Patent Publication Number: US-2015076822-A1

Title: Damping an oscillatory movement of a nacelle of a wind turbine

Description:
FIELD OF INVENTION 
     The present invention relates to a method for damping an oscillatory movement of a nacelle of a wind turbine. In particular, the oscillatory movement is coordinated with a yawing movement in an advantageous manner. The invention also relates to a control device for damping such an oscillatory movement. Furthermore, the invention relates to a wind turbine comprising such a control device. Finally, the invention relates to a computer program for damping an oscillatory movement of a nacelle of a wind turbine. 
     BACKGROUND OF INVENTION 
     A wind turbine, in particular a tower of a wind turbine, has to withstand considerable load during its lifetime. A tower may experience extreme loading or fatigue loading. Extreme loading means the maximum/minimum limit that the tower can withstand. Extreme loading may, for example, be experienced during a large, i.e. heavy, wind gust. Fatigue loading means progressive damage to the structure as a result of cyclic loading. Fatigue damage may occur as the tower oscillates in so-called side-side or fore-aft movements in normal operation. It would be highly advantageous to reduce the fatigue load of the tower, because then the tower could be made with less material, e.g. steel, and thereby cost and/or weight might be reduced. Alternatively, the same tower with a reduced fatigue load could have a longer lifetime. 
     Side-side tower oscillations may be induced by a wind gust, by a yaw movement of a nacelle of the wind turbine, or simply due to the natural variation of the wind. The European patent EP 2 146 093 B1 describes a method to damp side-side tower oscillations by adding a sinusoidal signal to an electrical torque reference or electrical tower reference. However, this involves significant processing and transformation of electrical signals. 
     Thus, there exists an urgent need to provide an improved method for damping an oscillatory movement of a nacelle of a wind turbine. 
     SUMMARY OF INVENTION 
     This objective is achieved by the independent claims. The dependent claims describe advantageous developments and modifications of the invention. 
     In accordance with the invention there is provided a method for damping an oscillatory movement of a nacelle of a wind turbine. The nacelle is attached to a tower of the wind turbine. The method comprises rotating the nacelle about a yawing axis with a yawing speed, wherein the yawing axis is aligned with a longitudinal axis of the tower. The method furthermore comprises changing the yawing speed, and coordinating the yawing speed with the oscillatory movement such that a torque resulting from the change of the yawing speed damps the oscillatory movement of the nacelle of the tower. 
     The oscillatory movement may also be denoted as a pivoting movement. It includes, for example, a pivoting movement of the tower about a pivot point. It also includes bending of the tower. 
     Advantageously, the nacelle is attached to the tower via a bearing. The wind turbine is a device that can convert wind energy, i.e. kinetic energy from wind, into mechanical energy. Advantageously, the mechanical energy is subsequently used to generate electricity. A wind turbine is also referred to as a wind power plant. 
     The change of the yawing speed includes acceleration as well as reduction of the yawing speed. Damping of the oscillatory movement includes reducing, mitigating or even eliminating the oscillatory movement. A damping of the oscillatory movement, e.g. side-side tower oscillations, is advantageous for the wind turbine, in particular for the tower, as this reduces load. Reducing e.g. fatigue load may allow for reduction of the design fatigue load or prolong the tower lifetime. Fatigue has to be understood as a progressive and localised structural damage that occurs when a material is subjected to cyclic loading. 
     Advantageously, a yaw bearing exists between the nacelle and the tower. The yaw bearing allows a rotation of the nacelle about the yawing axis, the yawing axis being aligned with the longitudinal axis of the tower. If the tower is substantially rotationally symmetric, then the longitudinal axis of the tower is advantageously identical to the axis of symmetry of the tower. One purpose of yawing the nacelle relative to the tower is to reposition, i.e. to follow up or to track, the nacelle with regard to a changing incoming wind direction. This is in particular done in order to reposition rotor blades which are attached to a hub, the hub being connected with the nacelle, with regard to the changing incoming wind direction. In other words, if the incoming wind changes its direction or angle, then advantageously the nacelle is repositioned or yawed into a new rotational position. 
