Abstract:
A method is provided for determining a deflection of a rotor blade of a wind turbine including the following steps: fixing at least one electrical conductor loop via fixing points to at least one surface of the rotor blade, wherein the at least one electrical conductor loop is arranged such that due to the deflection of the rotor blade an extension of the at least one conductor loop is forced between at least two of the fixing points, the extended electrical conductor loop keeps closed if the deflection of the rotor blade is below the defined threshold, the extended conductor loop opens if the deflection of the rotor blade is beyond the defined threshold. Further an arrangement, a sensor, a rotor blade and a wind turbine are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority European application No. EP 15189797.2 having a filing date of Oct. 14, 2015, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a method and to an arrangement for determining a deflection of a rotor blade. Further, a sensor and a rotor blade and a wind turbine are suggested. 
       BACKGROUND 
       [0003]    Due to increasing power to be provided by wind turbines, rotor blades representing essential driving components of such wind turbines are optimized with respect to, e.g., weight, aerodynamic characteristics and most possible generation of electrical power. 
         [0004]    Thereby, design and shape of the rotor blades is characterized or limited by several factors. As an example, in particular for long slender and flexible rotor blades, one of the key characteristics during operation of a wind turbine is a clearance between a tip of the rotor blade and a surface of a tower of the wind turbine during extreme situations in order to avoid collisions. The distance between the tower and the tip end of the blade is also referred to as “blade tip/tower clearance”. 
         [0005]    Tip deflections have limiting consequences on the structure of the rotor blades, the same as limiting economical consequences. Consequently, the level of loading which the blade can carry is often limited by this factor and thus the amount of energy extracted as well as the cost of energy. 
         [0006]    The problem of maintaining blade tip/tower clearance may be solved from the structural design perspective by, e.g., increasing stiffness of a main spar of the blade. This is a purely structural solution which is related to higher blade mass and thus higher blade cost as well as higher fatigue loads in both the blades and the hub components. 
         [0007]    From an aerodynamic design perspective the problem of high tip deflections may be solved according to one out of the following exemplary options:
       Increasing slenderness of the blades:   A disadvantage of this solution might be the negative affection of the aerodynamic performance of the wind turbine during normal operation. As a further drawback structural dimensions like “building height” may in some cases become very small;   Reduction of an aerodynamic twist of the blade towards the tip:   A disadvantage of that solution would be the loss of performance during normal operation;   Use of active flaps:   The disadvantage here is the necessary implementation of active elements in the blade;   Increasing a relative thickness of airfoils in order to increase the structural stiffness:   The drawback here is the poor performance of thinner airfoils in comparison with thicker airfoils.   Reduction of a pitch angle of the blade in critical loading situations which has negative consequences to Annular Energy Production, AEP.       
 
       SUMMARY 
       [0017]    An aspect relates to an improved approach for detecting and avoiding extreme deflection of a rotor blade. 
         [0018]    In order to overcome this problem, a method is provided for determining a deflection of a rotor blade of a wind turbine comprising the following steps:
       fixing at least one electrical conductor loop via fixing points to at least one surface of the rotor blade, wherein the at least one electrical conductor loop is arranged such that   due to the deflection of the rotor blade an extension of the at least one conductor loop is forced between at least two of the fixing points,   the extended electrical conductor loop keeps closed if the deflection of the rotor blade is below the defined threshold,   the extended conductor loop opens if the deflection of the rotor blade is beyond the defined threshold.       
 
