Abstract:
A wind turbine has a Lidar ( 20 ) device to sense wind conditions upstream of the wind turbine. Signals from the wind turbine are processed to detect an extreme change in wind direction. The detection is performed by differentiating the rate of change of wind direction and filtering for a period of time. On detection of extreme change the system controller takes the necessary evasive action which may include shutting down the turbine, commencing an immediate yawing action, and de-rating the turbine until the yawing action is complete.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to wind turbines, and in particular, to the advance detection of upstream extreme wind conditions and the control of wind turbines in response to such detected conditions. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is important for a wind turbine to have advance knowledge to the condition of the wind which will shortly arrive at the turbine. Such knowledge gives the turbine controller sufficient time to adjust operating parameters, such as blade pitch angle or rotor speed, to match the oncoming conditions. This may be done for a variety of reasons. At lower wind speeds it is important to maximize the energy that can be extracted by the wind by setting parameters such as blade pitch angle to an optimum position. At higher wind speeds it is important to adjust turbine parameters to avoid operation under conditions which might lead to damage. Wind turbines have a rated power and when this power output is exceeded, the blade pitch angle and other operating parameters will be adjusted to reduce the amount of energy that is extracted from the wind. Wind turbines also need to be designed to withstand extreme operating conditions. Typically, these extreme conditions are rare single events or a small number of cumulative events which cause large, often unbalanced loads on the wind turbine and will damage the turbine or reduce the overall lifetime of the turbine components, such as the blades or the gearbox by a significant amount. 
         [0003]    Wind turbines are required to meet extreme conditions as set out in International Standard IEC 61400-1 3 rd  Edition 2005. Section 6.3.2 defines a range of extreme wind conditions including extreme wind speed, extreme turbulence, and extreme direction change. In order to meet these requirements, wind turbines are required to be considerably over-engineered for normal use which greatly increases the amount of material used in turbine components, such as the blades, the gearbox, and tower. This in turn increases the cost of wind turbine installation and the cost of energy generation. 
         [0004]    Many proposals have been made for determining advance wind conditions. Generally these include sensors placed on the turbine which observe the upstream wind. 
         [0005]    One example is disclosed in EP-A-0970308 which discloses the use of a Lidar or similar remote sensing apparatus, mounted on the nacelle on the wind turbine, and sensing conditions several rotor diameters upstream of the turbine. Based on the sensed conditions the controller, which may be on board the turbine or may be a separate wind power controller, can instruct an individual turbine or group of turbines to change their operating parameters before the sensed wind conditions arrive at the turbine. Another example is disclosed in US-A-20060140764, in which the Lidar is mounted in the rotor hub and has a plurality of look directions that are inclined away from the rotational axis of the hub so that rotation of the hub ensures scanning. The multiple look directions may be achieved by using a number of dedicated Lidar systems and/or by using multiplexed Lidars or a beam splitter. 
         [0006]    The known art provides a number of examples of advance sensing of wind conditions at a wind turbine. However, none of the art enables the detection of an extreme change in wind direction and so none is able to protect the turbine from the consequence of such an extreme direction change. The present invention aims to address this deficiency in the prior art. 
         [0007]    Accordingly, the invention provides a control system for a wind turbine, comprising; a device mounted on the wind turbine to sense a property of wind conditions at a position upwind of the wind turbine; a controller for receiving and processing signals from the sensing device to detect an extreme change in wind direction upwind of the wind turbine, and for generating one or more control signals for varying an operating parameter of the wind turbine in response to the detected extreme change in wind direction, wherein the controller comprises a differentiator for measuring the rate of change of a wind parameter. 
         [0008]    The invention also provides a method of controlling a wind turbine, comprising; sensing a property of wind conditions at a position upwind of the wind turbine using a remote sensing device mounted on the turbine; receiving and processing signals from the sensing device at a controller to detect an extreme change in wind direction upwind of the wind turbine the processing comprising differentiating a measured wind parameter to determine the rate of change of that parameter; and generating one or more control signals for varying an operating parameter of the wind turbine in response to the detected extreme change in wind direction. 
         [0009]    Embodiments of the invention have the advantage that by differentiation of a suitable wind parameter, the rate of change of that parameter can be determined and the severity of a change of wind direction determined. This enables the system controller to determine whether there is an extreme direction change as defined by IEC 61400-1 discussed below and, thus, whether evasive action need be taken. 
         [0010]    In one preferred embodiment of the invention the differentiator differentiates the wind direction. A filter may determine whether the rate of change of wind direction exceeds a predetermined value for a predetermined period of time. This has the advantage that instantaneous changes in direction, which may be caused by turbulence, are not flagged as extreme events and an extreme event is only signalled when the detected change is maintained for a period of time. 
         [0011]    The predetermined value of the rate of change of wind direction may be dependent on the magnitude of the measured wind speed. This is advantageous as the higher the wind speed, the lower the change in direction required to cause over-loading and damage. 
         [0012]    In one preferred embodiment, the predetermined value or the rate of change of wind direction is 5°/s. The predetermined period of time may be at least 3 seconds. 