     If the yaw speed changes, then an angular momentum due to the yawing movement, which is a rotational movement, consequently changes, too. Thus, due to the changing angular momentum a torque is created. 
     The torque points in the same direction as the angular momentum. Thus, assuming a vertical tower, i.e. assuming a vertical yaw axis, the torque created by an acceleration or reduction of the yaw speed is pointing in vertical direction, too. If a center of mass of the wind turbine is distant from the yaw axis, then, a consequence of the vertical torque is a force which is pointing perpendicular to the yawing axis and perpendicular to a direction of a lever arm between the center of mass and the yawing axis. In this context, the lever arm is defined as a shortest distance from the center of mass to the yawing axis. The force, induced by the torque, may influence the oscillatory movements of the nacelle. 
     In other words, one aspect of the invention is coordinating the yawing speed such that the torque, which is generated by the change of the yawing speed, induces a force which points, at least partly, in an opposite direction compared to the oscillatory movement and thus is able to damp the oscillatory movement. 
     It has to be noted that a rotor with rotor blades of the wind turbine does not necessarily have to rotate for the method to work. However, the oscillatory movement can only be damped if the center of mass is distant from the yawing axis. 
     The method described above is particularly efficient with hard yaws. A hard yaw has a fixed yaw speed but a considerable initial yaw acceleration. This may induce or create high torques. Thus, side-side tower oscillations, for instance, can efficiently be damped. In general, a hard yaw is advantageous, as it is relatively cheap and simply built compared to e.g. a variable speed motor for a yaw drive. 
     A yaw acceleration is able to generate a force in a side-side direction of the tower and may excite or damp the tower in this direction. The yaw speed is often quite limited in generating a significant force. However, yaw acceleration, in particular yaw acceleration of a hard yaw, may be high enough to generate a considerable force. Thus, in particular for wind turbines with a hard yaw, the method described above is highly beneficial. 
     It is noted that one aspect of the present invention is based on a finding that nacelle yawing may have a significant impact on side-side tower oscillations. Thus, on the one hand, due to an advantageous scheduling of the yaw activity, for instance a slight postponing of a planned yaw activity, it is able to damp existing side-side tower oscillations. On the other hand, it is also possible to stop, i.e. brake or reduce, advantageously a yaw activity such that side-side tower oscillations which might just have been created by the acceleration of the yaw activity are eliminated. In other words, the yaw movement can be used to damp side-side tower oscillations and in particular timed yaw movements can damp the tower oscillations considerably. 
     In an advantageous embodiment, the oscillatory movement of the nacelle has a periodic time-dependency and the sign of the oscillatory movement changes periodically. Furthermore, the yawing speed and the oscillatory movement are coordinated such that the time-dependent oscillatory movement is damped. 
     In another advantageous embodiment, the periodic time-dependency of the oscillatory movement of the nacelle is at least approximately sinusoidal, and the yawing speed and the oscillatory movement are coordinated such that the at least approximately sinusoidal oscillatory movement is damped. 
     In other words, the method described above works particularly efficiently if the oscillatory movement is a periodic movement, in particular a sinusoidal movement. Side-side oscillations typically can be described by an at least approximately sinusoidal oscillatory movement. An amplitude of the oscillatory movement may be similar during a considerable time span, i.e. the amplitude may be substantially time-independent. Alternatively, the amplitude may change randomly or periodically. 
     In another advantageous embodiment, the method comprises a further step of measuring a first position of the nacelle with regard to a ground where the wind turbine is erected at a first moment, and measuring at least a second position of the nacelle with regard to the ground at a second moment. Subsequently the periodic time-dependency of the oscillatory movement is determined based on the measured first position and second position. 