         [0023]    The proposed solution enables detection of extreme deflection of a rotor blade mainly based on the fact that under extreme conditions and due to elastic deformations of the blade a suction side of the blade will be in compression whereas a pressure side of the blade will be in extension. Thus, any element like, e.g., an electrical conductor loop being assigned or fixed to the surface (which can be inner or outer surface) of the rotor blade will experience the same or similar elastic deformation. 
         [0024]    As an example, the extension of a blade segment will be proportional to the to the original length of the segment multiplied by the strain level. A strain level of, e.g., 3000 microstrains [10̂−6 mm/mm] valid for a blade segment with a length of 10 m would result in a total deformation, i.e. extension of 3000*10̂−6*10000=30 mm. 
         [0025]    In other words, for a blade length of, e.g., a couple of meters, the total deformation will result in a couple of centimeters. This variation in length can be used in different ways for monitoring provided that the element has the freedom to deform (i.e. not firmly attached to the blade surface) independently of the blade. 
         [0026]      FIG. 1  is exemplarily illustrating one possible embodiment of deformation of a rotor blade. According to  FIG. 1 , a nacelle  110  is rotatable mounted on top of a tower  120  of a wind turbine  100 . A rotor  130  is attached to an upwind side of the nacelle  110 . The rotor  130  includes a central rotor hub and a plurality of rotor blades  140  mounted to and extending radially from the rotor hub defining a rotor plane. 
         [0027]    Thereby, assuming an exemplary tip deflection of 5 meter for a 45 meter blade  140 , a inner surface of a pressure side of the blade  140  would be extended about 40 mm which would be sufficient to implement the proposed solution. In  FIG. 1  a corresponding rotor blade under pressure showing the aforementioned deflection is exemplary visualized by a rotor blade  140   a  wherein the pressure side of the blade  140   a  is indicated by an arrow  150  in  FIG. 1  and wherein a suction side of the blade  140   a  is indicated by an arrow  151 . Further, an extension of the pressure side  150  due to, e.g. deflection of the rotor blade is exemplarily illustrated by an arrow  160  wherein a corresponding contraction of the suction side of the rotor blade is indicated by an arrow  161 . 
         [0028]    Based on the exemplary scenario as shown in  FIG. 1 , one possible dimension or degree of the extension  160  of an electrical conductor loop (not shown in  FIG. 1 ) being assigned or fixed, e.g., to the inner surface of the pressure side  150  of the blade  140  may be about 40 mm which is a sufficient scale for applying the proposed solution. 
         [0029]    Electrical conductor loop my be any electrical conductible element which can be attached, mounted or fixed via several fixing points along a longitudinal section of a surface of a rotor blade. 
         [0030]    Fixing point may be any element or arrangement allowing an attachment, fastening, fixing or anchorage of at least a part of the electrical conductor loop to the surface of the rotor blade. As an example, a part of the electrical conductor loop may be glued to the surface of the rotor blade providing a proper fixation 
         [0031]    Advantageously the fixing points will follow a dynamic definition of the surface of the rotor blade and will also follow dynamic changes of the shape or form of the surface of the rotor blade. 
         [0032]    Surface may be the inner of outer surface of a suction side or pressure side of the rotor blade. 
         [0033]    Extension of the conductor loop means extracting, pulling apart or extending the conductor loop along its longitudinal direction, e.g. along the longitudinal direction of the rotor blade forced by the dynamic deflection of the rotor blade. 
         [0034]    A closed electrical conductor loop means a conductor loop maintaining electrical conductivity without electrical disconnection. 
         [0035]    An open electrical conductor loop means a conductor loop without or substantial minor electrical conductivity due to, e.g., an electrical disconnection located between the at least two fixing points. 
         [0036]    Several electrical conductor loops may be fixed to the inner and/or outer surface of the suction and/or pressure side of the rotor blade. Thereby the conductor loops may have different, i.e. individual dimensions. Such dimensions like, e.g., different lengths of the conductor loops may be aligned to different kinds of possible deflections of a rotor blade (also referred to as “different levels or degrees of deflection”). As an example, a root part of the rotor blade may comprise a minor degree of deflection than a tip part of the rotor blade showing a major degree of deflection. Using more than one conductor loop according to the proposed solution allows a more accurate determination of any kind of deflection of the rotor blade. 
         [0037]    A defined threshold may represent a certain degree of deflection like, e.g. an extreme deflection or bending condition/situation of a rotor blade during operation of the wind turbine. As an example, during extreme deflection of a rotor blade, a tip section of the rotor blade may hit the tower during operation of the wind turbine. 
         [0038]    Determining deflections of the rotor blade according to the proposed solution allows identification and prevention of extreme rotor blade deflections. In particular, by using several conductor loops which open or close at different levels, a more accurate estimation of the current blade deflection is possible. 
         [0039]    As an advantage, the proposed solution uses the deflection of the rotor blade as a triggering momentum in order to activate a mechanism to detect extreme deflections of a rotor blade. 
         [0040]    Methods for preventing extreme rotor blade deflections are an enabler for upscaling and upgrading rotors of existing wind turbines as well as designing larger future rotors, consequently allowing a significant reduction in cost of energy. 
         [0041]    In an embodiment, the method comprises
       monitoring at least one electrical characteristic of the at least one electrical conductor loop,   determining the deflection of the rotor blade based on the at least one monitored electrical characteristic.       
 
         [0044]    In another embodiment, the monitoring of the at least one electrical characteristic comprises a measurement of at least one out of the following electrical parameters concerning the conductor loop:
       an electrical resistance,   an electrical current through the conductor loop,   a voltage.       
 