         [0013]    In another preferred embodiment of the invention, the sensed wind parameter is wind speed, and wind speed signals are resolved into axial and lateral components. The differentiator acts on the lateral component to determine the rate of change of wind lateral wind speed. This approach is advantageous as it may be more accurate than determining a change in wind direction by looking at the angle of the wind to the axis of rotation of the wind turbine rotor. 
         [0014]    Preferably the sensing device senses the wind parameter at a plurality of distances upwind of the wind turbine. This has the advantage that changes in the oncoming wind front can be detected so that unnecessary evasive action is not taken when an extreme change in direction detected at a far distance degrades into an event that is not classed as extreme as it approaches the turbine. 
         [0015]    Preferably the sensing device is a multiple beam Lidar having a multiple range gate. 
         [0016]    The control signal generated by the controller in response to a detected extreme change of direction may comprise a turbine shutdown command, or a turbine yaw command. 
         [0017]    In the case of a yaw command the controller may additionally command the turbine to de-rate until the yawing action is complete. 
         [0018]    The invention also resides in a wind turbine having a control system as defined above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  is a graph of wind direction against wind speed showing an extreme change in direction; 
           [0021]      FIG. 2  is a similar graph to  FIG. 1  of wind direction against time at a wind speed of 6 m/s; 
           [0022]      FIG. 3  is a similar graph to  FIG. 2  at a wind speed of 10 m/s; 
           [0023]      FIG. 4  is a similar graph to  FIG. 3  at a wind speed of 25 m/s; 
           [0024]      FIG. 5  is a schematic overview of a wind turbine having a Lidar embodying the invention and showing an oncoming wind front with an extreme change in wind direction; 
           [0025]      FIG. 6  shows a schematic diagram of a first embodiment of the invention which measures the angle of wind direction; and 
           [0026]      FIG. 7  shows a schematic diagram of a second embodiment of the invention in which a change of direction is determined for the acceleration of a lateral velocity component. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
       [0027]    International Standard IEC 61400-1 3 rd  Edition, sets out design requirements for wind turbines. Chapter 6.3.2 referred to above sets out and defines extreme wind conditions including wind sheer events, peak wind speeds due to storms and rapid changes in wind speed and direction. The magnitude of an extreme direction change is given by: 
         [0000]    
       
         
           
             
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         [0000]    where θ θ  is the extreme direction change magnitude 
         [0000]      σ 1  is given by σ 1   =I   ref  (0.75 V   hub   +b );  b =5.6  m/s  
       /ref is the expected value of hub-height turbulence intensity at a 10 min average wind speed of 15 m/s   Vhub is the wind speed at hub height       
 
         [0030]    D is the rotor diameter 
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         [0031]    The extreme direction change transient, θ(t), shall be given by 
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         [0000]    where T=6 s is the duration of the extreme direction change. The sign shall be chosen so that the worst transient loading occurs. At the end of the direction change transient, the direction is assumed to remain unchanged. The wind speed shall follow the normal wind profile model. 
         [0032]    The transition into an extreme direction change is shown for a Vestas V90 1.8MVV wind turbines in  FIGS. 1 to 4 .  FIG. 1  is a graph of wind speed against wind direction with the thick line  10  showing the transition to an extreme direction change based upon the formula expressed in the equation above. Thus, a change of about +/−60° is considered to be extreme at a low wind speed of 5 m/s but at higher wind speeds, a much smaller wind change over the designed period t=6s is considered extreme. At 25 m/s a change of about 30° is treated as extreme. 
         [0033]      FIGS. 2 to 4  show this data as changes in wind speed over time at fixed wind speeds of 6 m/s ( FIG. 2 ); 10 m/s ( FIGS. 3 ); and 25 m/s ( FIG. 4 ). The figures show that the rate of change is more important than the actual magnitude of the change as the rate of change determines the ability of the turbine to act. A wind direction that changes slowly over time is less likely to cause a problem to the turbine controller as the controller will have time to adjust turbine operating parameters in accordance with the change, whereas a rapid change may not give the turbine controller sufficient time to react. As wind speeds increase, the rate of change of direction that the controller can handle adequately decreases as is reflected in the graphs which show the extreme transition occurring at a much lower angle for higher wind speeds that for lower wind speeds. In all three examples shown in  FIGS. 2 to 4 , the extreme transition is a gentle S-shaped curve, which is essentially a straight line over it&#39;s mid-portion between about 2 and 4 s. 
         [0034]      FIG. 5  illustrates an embodiment of the invention in which a Lidar or similar remote sensing apparatus  20  is mounted on a wind turbine  30 . It is presently preferred to mount the Lidar on the top surface of the turbine nacelle behind the rotor blades with a look direction extending generally in front of the blades. Alternative locations for the Lidar may be used, for example it may be mounted in hub to rotate with the hub to provide a conical scan. 
         [0035]    It is preferred, but not essential, that the Lidar is a multiple gate range Lidar. This means that the Lidar is capable of sensing wind conditions at a plurality of distances from the wind turbine. This makes it possible to monitor the progress of a detected extreme event which may reduce in intensity as it approaches the wind turbine. This is important as it prevents evasive action being taken which is unnecessary if the severity of the event diminishes as it approaches the turbine. Reacting to an extreme event is undesirable unless absolutely necessary and will cause a temporary loss in energy production. Sensing wind conditions relatively far from the turbine, however, is desirable as it gives more time for the turbine to react. 