     In practice, it is advantageous to detect and measure a whole set of positions of the nacelle. Thus, a reliable and meaningful time-dependency can be determined 
     One way to measure the position is by installing a detector working with a global positioning system (GPS) at the nacelle. 
     Another advantageous way to measure the position is by an accelerometer which is mounted at the wind turbine. Beneficially, the accelerometer is mounted in the nacelle or at the tower, especially near the top of the tower. The accelerometer is thus highly useful for evaluating the tower movement and time the yaw activity. 
     In another advantageous embodiment, the nacelle oscillates around a pivot point which is located in a bottom section of the tower. 
     The bottom section of the tower may be defined as a part of the tower which comprises  10  per cent of the mass of the whole tower. The bottom section of the tower may also be defined by a bottom volume, the bottom volume comprising  10  per cent of a total volume of the tower and being most distant to the nacelle. Advantageously, the bottom section of the tower is directly attached to the ground. In other words, the pivot point is located near the tower base. 
     The pivot point may lie on the yaw axis. More specifically, it may lie at an intersection of the yaw axis and the ground. If the wind turbine comprises a foundation, the pivot point may be a part of the foundation. 
     In another advantageous embodiment, the wind turbine comprises a rotor which is rotatably mounted about a rotor axis of rotation, and the nacelle oscillates in a plain which is substantially perpendicular to the rotor axis of rotation. 
     This embodiment is also referred to as side-side tower oscillations. The notion “side-side” refers to a view of the hub and the rotor blades as viewed from the front. 
     The invention is also directed towards a control device for damping an oscillatory movement of a nacelle of a wind turbine, the nacelle being attached to a tower of the wind turbine. The control device is configured to coordinate a rotation of the nacelle about a yawing axis with a yawing speed, wherein the yawing axis is aligned with a longitudinal axis of the tower. Furthermore the control device is configured to coordinate a change of the yawing speed, such that a torque resulting from the change of the yawing speed damps the oscillatory movement of the nacelle. 
     The control device may be located at the tower or the nacelle. The control device advantageously works fully automatically. 
     The control device is able to perform the method for damping the oscillatory movement of the nacelle described above. Thus, specific details and features of the method also apply to the control device. 
     The invention is also directed towards a wind turbine for generating electrical power, wherein the wind turbine comprises a control device as described above. 
     Finally, the invention is also related to a computer program for damping an oscillatory movement of a nacelle of a wind turbine, wherein the computer program, when being executed by a data processor, is adapted for controlling and/or carrying out the method described above. 
     The aspects defined above and further aspects of the present invention are apparent from the examples of embodiments to be described hereinafter and are explained with reference to the examples of embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  shows a wind turbine with a control device, 
         FIG. 2  shows an oscillatory movement of a hub of a wind turbine, 
         FIG. 3  shows a location of a center of mass of a wind turbine, and 
         FIG. 4  shows an example of a load of a tower of a wind turbine due to yawing. 
     
    
    
     The illustrations in the drawings are schematically. 
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a wind turbine  10  which is erected on a ground  22 . The wind turbine  10  comprises a substantially cylindrical tower  11  which comprises a longitudinal axis (not explicitly shown). A nacelle  12  is mounted upon the tower  11 . An accelerometer  121  for measuring the position of the nacelle  12  relative to the ground  22  is mounted on top of the nacelle  12 . The nacelle  12  can be rotated about a yawing axis  18 . Furthermore, the wind turbine  10  comprises a main shaft  15  which, on the one side, is connected to a generator  19  for generating electricity and, on the other side, connected to a hub  13 . Rotor blades  14  are attached to the hub  13 . The main shaft  15 , the hub  13  and the rotor blades  14  together are referred to as the rotor of the wind turbine  10 . The rotor is mounted about a rotor axis of rotation  16 . Finally, the wind turbine  10  comprises a control device  17  for damping an oscillatory movement of the nacelle  12 . 