         [0048]    In a further embodiment, an extreme deflection of the rotor blade is determined
       if the measured electrical resistance of the at least one electrical conductor loop exceeds a predetermined threshold, or   if the electrical current through the at least one electrical conductor loop drops below a predetermined threshold.       
 
         [0051]    In a next embodiment, the at least one electrical conductor loop is fixed at an inner surface of a pressure side of the rotor blade. 
         [0052]    It is also an embodiment that a control signal is provided for controlling the operation of the wind turbine based on the determined deflection of the at least one rotor blade. 
         [0053]    The problem stated above is also solved by an arrangement for determining a deflection of a rotor blade of a wind turbine comprising:
       at least one electrical conductor loop being fixed via fixing points to at least one surface of the rotor blade, wherein the at least one electrical conductor loop is arranged such that
           due to the deflection of the rotor blade an extension of the at least one conductor loop is forced between at least two of the fixing points,   the extended electrical conductor loop keeps closed if the deflection of the rotor blade is below a defined threshold,   the extended conductor loop opens if the deflection of the rotor blade is beyond the defined threshold.   
               
 
         [0058]    Pursuant to another embodiment, a processing unit is arranged for
       monitoring at least one electrical characteristic of the at least one electrical conductor loop, and   determining the deflection of the rotor blade based on the at least one monitored electrical characteristic       
 
         [0061]    According to an embodiment, the processing unit is arranged for providing a control signal for controlling the operation of the wind turbine based on the determined deflection of the at least one rotor blade. 
         [0062]    According to another embodiment, the at least one electrical conductor loop comprises or is assigned to at least one sliding contact capable of being switched in an open loop circuit or in a closed loop circuit dependent on the deflection of the rotor blade. As an example, a first part of the electrical conductor loop may represent a first sliding element of the sliding contact and a second part of the electrical conductor loop may represent a second sliding element of the sliding contact. Due to forces acting on both parts of the electrical conductor loop via the fixing points both sliding elements may be pulled apart (“open sliding contact”) or keep in contact (“closed sliding contact”) dependent on the current deflection of the rotor blade. 
         [0063]    The problem stated above is also solved by an sensor suitable for determining a deflection of a rotor blade of a wind turbine according to the steps of the method described herein, comprising
       at least one electrical conductor loop suitable to be fixed via fixing points to at least one surface of the rotor blade, such that
           due to the deflection of the rotor blade an extension of the at least one conductor loop is forced between the at least two fixing points,   the extended electrical conductor loop keeps closed if the deflection of the rotor blade is below the defined threshold,   the extended conductor loop opens if the deflection of the rotor blade is beyond the defined threshold.   
               
 
         [0068]    In yet another embodiment, the sensor comprises a processing unit that is arranged for
       monitoring at least one electrical characteristic of the at least one electrical conductor loop, and   determining the deflection of the at least one rotor blade based on the at least one monitored electrical characteristic.       
 
         [0071]    The solution provided herein further comprises a rotor blade for a wind turbine comprising at least one sensor as described herein. 
         [0072]    The problem stated above is also solved by a wind turbine comprising an arrangement as described herein. 
       BRIEF DESCRIPTION 
       [0073]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         [0074]      FIG. 1  exemplarily illustrates an extension of the pressure side of a wind turbine blade upon deflection of the tip; 
         [0075]      FIG. 2  shows a schematic view of an exemplary embodiment of a rotor blade using an electrical conductor loop; 
         [0076]      FIG. 3  shows in a schematic view an exemplary embodiment a sliding contact; and 
         [0077]      FIG. 4  shows in a schematic view an alternative exemplary embodiment of a sliding contact. 
     