         [0036]    The Lidar is a multiple beam Lidar having at least two beams enabling it to sense the direction of movement. Although not essential, the Lidar preferably has between three and five beams. These beams may be produced by any suitable method, for example using a single Lidar device with a beam splitter or multiplexer or by using a plurality of devices. 
         [0037]    In  FIG. 5  the Lidar  20  senses wind conditions at two ranges: 50 m and 100 m. This is exemplary only and different distances and a different number of distances may be chosen depending on the site and the number of ranges the chosen Lidar can measure. A wind front  40  is shown advancing on the turbine. This front changes direction at a point between the two ranges with the direction change front being shown by dotted line  50 . At the 50 m range the Lidar detects zero wind direction, that is, the wind direction is parallel to the axis of rotation of the turbine. At 100 m the Lidar and associated processor detects an angle of about 30° over a 6 second period or 5° per second. Depending on the wind speed, this change in angle could represent an extreme change. To enable this to be determined, the controller differentiates the signal provided by the Lidar to determine the rate of change of direction. In practice, wind direction signals will frequently change instantaneously by this amount. However, in determining whether the change may be treated as an extreme event, it is important to determine whether this rate of change is maintained over a period of time, for example between about 2 to 5 seconds, preferably for at least three seconds and more preferably at least four seconds. This may be achieved by filtering the differentiated signal. If the signal reaches the threshold of 5° per second, then the controller can command evasive action. 
         [0038]    The controller may determine the angle of the wind direction with respect to the axis of rotation of the wind turbine rotor. Alternatively it may look at the detected wind velocity and resolve that velocity into lateral and axial components with the lateral component representing the velocity of travel in a direction parallel to the plane of rotation of the wind turbine rotor, or normal to its axis of rotation. Once the lateral component has been determined it is differentiated to give the acceleration or rate of change of the lateral component. If that acceleration exceeds the given threshold then action is taken. The threshold may be exceeded for a time period as mentioned above for the change in direction to be treated as an extreme change. The time period may depend on the magnitude of the acceleration so that a more rapidly changing wind front may need to be detected for a shorter time than one which only just exceeds the extreme event threshold for evasive action to be taken. 
         [0039]    Once the controller detects that the threshold has been exceeded for the predetermined time it commands the turbine to take evasive action. This may require a controlled shut down or an emergency shut down of the turbine or some other action such as varying the blade pitch angle for the output power. Alternatively, or additionally, the controller may override the turbine yaw to start an immediate yawing procedure. 
         [0040]    The turbine may be de-rated until the yawing action is complete. The choice of evasive action will depend on the severity of the extreme event. 
         [0041]      FIGS. 6 and 7  illustrate the two embodiments described. In  FIG. 6 , a Lidar  20  mounted on a wind turbine emits a plurality of beams  48  to detect a parameter of the upwind wind front  40 . In this embodiment the Lidar is a multibeam Lidar which has a plurality of beams or look directions three being shown in the figure and which detect the wind direction which may be expressed as an angle ⊖ with respect to a known direction such as the axis of rotation of the wind turbine rotor. A differentiator  55  differentiates the measured angle with respect to time to give a value d⊖/dt and a filter  60  filters that signal over a predetermined period of time, here 4 seconds as discussed above. A threshold detector  65  receives the output from the filter and an indication of windspeed and determines whether the treshold hes been exceeded. The threshold detector includes a look up table of thresholds at different windspeeds. At  70 , where the output of the threshold detector indicates an extreme event, the controller commands an evasive action and causes a parameter of the wind turbine to be adjusted accordingly. This parameter may be a total shut down command or a nacelle yaw command. 
         [0042]    The embodiment of  FIG. 7  is similar to that of  FIG. 6  except that the controller acts on the output of the Lidar  20  at  80  to determine the wind velocity and resolves that velocity into two components: an axial velocity in the direction of the axis of rotation of the wind turbine rotor, and a lateral velocity being the velocity in the plane of the rotor or normal to the axis of rotation. The differentiator  55 A acts on the lateral velocity to provide an output to the filter  60  dV lateral /dt which is the lateral acceleration of the wind and therefore indicative of a change in direction. 
         [0043]    Thus, embodiments of the invention enable extreme changes of direction to be detected and evasive action taken before the events arrive at the wind turbine. This enables the design constraints on the turbine to be changed so that they do not have to withstand loading caused by extreme changes in wind direction This in turn enables wind turbine designers to use lighter components reducing the cost of wind turbines and thus the cost of producing energy. Alternatively, it enables existing components to be operated at higher rated output powers thus boosting the energy that can be extracted by a given turbine. 
         [0044]    Many modifications to the embodiments described above are possible and will occur to those skilled in the art without departing from the invention. 
         [0045]    For example, the controller may be mounted on, and be part of, an individual turbine, or it may be a remote controller which controls multiple turbines which form a wind park or a part of a wind park. The scope of which is defined by the following claims.