Patent Publication Number: US-2021180565-A1

Title: Method for operating a wind turbine in emergency mode, and controller and wind turbine

Description:
BACKGROUND 
     Technical Field 
     The invention concerns a method for a wind turbine in an emergency mode as well as a control means and a wind turbine. 
     Description of the Related Art 
     In accordance with the state of the art wind turbines have a plurality of components which can be adjusted in dependence on the prevailing wind factors in order to maximize the energy output of a wind turbine. Energy is required for setting or adjusting the adjustable components. 
     Accordingly in modern wind turbines for example the rotor blades of an aerodynamic rotor can be rotated about their longitudinal axis, therefore displaced, so that the rotor blades ideally receive the afflux flow of the wind. A lift effect resulting therefrom of the rotor blades serves to generate a torque in a rotor of a generator that is driven by the aerodynamic rotor. Accordingly the kinetic energy of the wind can be converted into electrical energy with the generator of the wind turbine. 
     In addition modern wind turbines include a wind direction tracking arrangement in order to orient the entire aerodynamic rotor into the prevailing wind direction. The wind direction tracking arrangement is also referred to as yaw adjustment. 
     Accordingly the rotor blade plane, namely a plane in which the rotor blades of a wind turbine rotate, is always oriented substantially perpendicularly to the wind afflux flow so that this ensures a symmetrical afflux flow to the rotor. Bending moments are reduced and energy output is maximized by the symmetrical afflux flow. 
     An afflux flow for the rotor accordingly denotes the relationship between the wind direction and the rotor position. In the case of a symmetrical afflux flow the rotor plane is oriented substantially perpendicularly to the wind direction by the wind direction tracking arrangement, or, expressed in other terms, the axis of the rotor in the case of a symmetrical afflux flow is oriented parallel to a directional vector of the current wind. The term parallel afflux flow is therefore also used for the symmetrical afflux flow. Furthermore, with a symmetrical afflux flow the wind meets the rotor plane from the front, that is to say from the side of the rotor, that does not face towards the pod of the wind turbine. 
     There are situations however in which it is not possible to adapt the components of the wind turbine to the changing wind conditions. Mention may be made here for example of the emergency operation of a wind turbine, in which the wind turbine is separated from the grid and thus cannot take any power from the grid for adjustment of the mechanical components of the wind turbine. 
     Accordingly the mechanical components of the wind turbine, in particular the rotor shaft with which the rotor is mounted on the stationary part of the wind turbine are designed to be oversized for those situations, to prevent damage due to loadings, for example in the event of a transverse flow to the rotor blades. 
     Accordingly therefore wind turbines are designed to be more stable than would be necessary for normal operation in which power can be taken from the grid and fed in. Such additional stability however gives rise to additional manufacturing costs and also in normal operation in part reduces the efficiency and thus the output of a wind turbine. 
     It is therefore known from the state of the art to provide energy storage means in the wind turbine in order to move the rotor blades into a predefined position even in the emergency mode. That can permit spinning of the rotor blades so that at least the speed of rotation of the rotor in the emergency mode when there is no braking device provided for the rotor can be reliably kept below a predetermined speed of rotation upper limit. 
     An emergency mode which is started as soon as the wind turbine is separated from the grid accordingly provides that the rotor blades are transferred as a one-off operation from the current position into a predefined position. Suitable emergency power sources are designed for that one-off adjustment. Adjustment of the wind direction tracking arrangement or yaw setting, that is to say setting the yaw angle, has hitherto not been provided in the emergency mode because the power required for that, which would also have to be provided in emergency power storage means, would be very high and therefore the provision of an emergency power supply of many times the magnitude of an emergency power supply for adjustment of the rotor blades would be necessary. Accordingly oversizing of the wind turbine is still necessary to avoid damage in an emergency situation with an inclined flow to the blades. 
     On the German patent applications from which priority is claimed for the present application the German Patent and Trade Mark Office searched the following documents: US 2011/0076142 A1, WO 2007/132303 A1, US 2010/0078939 A1 and US 2011/0280725 A1. 
     BRIEF SUMMARY 
     Stability requirements of wind turbines are to be reduced to preferably avoid oversizing of the wind turbines, which has to be designed for situations involving an inclined afflux flow in an emergency mode or at least to minimise the degree of oversizing. 
     Provided is a method for a wind turbine in an emergency mode. Accordingly the wind turbine is in an emergency mode and is thus separated from the power grid. Separation from the grid does not necessarily mean here that a physical separation has to be implemented, but it also includes that situation. At any event separation from the grid means that no power for operation of the wind turbine can be taken from the grid. The term grid is used to mean a power supply grid, by way of which the wind turbine in normal operation provides the generated energy to feed that for example to consumers. 
