Patent Publication Number: US-2023147218-A1

Title: Methods and systems for determining roughness of wind turbine blades and wind turbine control

Description:
The present disclosure relates to methods for controlling and operating wind turbines. More particularly, the present disclosure relates to methods for determining or detecting wind turbine blade roughness and methods of operating a wind turbine, as well as to wind turbine controllers and wind turbines. 
     BACKGROUND 
     Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly (“directly driven” or “gearless”) or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid. 
     The wind turbine hub may be rotatably coupled to a front of the nacelle. The wind turbine hub may be connected to a rotor shaft, and the rotor shaft may then be rotatably mounted in the nacelle using one or more rotor shaft bearings arranged in a frame inside the nacelle. The nacelle is a housing arranged on top of a wind turbine tower that may contain and protect the gearbox (if present) and the generator (if not placed outside the nacelle) and, depending on the wind turbine, further components such as a power converter, and auxiliary systems. 
     During operation of a wind turbine, the outer surface of the wind turbine blades, and in particular the leading edges and adjacent surface areas, may get dirty. For example dust, pollen, insects, salt or ice may accumulate on an outer surface of a wind turbine blade. Wind turbine blades may also erode due to impacts received on the blades, e.g. by rain, hail and particles in the wind. Dirty and/or eroded blades have a more irregular surface than clean blades which can have a significant impact on the air flow around the blades. Roughened blades generally produce less lift and more drag for a given wind flow, which reduces the power produced by the wind turbine. For example, roughened blades may cause a reduction of annual energy production (AEP) between 2% and 5%, which is a non-negligible loss. 
     Blade roughness may be monitored to check whether it may be necessary to trigger some corrective action, e.g. to clean or to repair the blades, or whether the wind turbine may be instructed to increase its power production, e.g. after a rainfall which may have removed some dirt from the blades. Blade inspection may be time consuming and expensive to perform, and it generally requires the presence of one or more operators. In addition, access to possibly affected blade regions may be difficult, and the monitoring equipment may be sensible to external conditions. In some examples, drones may be used. In these or other examples, pictures may be taken, e.g. with an infra-red camera, but a lot of post-processing of the images may be required. 
     The present disclosure aims to provide an improved determination or detection of blade roughness. 
     SUMMARY 
     In an aspect of the present disclosure, a method for determining a surface condition of one or more wind turbine blades of a wind turbine comprising rotor including a first wind turbine blade and one or more additional wind turbine blades is provided. The method comprises rotating the wind turbine rotor under the influence of a wind in predetermined rotation conditions, wherein the predetermined rotation conditions include at least a predetermined pitch angle of the additional wind turbine blades. The method further comprises determining a current value of one or more parameters of the wind turbine when rotating in the predetermined rotation conditions. The method further comprises comparing the current value of the one or more parameters of the wind turbine with one or more reference values to determine the surface condition of one or more of the wind turbine blades. 
     According to this aspect, one or more values of one or more parameters may be determined while the wind turbine rotor is rotating in predetermined rotation conditions and then compared to corresponding reference values. Reference values of parameters are known, e.g. they may have been determined at a previous time at a specific wind turbine rotor configuration. 
     Comparing current values of one or more wind turbine parameters determined in predetermined rotation conditions, wherein the predetermined rotation conditions include at least a predetermined pitch angle of the wind turbine blades other than the first wind turbine blade, to reference values may help to detect a condition of the blade surface, and particularly an indication of roughness without the use of blade sensors or drones. The presence of operators may also be avoided. As taking images may be omitted, a time-consuming post processing may also be dispensed with. A faster and more autonomous detection of blade roughness may therefore be provided. 
     Throughout this disclosure, a pitch angle of a wind turbine blade may be understood as an angle that may be measured, in cross-section, between a reference line and a chord of the blade. The reference line may be substantially parallel, e.g. included, in a wind turbine rotor plane in some examples. 
