METHOD AND DEVICE OF CONTROLLING AN OPERATION OF A FLOATING WIND TURBINE

Field of invention

The present invention relates to a method and a device of controlling an operation of a floating wind turbine to con trol or protect rotation conditions referring to speed, ac celeration or amplitude of roll and pitch motions of the floating wind turbine.

Floating wind turbines (FWTs) are known to be subjected to additional motions due the additional degrees of freedom (DOF) compared to conventional bottom fixed turbines. For in stance, FWTs experiences floater pitch rotational motion which might be amplified due to the coupling with a standard rotor speed control through blade pitch angle actuation. Therefore, high rotor speed arises, high blade pitch actua tion as well as excessive tower bottom/floater loads, which are seen increasing the material costs involved in tow er/foundation design.

Thus, the aim of maximizing controller performance and mini mizing the floater pitch motion is a challenge of great in terest that has drawn attention in both industrial and aca demic communities.

Floater pitch motion further introduces challenges in exist ing estimates, such as determining the rotational speed from hub accelerations and other components in the tower accelera tion. For instance, the heave motion might interfere with the accelerometer and gyroscopes creating wrong estimates used in the controller. Moreover, other floater structural modes such as the roll motion may be excited during the turbine opera tion. The controller should also be able to cope with any floater natural motion that is undesirably amplified. The floater pitch motion instability has previously been solved by adding an additional reference speed signal to the speed-pitch control in such a way that the controller refer ence does not interfere with the floating system. The input comes from a gyroscope measuring the tilt angle/angular ve locity and/or an accelerometer measuring the tower and/or na celle accelerations. Also, other solutions have been made to detune the speed pitch controller to lower the bandwidth (frequency), such that it will not react on the floater pitch frequency . However, this comes with poor rotational speed tracking capabilities. Detuning is trade-off between stabil ity and control performance. Nonetheless, small focus has been given to include the floater roll natural mode damping.

A rotor speed variation caused by the aerodynamic torque changes due to the floater pitch motion is still a problem and thus speed regulation is still necessary.

Summary of the Invention

It is the object to provide an improved method and an im proved device of controlling an operation of a floating wind turbine . This object may be achieved by the subject matters according to the independent claims. The present invention is further developed as set forth in the dependent claims.

According to a first aspect of the invention, a method of controlling an operation of a floating wind turbine is pro vided. The floating wind turbine comprises a tower, a na celle, which is mounted to the tower rotatable about a na celle yaw axis, a hub, which is mounted to the nacelle rotat able about a hub rotational axis, and blades being mounted to the hub.

The wind turbine is floated such that the wind turbine can perform a wind turbine yaw rotation about a wind turbine yaw axis essentially parallel to the nacelle yaw axis, a wind turbine roll rotation about a wind turbine roll axis essen tially perpendicular to the wind turbine yaw axis and essen tially parallel to the hub rotational axis, and a wind tur bine pitch rotation about a wind turbine pitch axis essen tially perpendicular to the wind turbine roll axis and the wind turbine yaw axis.

The method controls at least one parameter of the operation of the floating wind turbine by the following steps: deter mining a pitch rotation of the wind turbine; determining a roll rotation of the wind turbine; calculating a pitch dif ference between the determined pitch rotation and a wind tur bine pitch reference; calculating a roll difference between the determined roll rotation and a wind turbine roll refer ence; determining a pitch and roll impact value based on the pitch difference and the roll difference; determining a ref erence of the parameter based on a predefined reference of the parameter and the pitch and roll impact value; and con trolling the parameter of the wind turbine based on the ref erence of the parameter. The at least one parameter can be any parameter which is used in operating the floating wind turbine.

Instead of modifying blade pitch angles through a convention al speed-pitch controller to keep the rotor speed close to its reference, the present invention proposes improved con trol handles to achieve the object. Advantageously, rotation conditions referring to speed, acceleration or amplitude of roll and pitch motions of the floating wind turbine can be controlled or protected.

In an embodiment, the reference of the parameter is a target output power of the wind turbine. In an embodiment, the out put power of the wind turbine is controlled based on the tar get output power for a speed regulation which minimizes blade pitch angle variations and/or aerodynamic damping variations; and/or the output power of the wind turbine is controlled based on the target output power for generating a counter torque to damp the wind turbine roll rotation.

In an embodiment, the reference of the parameter is a target position of an add-on member on at least one of the blades, which add-on member changes an aerodynamic property of the blade. In an embodiment, the position of add-on members on the blade is controlled based on the target position of add on members on the blades to implement a positive damping into the wind turbine for cancelling a negative damping from a speed-pitch controller and thereby damping wind turbine pitch rotation; and/or the position of add-on members on the blade is controlled based on the target position of add-on members on the blades individually for each blade to implement a counter-torque for damping the wind turbine roll rotation.