     A change in a wind direction is detected. In addition or alternatively a change in at least one force acting on the wind turbine is detected. Thereupon, while maintaining the current yaw angle of the yaw setting, at least one of the rotor blades is adjusted to a different angle, that angle being dependent on the detected change. 
     Detection here is not intended to be interpreted as measurement. Rather, preferably for detecting a change in wind direction, firstly the wind direction is measured and/or for detecting a change in force firstly the force is measured, continuously or in interval relationship, preferably absolutely. It is only then that a change is detected from that measurement, taking account of previous measurements, for example in a control means, if the measurement values for the currently measured wind direction and/or force differ from a previously measured wind direction or force. 
     A change in the wind direction means that firstly there is a first wind direction and that changes to a second wind direction. That change from the first wind direction to the second wind direction then corresponds to a detectable change in the wind direction. A change in a force acting on the wind turbine concerns a change in a force which is preferably exerted by the current or a prevailing wind. If the wind changes, that is to say in particular the wind strength or the wind direction, then certain forces which act on the wind turbine, in particular the components thereof, change. At least one of those changes in force which is preferably predefined therefore changes in dependence on the wind, with that change being detected. 
     That purpose is served for example by one or more sensors like strain gauges which are arranged for example on the turbine tower and/or at the rotor blades. Preferably three or four sensors are arranged on the tower uniformly in the peripheral direction for that purpose. The term force is also used here generally to denote any force parameter which, besides directing acting forces, also embraces moments, in particular flexural moments, or other loads. 
     According to the method therefore the wind direction or at least one force is monitored, measured or detected, and in the situation where the wind direction or force changes, that change is detected so that the further steps in the method are then carried out. Accordingly at least one of the rotor blades is set differently from the position which was adopted before the change in wind direction or the change in force. If therefore the rotor blade has a first position with the first wind direction or force then the rotor blade is adjusted in such a way that it is in a second position after the change, that is to say with the second wind direction or force. 
     That makes it possible that, in the situation where an inclined afflux flow to the wind turbine occurs and adjustment of the yaw angle is not possible by virtue of the emergency mode then nonetheless the force acting on the wind turbine by the wind can be minimized. Loading on the wind turbine is thus reduced with the inclined afflux flow and the wind turbine therefore does not have to be oversized or can be at least to a lesser degree oversized. 
     According to a first embodiment the at least one rotor blade is adjusted to an angle which differs from an angle for a feathered position, in particular in the case of a symmetrical afflux flow, with the retained yaw angle, namely the current yaw angle. 
     According to a further embodiment adjustment of at least one of the rotor blades is effected after a change in the wind direction and/or a change in the force was detected such that an angle of the rotor blade is set to the angle from which there result the forces and/or the flexural moments on the at least one rotor blade and/or the rotor and/or the rotor shaft, which are minimized in comparison with one or more other angles. Preferably the rotor blade is moved into a feathered position relative to the fresh wind direction, that is to say the wind direction after the change, so that forces occurring by virtue of that wind direction substantially cancel each other out on the suction and pressure sides. 
     Accordingly the forces resulting on the wind turbine are reduced when there is an inclined afflux flow. 
     According to a further embodiment a change in the wind direction is detected when a horizontal differential angle between a first wind direction and a second wind direction is above a predefined differential angle threshold value. Accordingly no change in the wind direction is detected and the rotor blades remain in the current position as long as the differential angle is at or below the differential angle threshold value. That avoids blade adjustment already being effected at low differential angles, although there are still no high loads exerted on the rotor blade or the wind turbine, due to the inclined afflux flow which is thus only slight. Unnecessary reciprocating adjustment of the rotor blades is thus avoided. The differential angle threshold value is preferably in a range of 1 to 10 degrees, particularly in a range of 2 to 5 degrees, and can be predetermined by simulation. 
     Additionally or alternatively in this embodiment a change in the force exerted on the wind turbine is detected if a differential force between a first detected force and a second detected force is above a first predefined differential force threshold value. Equally no change in the force exerted on the wind turbine is detected if the differential force is at or below the differential force threshold value. 
     According to a further embodiment a wind speed is determined and compared to a predefined wind speed threshold value. If the wind speed is above the predefined wind speed threshold value then in the situation where a change in the wind direction or the force on the wind turbine is detected, the yaw angle is maintained and at the same time the at least one rotor blade is adjusted to another angle. If the wind speed is below or at the wind speed threshold value the rotor blades remain in the current position. That procedure of determining the wind speed and comparing it to the wind speed threshold value is preferably effected prior to the step of detecting a change in the wind direction. The wind speed threshold value can also be predetermined by simulation. 