       FIG. 2  shows a wind turbine  10  in a front view. The wind turbine  10  is erected on a ground  22 . The wind turbine  10  comprises a tower  11 , a nacelle (not shown) and a hub  13 . The hub  13  is connected to a main shaft (not shown) and is rotatably mounted about a rotor axis of rotation  16 . Three rotor blades  14  are attached to the hub  13 . Furthermore, in  FIG. 2  an oscillatory movement  20 , in particular side-side oscillations, of the hub  13  are shown. This oscillatory movement  20  may for instance be present because of a previous yawing activity of the wind turbine  10 . 
       FIG. 3  shows a similar wind turbine  10  to the wind turbine  10  shown in  FIG. 1 . Again, the wind turbine  10  comprises a tower  11 , a nacelle  12 , a hub  13 , rotor blades  14 , a yawing axis  18  and a rotor axis of rotation  16 . A nacelle  12  is mounted upon the tower  11 . Again, an accelerometer  121  for measuring the position of the nacelle  12  relative to the ground  22  is mounted on top of the nacelle  12 . The wind turbine  10  is erected on a ground  22 . Additionally, the wind turbine  10  comprises a control device  17  which is configured to damp an oscillatory movement  20  of the nacelle  12 . Additionally,  FIG. 3  shows a center of mass  30  of the wind turbine  10 . As can be seen, the center of mass  30  is shifted, with regard to the yawing axis  18 , towards the rotor blades  14  and along the rotor axis of rotation  16 ,. In other words, there is a lever-arm distance  31  between the center of mass  30  and the yawing axis  18 . 
     Assuming side-side oscillations in a plane which is perpendicular to the rotor axis of rotation  16 , these side-side oscillations originate in a force which is perpendicular to the rotor axis of rotation  16  and the yawing axis  18 . If in a yawing movement along the yawing axis  18  the yawing speed is changed, then a torque  32  in the same direction as the yawing axis  18  is induced. This, however, induces another force, which is directed in the same direction or in the opposite direction as the force which is responsible for the side-side oscillations. Thus, due to an advantageous timing of the yawing activity, the oscillatory movement  20 , i.e. the side-side oscillations, may be damped. 
       FIG. 4  illustrates how yaw activity may affect the movement of the tower. Exemplarily, three yaw movements during a time period of ten minutes are assumed. At the axis of abscissas, i.e. the x-axis, time  40  in minutes is shown. As mentioned, an interval of ten minutes is depicted as an example. 
     The upper graph (a) shows a yaw direction, characterized by a yawing angle  42 . As can be seen, a first yaw movement occurs at approximately 0:50 minutes, a second yaw movement occurs at approximately 1:15 minutes and a third yaw movement occurs at approximately 2:20 minutes. The yaw movements themselves may only comprise relatively small changes in the yawing angle  42 , e.g. only comprising a few degrees. 
     The following graph (b) depicts a torsion moment  43  of a top of the tower in arbitrary units. Each of the three yaw movements induces a distinctive spike in the torsion moment which as a consequence leads to side-side oscillations of the tower as will be described in the following. 
     Note that in  FIG. 4  oscillatory movements of the tower of the wind turbine are shown. A nacelle of the wind turbine oscillates likewise, showing a similar time dependency of the oscillatory movements. Thus, the results presented in  FIG. 4  may also be applied to an oscillatory movement of the nacelle. 
     The following graph (c) shows a moment of the oscillatory movement of a bottom section of the tower  44  in arbitrary units. Likewise, the lower graph (d) shows a moment of the oscillatory movement of a top section of the tower  45  in arbitrary units. It can be seen that the third yaw movement, occurring at a time of approximately 2:20 minutes, damps the side-side tower oscillations efficiently and almost instantaneously. This is due to the fact that a phase of the excitation makes it act as damping with regard to the ongoing tower oscillatory movement.