In an embodiment, the reference of the parameter is a target rotational speed of the hub or a blade pitch angle of the blades.

In an embodiment, the pitch rotation of the wind turbine is determined based on a tower top acceleration which is meas ured by a G-sensor; the roll rotation of the wind turbine is determined based on the tower top acceleration which is meas ured by the G-sensor; the pitch rotation and/or the roll ro tation is determined from an acceleration in pitch and/or roll coordinates which are measured by a G-sensor; the pitch rotation and/or the roll rotation is determined from a veloc ity in the pitch and/or roll coordinates which are measured by a gyroscope; the pitch rotation and/or the roll rotation is determined from angles in the pitch and/or roll coordi nates which are measured by an inclinometer; and/or the pitch rotation and/or the roll rotation is determined from a load on the blade measured by a blade load sensor which measures a load on at least one of the blades, for example with suitable coordinate transformation and angle of attack. In an embodiment, the method further includes at least one of the following steps: estimating a natural frequency of the wind turbine roll rotation and/or the wind turbine pitch ro tation; a filtering step to provide a signal-to-noise ratio between a frequency of the wind turbine roll rotation and/or the wind turbine pitch rotation on the one hand, and a fre quency of an input sensor signal on the other hand; a para metrical updating step to verify that the filtering step is targeting an expected frequency.

In an embodiment, the pitch and roll impact value is deter mined from a lookup-table where values of the pitch differ ence and the roll difference are stored as arguments.

In an embodiment, the reference of the parameter is deter mined from a lookup-table where values of the predefined ref erence and the pitch and roll impact value are stored as ar guments .

In an embodiment, the parameter of the wind turbine is con trolled for controlling at least one of a speed, an accelera tion, and an amplitude of roll and/or pitch motions of the floating wind turbine.

According to a second aspect of the invention, a control de vice for controlling an operation of a floating wind turbine is provided. The floating wind turbine comprises a tower; a nacelle, which is mounted to the tower rotatable about a na celle yaw axis and comprises a hub, which is mounted to the nacelle rotatable about a hub rotational axis; and blades be ing mounted to the hub. The wind turbine is floated such that the wind turbine can perform a wind turbine yaw rotation about a wind turbine yaw axis in parallel to the nacelle yaw axis, a wind turbine roll rotation about a wind turbine roll axis perpendicular to the wind turbine yaw axis and in paral lel to the hub rotational axis, and a wind turbine pitch ro tation about a wind turbine pitch axis perpendicular to the wind turbine roll axis and the wind turbine yaw axis. The control device is configured to control at least one parame ter of the operation of the floating wind turbine by deter mining a pitch rotation of the wind turbine; determining a roll rotation of the wind turbine; calculating a pitch dif ference between the determined pitch rotation and a wind tur bine pitch reference; calculating a roll difference between the determined roll rotation and a wind turbine roll refer ence; determining a pitch and roll impact value based on the pitch difference and the roll difference; determining a ref erence of the parameter based on a predefined reference of the parameter and the pitch and roll impact value; and con trolling the parameter of the wind turbine based on the ref erence of the parameter.

Instead of modifying blade pitch angles through a convention al speed-pitch controller to keep the rotor speed close to its reference, the present invention uses a so-called Uni- fied-Motion-Control (UMC) based on the pitch and roll impact value to achieve the goal.

The controller can therefore minimize aerodynamic damping variations over the rotor and damp the floater motion in the roll direction. It is noteworthy that the IMC controller can obtain the necessary control action (concerning the output power or the operation of active add-on members on the blades) based on two error signals characterizing a multivar iable control strategy.

Advantageously, the present invention can minimize the float er motion instability by the UMC control which covers both roll and pitch directions together.

In addition, tower and floater involved costs in design, man ufacturing, etc. can be reduced because of driving down the involved mechanical fatigue loads of those components. Also, the present invention leverages an extended blade bearing life compared with the prior art, since its main control in- put can be a generator torque alleviating the blade pitch an gle actuation.

Also an electrical torque as manipulated variable (instead of the blade pitch angle) can be used in order to eliminate a coupling between the floater pitch motion and the rotor speed as well as reducing the floater roll motion that is directly related to the side-side tower top motion.

According to a further aspect of the invention, a floating wind turbine having a a control device for controlling an op eration of a floating wind turbine according to any of the preceding embodiments is provided.

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref erence to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other noti fied, in addition to any combination of features belonging to one type of subject matter also any combination between fea tures relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application.