    
     DETAILED DESCRIPTION 
       [0078]      FIG. 2  shows a schematic view of a rotor blade  200  with an assigned electrical conductor loop  210  according to the proposed solution. Thereby, the electrical conductor loop  210  is assigned to an inner surface (indicated by an arrow  225 ) of a pressure side (indicated by an arrow  220 ) of the rotor blade  200 . According to the exemplary embodiment as shown in  FIG. 2 , the electrical conductor loop  210  is fixed via two fixing points  230 ,  231  to the inner surface  225 . According to an alternative embodiment, the electrical conductor loop may be fixed to the inner surface  225  via more than two fixing points (not shown in  FIG. 2 ). 
         [0079]    The electrical conductor loop  210  comprises two parts  232 ,  233  wherein a first part  232  is fixed via the fixing point  230  to the inner surface of a root end section of the rotor blade and the second part  233  is fixed via the fixing point  231  to the inner surface of a tip end section of the rotor blade. Both parts  232 ,  233  of the conductor loop  210  are assigned to a sliding contact  240  providing an electrical connection or disconnection of both parts  232 ,  233  of the electrical conductor loop  210  dependent on the current deformation of the rotor blade, i.e. dependent on the actual contraction or extension of the surface of the blade  200  causing contraction or extraction forces between the fixing points  230 ,  231  und thus between both parts  232 ,  233  of the conductor loop. 
         [0080]    It should be noted that according to a further embodiment of the proposed solution several electrical conductor loops may be fixed to the rotor blade opening or closing at different levels or degrees of deflection or deformation of the rotor blade. 
         [0081]    The both parts  232 ,  233  of the electrical conductor loop  210  may be connected with an electrical source  205  providing electrical voltage and/or current to the conductor loop  210 . The electrical source  205  may be located in a root section of the rotor blade  200  or in the rotor hub. The electrical conductor loop  210  may be further connected with a monitoring device  206  monitoring electrical characteristics of the conductor loop  210 . 
         [0082]    In  FIG. 2  a dotted line  260  shows a more detailed view of the sliding contact  240  dependent on various deformation scenarios of the rotor blade  200 . Thereby, a one end of the first part  232  of the electrical conductor loop  210  represents a first sliding element  234  of the sliding contact  240  and one end of the second part  233  of the electrical conductor loop  210  represents a second sliding element  235  of the sliding contact  240 . 
         [0083]    According to a first deformation scenario (indicated by an arrow  241 ), showing the sliding contact  240  during normal (i.e. minor) blade deflections, the sliding contact  240  is closed (“closed loop circuit”) providing an electrical connection (indicated by a darkened field  244  in the first deformation scenario  241 ) between both sliding elements  234 ,  235 , i.e. between both parts  232 ,  233  of the electrical conductor loop  210  with the consequence that electrical current may flow through the conductor loop  210 . That flowing of electrical current thought the loop  210  may be recognized by the monitoring device  206  thereby identifying a closed sliding contact  240 . Thus, a closed sliding contact  241  is indicating a minor deflection of the rotor blade  200 . 
         [0084]    According to a second scenario (indicated by an arrow  242 ), showing the sliding contact  240  during extreme blade deflections, the sliding contact  240  is in extension (“open loop circuit”) due to extracting forces (indicated by an arrow  243 ) acting on both parts  232 ,  233  of the electrical conductor loop  210  thereby disconnecting both parts  232 ,  233  of the electrical conductor loop  210 , i.e. both sliding elements  234 ,  235  und thus preventing flow of electrical current through the conductor loop  210 . That “non-flow” of electrical current thought the loop  210  is recognized by the monitoring device  206  thereby identifying an open sliding contact  240 . Thus, an open sliding contact  241  is indicating a major or extreme deflection of the rotor blade  200 . 
         [0085]    The detection of an open or closed sliding contact or conductor loop may be based on monitoring at least one electrical characteristic of the electrical conduction loop  210  like, e.g., monitoring an electrical resistance. As an example, an increase of the resistance of the electrical conduction loop  210  by several orders of magnitude within a short time period indicates on open loop circuit and consequently indicates an exceed of a blade deflection beyond an allowable, i.e. defined threshold. 
         [0086]    Alternatively or in addition to monitoring the electrical resistance other electrical characteristics of the electrical conductor loop  21  may be monitored by the monitoring device  206  allowing an identification of an open or closed sliding contact  240 , i.e. determining a connected or disconnected conductor loop  210 . 
         [0087]    Alternative methods for monitoring an electrical characteristic of the conductor loop may be, e.g., a measurement of the electrical current through the electrical conductor loop or a measurement of the actual voltage being effective on the electrical conductor loop. 
         [0088]    Based on the outcome or result of the measurement, i.e. detection of an open or closed sliding contact  240  and thus detection of an extreme deflection of the rotor blade, operation of the wind turbine will be controlled accordingly thereby avoiding high deflections of the rotor blade. 
         [0089]    As an example, in case of a determination of an extreme deflection of the rotor blade, in particular in case of a determination of a extreme deflection of a tip part of the rotor blade several exemplary control options for a proper operation of the wind turbine might be possible:
       switching to a less aggressive mode of operating the wind turbine;   increasing the blade pitch angle (“pitching out”) of at least one of the rotor blades in order to reduce the loading on the blades. This can be, e.g., during gusts with corresponding high blade loading;   triggering an emergency shutdown;   a filtered measurement of tip deflection can be used in order to apply a more aggressive pitch control in case, e.g., the deflection levels of the rotor blades are lower than expected for specific ranges of wind speed.       
 