     Account is taken of the fact here that damage to the wind turbine can only occur as from the occurrence of given forces caused by high wind speeds. If wind speeds remain below that limit, which is referred to here as the wind speed threshold value, there is no danger to the wind turbine and there is therefore also no need for adjustment. As previously, in this embodiment therefore it is possible to avoid unnecessary adjustment procedures for the rotor blade or blades. 
     According to a further embodiment a change in the at least one force acting on at least one component of the wind turbine, in particular on one or more rotor blades, the rotor or the rotor shaft, is detected when a monitored force exceeds a predefined force threshold value. For example a sensor like for example a strain gauge serves to monitor the force. In that way for example bending forces on the rotor shaft or one or more rotor blades are measured and a force which is acting on at least one component of the wind turbine is determined from the measurement. That force or those forces are then compared to a force threshold value. 
     Detection of a change and retention of the yaw angle of the yaw setting and adjustment of at least one of the rotor blades is effected only when the given force is above a force threshold value. If the force is below the force threshold value or at that value then the rotor blades remain in the current position. That operation of determining the at least one force and comparing it to the force threshold value is preferably effected prior to the step of detecting a change in the wind direction. The force threshold value can also be predetermined by simulation. 
     Alternatively or additionally to the wind speed therefore it is possible to determine a force exerted on the wind turbine by the wind and by establishing the force threshold value at least one of the rotor blades is then adjusted only as soon as that force could lead to damage to the wind turbine. If on the other hand accordingly there is no risk of damage due to the given force which occurs it is possible to avoid adjustment of the rotor blades. 
     According to a further embodiment the power for adjustment of the rotor blades is taken from an emergency power supply, in particular an accumulator, of the wind turbine. 
     The method is therefore carried out in the emergency mode when therefore no energy can be taken from the grid to which the wind turbine is connected. Accordingly adjustment of the rotor blades can be effected without having to take power from the grid by the provision of the emergency power supply and supplying the motors for adjustment of the rotor blades. 
     According to a further embodiment the emergency power supply in the emergency mode is designed or sized for two or more adjustment operations of all rotor blades, in particular when it is assumed that each adjustment operation includes a rotation of each rotor blade about its longitudinal axis through at least 90 degrees. Multiple adjustment of the rotor blades with an inclined afflux flow which change a plurality of times during the emergency mode of operation is thus possible. Accordingly, after for example a first change in a wind direction has been detected and the rotor blades have been correspondingly adjusted, the rotor blades can be freshly set upon a further change in the wind direction. 
     According to a further embodiment the rotor blades are rotatable through 360 degrees about the longitudinal axis. If accordingly the wind direction changes through 180 degrees in relation to a symmetrical afflux flow from the front side of the rotor then the directional rotor of the wind after the change in wind direction accordingly points to the rear side of the rotor blade plane. In that case the rotor blades can be adjusted through 180 degrees. In the case of transverse afflux flows the rotor blades can be respectively adjusted accordingly through 90 degrees or 270 degrees in relation to a feathered position when there is a parallel afflux flow, that is to say with a symmetrical afflux flow, as viewed at the current yaw angle. Irrespective of the wind direction the rotor blades can thus be so adjusted that a force which is as low as possible acts on the rotor blades due to the wind independently of the yaw angle. 
     According to a further embodiment the rotor blades are individually adjustable. Accordingly each rotor blade can be set in such a way that a force which is as low as possible due to the wind acts on the respective rotor blade. 
     Provided is control means for a wind turbine, adapted to carry out the method according to one of the above-mentioned embodiments. Preferably for that purpose the control means includes a computer unit and one or more sensors, in particular for detection of the wind direction, the wind speed, at least one force acting on a component of the wind turbine, and/or further parameters as sensor data. The wind turbine is then adapted to transmit the sensor data to the control means. 
     Furthermore the control means serves to operate the wind turbine in an emergency mode with an emergency power supply for adjustment of the rotor blades. In that situation the control means is adapted to cause actuators for adjustment of the rotor blades to be adjusted in accordance with a position which is determined in the control means. Adjustment is determined in dependence on a detected change in the wind direction. 
     The invention further concerns a wind turbine having a control means according to one of the above-described embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Further configurations will be apparent from the embodiments by way of example described in greater detail with reference to the figures. 
