref=T.sub.ref 0(1+K.sub.tc{dot over (y)}.sub.n), [9]

(30) where T.sub.ref 0 is the original generator torque reference signal, and K.sub.tc is the in-plane damping generator torque controller gain.

(31) Similarly, as in the previous section, an in-rotor-plane control scheme with low pass filtering can be desirable to avoid high frequency disturbances and use of the low pass filtering scheme in equation (8) leads to the following generator torque control scheme for active in-plane damping:
T.sub.ref=T.sub.ref 0(1+K.sub.tc{dot over (y)}.sub.nf) [10]

(32) The active in-plane damping by use of generator torque control is particularly suitable below the rated wind speed and the measured in-rotor-plane velocity could also be an estimate based on other measurements. A simulation snapshot plot with and without active in-plane damping with use generator torque control below the rated wind speed is shown in FIG. 2. FIG. 2 shows a time domain simulation snapshot plot of the in-plane motion at the mean water level for an environmental condition with significant wave height 2 m, peak period 8.5 s and mean wind speed 8 m/s. Conventional wind turbine control system (CC) and conventional system with active in-plane damping by use of generator torque control (CC+ARTC) are shown. The simulation is carried out with measurement of the nacelle sway velocity, K.sub.tc=3.0 and

(33) ${?}_c=\frac{2?}{20}.$

(34) The principle of desired blade pitch angle for generating a yaw harmonic force given in equation [3] can be transferred to a control scheme with a generally non-harmonic behaviour based on a measurement of the yaw velocity of the floating wind turbine. However, it is desirable to include proportional and/or integral control actions in addition to, or instead of, the derivative control actions indicated in equation [3]. This because the yaw motions are slowly varying and because the yaw motions are little affected by the waves.

(35) FIG. 3 shows the rotational angle of the blades as seen in the positive x-direction (wind direction). From FIG. 3 it can be seen that a restoring yaw moment for a positive yaw motion is obtained by positive individual pitching of the rotor blades with rotor blade azimuth angles from 0 to 180 degrees and negative individual pitching of the rotor blades with rotor blade azimuth angles from 180 to 360 degrees, and opposite for negative yaw motion.

(36) It is assumed that a smooth cyclic variation of the rotor blade pitch angles is desirable during one revolution of the rotor, and on this basis a control schedule incorporating proportional, integral, and derivative yaw motion control actions can be formulated as:
?.sub.ref,i=?.sub.c+?.sub.iy, [11]

(37) where ?.sub.ref,i is the blade pitch angle reference signal for blade i, ?.sub.c is the collective blade pitch angle reference signal for all blades (including possibly active pitch motion damping control), while ?.sub.iy is the additional yaw individual blade pitch angle reference for blade i, represented as

(38) $\begin{array}{cc}{?}_{\mathrm{iy}}=\left(K_{\mathrm{py}}?+K_{\mathrm{iy}}{?}_{t_0}^t?\left(?\right)d?+K_{\mathrm{vy}}\stackrel{.}{?}\right)\sin \left({?}_i\right),& \left[12\right]\end{array}$

(39) where ? is the tower yaw angle, {dot over (t)} is the tower yaw angular velocity, K.sub.iy is the integral controller gain, K.sub.py is the proportional controller gain and K.sub.vy is the derivative controller gain and ?.sub.i is the azimuth angle of rotor blade i.

(40) The active yaw motion control by use of individual blade pitch control can be applied both above and below the rated wind speed. The yaw motion may be measured directly or could also be an estimate based on other measurements. A simulation snapshot plot with and without active yaw motion control with use of individual blade pitch control is shown in FIG. 4. FIG. 4 shows a time domain simulation snapshot plot of the yaw motion at the mean water level for an environmental condition with significant wave height 5 m, peak period 10.7 s and mean wind speed 16.5 m/s, i.e. above rated wind speed. Conventional floating wind turbine collective blade pitch control system (ADC) and conventional system with yaw motion control by use of individual blade pitch control (ADC+AYIC) are shown. The simulation is carried out with K.sub.py=2.5, K.sub.iy=0.25 and K.sub.vy=0.

(41) In this example the derivative controller gain is set to zero. This is because it was realised that the effect of the derivative controller action on yaw motions in floating wind turbines is negligible. The yaw motion is slowly varying with changes in the wind field so, as a result, a damping force (provided by the derivate control action) has little effect. As a result, the controller does not need to comprise a derivative control action.

(42) Due to the nonlinear nature of the aerodynamic thrust force, it may be advantageous to apply gain scheduling techniques to schedule the yaw motion controller gains with the actual operational condition based on measurements of e.g. rotor speed, blade pitch angle and/or wind speed.

(43) The control schemes presented above for the in-plane motion (either by blade control or torque control) can be combined (multiplied by suitable scaling factors and added together) with the control schemes for yaw motion to obtain both active in-plane damping and yaw motion control. The control schemes above are examples on implementation only and are not optimized. Optimization will employ controller settings that reduce the motions sufficiently and at the same time do not cause too large negative effects on e.g. blade loads.