Brief Description of the Drawings

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited. Fig. 1 shows a wind turbine and the different elements thereof;

Fig. 2 shows lateral movements of the floating wind tur bine;

Fig. 3 shows rotational movements of the floating wind turbine; and

Fig. 4 shows a block diagram for controlling an opera tion of the wind turbine.

Detailed Description

The illustrations in the drawings are schematically. It is noted that in different figures, similar or identical ele ments are provided with the same reference signs.

Fig . 1 shows a wind turbine 1. The wind turbine 1 comprises a nacelle 3 and a tower 2. The nacelle 3 is mounted at the top of the tower 2. The nacelle 3 is mounted rotatable with re gard to the tower 2 by means of a yaw bearing. The axis of rotation of the nacelle 3 with regard to the tower 2 is re ferred to as the nacelle yaw axis 9.

The wind turbine 1 also comprises a hub 4 with three rotor blades 6 (of which two rotor blades 6 are depicted in Fig.

1). The hub 4 is mounted rotatable with regard to the nacelle 3 by means of a main bearing 7. The hub 4 is mounted rotata ble about a rotor axis of rotation 8.

The wind turbine 1 furthermore comprises a generator 5. The generator 5 in turn comprises a rotor connecting the genera tor 5 with the hub 4. If the hub 4 is connected directly to the generator 5, the wind turbine 1 is referred to as a gear less, direct-driven wind turbine. Such a generator 5 is re ferred as direct drive generator 5. As an alternative, the hub 4 may also be connected to the generator 5 via a gear box. This type of wind turbine 1 is referred to as a geared wind turbine. The present invention is suitable for both types of wind turbines 1.

The generator 5 is accommodated within the nacelle 3. The generator 5 is arranged and prepared for converting the rota tional energy from the hub 4 into electrical energy in the shape of an AC power.

The floating wind turbine 1 comprises a floating foundation 10 which floats on the water surface. The floating foundation 10 is fixed to the sea ground by means of ropes 11. In an em bodiment, the length of the ropes 11 can be variably con trolled to stabilize the floating wind turbine 1 at high wind speeds or high waves.

Fig. 2 shows lateral movements of the floating wind turbine 1, and Fig. 3 shows rotational movements of the floating wind turbine 1. The floating wind turbine l is a complex system with multiple variables and multiple degrees of freedom (DOFs). Wind, wave and current loadings, which all are irreg ular by nature along with aerodynamical, structural and hy- drodynamical couplings, as well as control actuations, all contribute to a highly complex dynamical behavior.

In a global coordinate system having the axes x, y and z, the floater motion can be divided into six individual DOF namely three translations: surge along the x-axis, sway along the y- axis and heave along the y-axis, and three rotations, i.e., floater roll about the x-axis, floater pitch about the y-axis and floater yaw about the z-axis. Due to the similarities in the naming between the pitch of the blades 6 and the pitch of the wind turbine 1, a clear distinction is made by dividing it into wind turbine pitch and blade pitch. That is, the wind turbine pitch is the rotation of the floating wind turbine 1 around its point of rotation, while blade pitch defines the controlled pitching of the blades 6. Likewise, wind turbine yaw defines the rotation of the wind turbine 1 around its vertical axis z, while rotor yaw is the rotation of the ro tor-nacelle-assembly (RNA) around the nacelle yaw axis 9.

In general terms, the wind turbine 1 is floated such that the wind turbine 1 can perform a wind turbine yaw rotation about the wind turbine yaw axis z in parallel to the nacelle yaw axis 9, a wind turbine roll rotation about the wind turbine roll axis x perpendicular to the wind turbine yaw axis z and in parallel to the hub rotational axis 8, and a wind turbine pitch rotation about the wind turbine pitch axis y perpendic ular to the wind turbine roll axis x and the wind turbine yaw axis z.

Fig. 4 shows a block diagram for controlling an operation of the wind turbine 1. The wind turbine 1 comprises a control device (not shown) which controls a parameter a of the opera tion of the floating wind turbine 1 by the following steps, wherein the UMC is applied by using a power reference as con trol handle:

A pitch rotation b of the wind turbine 1 and a roll rotation c of the wind turbine 1 are determined. A pitch difference d between the determined pitch rotation b and a wind turbine pitch reference f is calculated, and a roll difference g be tween the determined roll rotation c and a wind turbine roll reference h is calculated.

In a block 20, a pitch and roll impact value i is determined based on the pitch difference d and the roll difference g.