         FIG. 1  shows a side view of a wind turbine, 
         FIG. 2  shows a plan view of a wind turbine with rotor blades in the feathered position, 
         FIG. 3  shows the view of  FIG. 2  with changed rotor blade position, 
         FIG. 4  shows the plan view from  FIGS. 2 and 3  with the rotor blade position again changed, and 
         FIG. 5  shows the steps of an embodiment of the method. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a wind turbine  10  having a tower  12  and a pod  14 . Arranged on the pod  14  is an aerodynamic rotor  16  having three rotor blades  18  and a spinner  20 . In operation the rotor  16  is driven in rotation by the wind and thereby drives a generator in the pod  14 . 
     The pod  14  is adjustable about a perpendicular axis  22  in the direction of the arrow  24 . That adjustment is also referred to as the yaw adjustment or wind direction tracking and thus serves for setting the yaw angle of the wind turbine  10 . The yaw setting accordingly serves to orient the rotor  16  to the prevailing wind direction  26 . The yaw or yaw angle shown here is defined for example as the yaw angle of zero degrees. With the illustrated yaw angle and the prevailing wind direction  26  reference is made to a parallel afflux flow or a symmetrical afflux flow. That is the case when the wind  26  impinges on the rotor  16  substantially parallel to a rotor shaft  28  from the front side  30 . 
     The rotor blades  18  can also be rotated about their longitudinal axis  32 . That is illustrated by the arrow  34 . In the present case the rotor blades  18  are oriented in a feathered position. This means that the wind which is incident on the rotor  16  from the wind direction  26  produces no or only a slight torque on the rotor shaft  28 . The reason for this is that the position of the rotor blades  18  provides that the forces acting on the respective rotor blade  18  substantially cancel each other out or add up to make zero. 
     That feathered position is preferably assumed when the wind turbine  10  is not to feed any energy into the grid or the wind turbine  10  is in an emergency mode of operation in which no power for further controlled adjustment of the wind turbine  10  can be taken from the grid. 
       FIG. 2  shows a plan view of the wind turbine  10  of  FIG. 1 . The identical references denote the same features. In  FIG. 2  the wind turbine  10  is also oriented in relation to the wind and this therefore involves a parallel afflux flow. The rotor blades  18  are also set in the feathered position. The loading on the wind turbine  10  is thus minimal. 
       FIG. 3  now shows a first example for adjusted rotor blades  18  in accordance with the method. In this case the wind direction  26  has now turned through 90 degrees from the first wind direction  26   a  as shown in  FIG. 2  into a second wind direction  26   b  in the clockwise direction in relation to the wind direction in  FIG. 2 . Accordingly this now no longer involves a parallel afflux flow as shown in  FIG. 2  but an inclined afflux flow generally or here specifically a transverse afflux flow. Accordingly the wind turbine  10  detects that there has been a change in the wind direction from the first wind direction  26   a  into the second wind direction  26   b . Alternatively or additionally that change in wind direction can also be detected by a change in a force acting on the wind turbine  10 . Loads or changes in load are for that purpose monitored for example with strain gauges. 
     As shown in  FIG. 3  the yaw angle of the yaw setting is maintained in spite of the change in the wind direction and the rotor blades  18  are moved to a different angle which can also be referred to as the second angle. The second angle differs from an angle for a feathered position in relation to a parallel afflux flow with the current yaw orientation. More specifically accordingly the rotor blades  18  are now set in relation to the wind in such a way that as low a force as possible acts on the rotor blade  18  due to the wind. It can be seen from  FIG. 3  that the rotor blades  18  are adjusted through 90 degrees in relation to the position in  FIG. 2 . 
     A further example for a detected changed wind direction  26  and/or changed force on the wind turbine  10  is shown in  FIG. 4  in which the wind direction  26  is now directed on to the rear side of the wind turbine  10 . Here too the rotor blades  18  are adjusted with the yaw angle being maintained as a change in the wind direction or force, namely a wind direction, was detected, which is different from the parallel afflux flow with the current yaw orientation. The rotor blades  18  are here adjusted in such a way that as viewed from the wind direction  26  they are in a feathered position, wherein that feathered position differs from the feathered position with the parallel afflux flow with the current yaw orientation, as in  FIG. 2 . 
     Accordingly the position of the rotor blades  18 , namely the pitch angle, is adapted to the changing wind directions  26 , in dependence on the wind direction  26 . 
       FIG. 5  shows an embodiment of the steps in the method according to an embodiment. In step  50  the wind direction  26  or a force on the wind turbine  10  is monitored while step  52  detects that the wind direction  26  or the force has changed. In step  54  therefore the yaw angle of the yaw setting of the wind turbine is maintained and in step  56  the rotor blades  18  are adjusted in dependence on the changed wind direction  26 .