         [0094]      FIG. 3  shows in a schematic view an exemplary possible embodiment of a sliding contact  300  which might be used according to the proposed solution. Thereby, a strip element  340  which may be embedded inside the blade comprises a cavity  330  which may be, e.g., filled with a dielectric fluid. A first part of the strip element  340  (representing a first part of the sliding contact  300 ) comprises an end  311  of a first part  310  of an electrical conductor loop (as exemplarily shown in  FIG. 2 ). A second part of the strip element  340  (representing a second part of the sliding contact  300 ) comprises an end  321  of a second part  320  of the electrical conductor loop. Both ends  311 ,  321  are correlating to each other thereby providing an “open” or “closed” condition between both parts  310 ,  320  of the electrical conductor loop. 
         [0095]    One or several strip elements  340  may be assigned, fixed or attached to the surface of the rotor blade which may be, e.g., the inner surface of the pressure side of the rotor blade. According to the description above, in case of an extreme deflection of the rotor blade, the surface of the pressure side of the rotor blade will be extended or extracted and thus the fixing points attached to the inner surface will be pulled apart in longitudinal direction. As a consequence, both parts  310 ,  320  of the conductor loop located in the cavity  330  of the strip element  300  will be pulled apart, switching the sliding contact  300  in “open circuit”, thereby providing an electrical disconnection in the electrical conductor loop. That disconnection is indicating an extreme deflection of the rotor blade. 
         [0096]      FIG. 4  shows in a schematic view an alternative exemplary embodiment of a sliding contact  400  which might be used according to the proposed solution. Thereby, a first part  410  of the sliding contact  400  is representing or is part of a first part of an electrical conductor loop (as exemplarily shown in  FIG. 2 ). That part  410  comprises a first section  411  comprising electric conductive material. A second section  412  of the first part  410  consist of dielectric material. Correspondingly, a second part  420  of the sliding contact  400  is representing or is part of a second part of the electrical conductor loop. That part  420  has the shape of a “Y” comprising two ends  420   a,    420   b,  both of them being of electric conductive material. The form or shape of the second part  420  is corresponding to the form or shape of the first part  410  of the sliding contact  400 . 
         [0097]    In a “closed circuit” condition as exemplarily shown in a scenario  401  in  FIG. 4 , both ends  420   a,    420   b  of the second part  420  of the sliding contact are compassing the second section  412  of the first part  410  of the sliding contact  400  thereby providing an electrical connection between the first section  411  and both ends  420   a,    420   b  of the second part  420  of the sliding contact  400 . Thus, in the closed circuit scenario  401  of  FIG. 4  an electrical resistance of the sliding contact  400  is almost zero indicating a minor deflection of the rotor blade. 
         [0098]    In an “open circuit” condition as exemplarily shown in a scenario  402  in  FIG. 4 , both parts  410 ,  420  of the sliding contact  400  are extended or extracted (indicated by an arrow  403 ) forced by the elastic deformation of the surface of the blade wherein both ends  420   a,    420   b  of the second part  420  of the sliding contact  400  are electrically separated or isolated by the second section  412  of the first part  410  consisting of dielectric material. By disconnecting the electrical conductor loop, the electrical resistance of the sliding contact  400 , e.g., has a substantial high value or is almost endless which is indicating an extreme deflection of the rotor blade. 
         [0099]    As an advantage, the proposed solution reacts passively to the deflection of the rotor blade. Being activated by bending, the proposed solution works at high bending levels, thus affecting only extreme load situations of the respective rotor blade while not affection energy production during normal wind turbine operation. As a further advantage, Annual Energy Production (AEP) can be increased during normal turbine operation in combination with different setting of the turbine controller. 
         [0100]    The proposed solution can be implemented with minor effort, in particular by using cheap material. 
         [0101]    Applying the proposed solution, deflection of the rotor blade, in particular deflection of the tip section of the rotor blade can be reduced. This beneficially allows
       a more aggressive operation of the rotor blades,   an up scaling or upgrading of rotor blades,   an minimization or removal of limiting structural constraints.       
 
         [0105]    Although the present invention has been described in detail with reference to the preferred embodiment, it is to be understood that the present invention is not limited by the disclosed examples, and that numerous additional modifications and variations could be made thereto by a person skilled in the art without departing from the scope of the invention. 
         [0106]    It should be noted that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. 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 construed as limiting the scope of the claims.