The block 20 uses the Unified-Motion-Control (UMC) as it con siders the combination of the pitch difference d and the roll difference g. In a block 21, a reference j of the parameter a is determined based on a predefined (for example power) ref erence k of the parameter a and the pitch and roll impact value i. The block 21 can be a controller that aims to coor dinate/combine the nominal reference k of the parameter a and pitch and roll impact value i computed by the unified motion controller. The block 21 can be a controller that will ensure that these two signals are compatible in the sense that the reference j of the parameter a is the result of non conflicting objectives and/or the highest priority is target ed. In the simplest case, it can be described as a reference summation.

Eventually, the parameter a of the wind turbine 1 is con trolled based on the reference j of the parameter a. For ex ample, the parameter a of the wind turbine can be controlled such that at least one of a speed, an acceleration and an am plitude of roll and pitch motions of the floating wind tur bine 1 does not exceed a predetermined threshold value.

Blocks 23 and 24 designate a pitch event scheduler and a roll event scheduler, respectively. A scheduler can be described as a general block that determines the desired reference like a supervisory control layer. Some examples can be listed as follows:

- event-triggered schedulers so that operational condi tion and states of the turbine (e.g. loads measure ment/estimates) or wind/wave conditions can be used as inputs to determine the reference. A simple example can be a look-up table.

- time-triggered schedulers so that a time instant can be a variable defining a different desired level of refer ence.

- manual-triggered schedulers so that the values can be manually set.

In the embodiment of Fig. 1, the reference j of the parameter a is a target output power of the wind turbine 1. The output power of the wind turbine 1 is controlled based on the target output power for a speed regulation which minimizes blade pitch angle variations and/or aerodynamic damping variations. In addition or alternatively, the output power of the wind turbine 1 can be controlled based on the target output power for generating a counter torque to damp the wind turbine roll rotation.

In another embodiment, the reference j of the parameter a can be a target position of an add-on member on at least one of the blades 6, which add-on member changes an aerodynamic property of the blade 6. The position of add-on members on the blade 6 can be controlled based on the target position of add-on members on the blades 6 to implement a positive damp ing into the wind turbine 1 for cancelling a negative damping from a speed-pitch controller and thereby damping wind tur bine pitch rotation. In addition or alternatively, the posi tion of add-on members on the blade 6 can be controlled based on the target position of add-on members on the blades 6 in dividually for each blade 6 to implement a counter-torque for damping the wind turbine roll rotation.

In another embodiment, the reference j of the parameter a can be a target rotational speed of the hub 4 or a blade pitch angle of the blades 6.

The physical parameters can be measured or determined based on the following: the pitch rotation of the wind turbine 1 can determined based on a tower top acceleration which is measured by a G-sensor; the roll rotation of the wind turbine 1 can be determined based on the tower top acceleration which is measured by the G-sensor; the pitch rotation and/or the roll rotation can be determined from an acceleration in pitch and/or roll coordinates which are measured by a G-sensor; the pitch rotation and/or the roll rotation can be determined from a velocity in the pitch and/or roll coordinates which are measured by a gyroscope; the pitch rotation and/or the roll rotation can be determined from angles in the pitch and/or roll coordinates which are measured by an inclinome ter; and/or the pitch rotation can be determined from a load on the blade 6 measured by a blade load sensor. According to Fig. 3, the tower top acceleration is an accel eration at the top 12 of the tower 2. The tower top accelera tion is induced by forces Fgx, Fgy, Fgz which can be measured by the G-sensor. In Fig. 3, the pitch rotation and/or the roll rotation is designated by the term "offset" and can be determined by the inclinometer, the G-sensor or a gyroscope, for example from measured pitch and/or roll coordinates.

In an implementation, the pitch and roll impact value i can be determined from a lookup-table where values of the pitch difference d and the roll difference g are stored in advance as arguments. The pitch and roll impact value i can be con sidered as a control action which is determined by a control algorithm of the block 20. In the same manner, the reference j of the parameter a can be determined from a lookup-table where values of the predefined reference k and the pitch and roll impact value i are stored in advance as arguments. The pitch and roll impact value i and the reference j of the pa rameter can be determined by employing different control strategies such as classical ones like PI, PID, and/or ad vanced ones like MPC, sliding mode, state-feedback, LQR, etc.

The present invention can additionally include at least one of the following steps: estimating a natural frequency of the wind turbine roll rotation, for example of the roll rotation c, and/or the wind turbine pitch rotation, for example the pitch rotation b; a filtering step to provide a signal-to- noise ratio between a frequency of the wind turbine roll ro tation, for example of the roll rotation c, and/or the wind turbine pitch rotation, for example the pitch rotation b, on the one hand, and a frequency of an input sensor signal on the other hand; a parametrical updating step to verify that the filtering step is targeting an expected frequency.

It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.