     Throughout this disclosure, blade roughness may refer to how irregular an outer surface of a blade, or a specific region of the outer surface of the blade, may be. Herein, a rough blade may refer to a blade whose surface differs from the surface of the blade when it was clean or cleaner and wherein the effect of the roughness is noticeable on the wind turbine performance. I.e. the air flow around the blade is affected to such an extent that lift and/or drag for a blade differs at a given angle of attack and given wind speed. For example, a rough blade may include irregularities that the blade did not include when a reference of cleanliness was obtained, e.g. protrusions and/or recesses, and these irregularities may affect the power produced by the wind turbine, generally in a negative way. For example, irregularities may e.g. create a more turbulent flow, a bigger wake, or a different point of separation of the air flow from the blade. 
     Throughout this disclosure, a clean blade may be understood as a smooth blade, i.e. a blade whose outer surface has not yet been affected, or at least not significantly affected, by matter accumulation and/or erosion. Matter accumulation may for example include dirt and ice. A surface of a clean blade may be a surface as designed and manufactured. A clean blade may be mounted atop of a wind turbine tower, e.g. during installation of a wind turbine. As the clean blade is at the beginning of its service life, it may not yet have accumulated dirt, ice or other and/or been eroded, at least in a significant manner. It may be understood that a clean blade is able to provide a maximum power output according to design specifications under optimum conditions. During blade installation, it might accumulate some dirt and/or it might be slightly eroded, but this will generally not have a significant effect and therefore the blade in the context of the present disclosure would be considered to be a clean blade. A clean blade may be used for obtaining reference values to which to compare values determined a certain period of time after the wind turbine has started to operate, e.g. days, weeks or months subsequent to the start of operation of the wind turbine. 
     Throughout this disclosure, a semi-clean blade may refer to a blade which has an outer surface differing from a blade surface when the blade was new, i.e. “clean”, for example due to erosion or the fact that the blade might have been repaired blade and its surface may be slightly different from when it was new. A semi-clean blade may for instance be a cleaned blade or a repaired blade. Semi-clean blades may also serve for determining reference values of one or more parameters. 
     Throughout this disclosure, it may be understood that a wind turbine is in operation when its rotor is rotating at a speed high enough to produce energy and the generator of the wind turbine is producing electrical power. 
     In a further aspect of the disclosure, a wind turbine controller is provided. The controller comprises a communications module, a processor, and a memory. The memory comprises instructions that, when executed by the processor, cause the processor to execute one or more of the method steps disclosed herein. 
     Still in a further aspect of the disclosure, a method for controlling a wind turbine is provided. The method comprises determining one or more reference values of one or more wind turbine parameters with a first blade positioned at a reference value of a first pitch angle and the remaining blades positioned at a reference value of a second pitch angle for an idling wind turbine rotor before the wind turbine starts to operate. The method further comprises starting wind turbine operation; and after a period of time, starting to idle the wind turbine rotor and positioning the first blade at the reference value of the first pitch angle and the remaining blades at the reference value of the second pitch angle. The method further comprises, determining one or more current values of one or more wind turbine parameters and compare them to the corresponding reference values. The method further comprises adapting the wind turbine operation based on the comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a perspective view of one example of a wind turbine; 
         FIG.  2    illustrates a simplified, internal view of one example of the nacelle of the wind turbine of the  FIG.  1   ; 
         FIG.  3    schematically illustrates an example of a controller for a wind turbine; 
         FIG.  4    shows a flow chart of an example of a method for operating a wind turbine for detecting blade roughness; 
         FIGS.  5 A,  5 B and  5 C  schematically illustrate different pitch angles of a wind turbine blade according to an example; 
         FIG.  6    schematically illustrates a frontal view of an example of a wind turbine with a first blade positioned at a first pitch angle and with a second blade and a third blade positioned at a second pitch angle, the value of the first pitch angle being higher than the value of the second pitch angle; 
         FIG.  7    schematically illustrates a set of reference values; 
         FIG.  8    schematically illustrates an example of an evolution of a reference wind turbine rotor speed as a function of wind speed as well as an example of a determined current value of rotor speed; and 
         FIG.  9    shows a flow chart of an example of a method for controlling a wind turbine. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLES 
     Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation only, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIG.  1    is a perspective view of an example of a wind turbine  10 . In the example, the wind turbine  10  is a horizontal-axis wind turbine. Alternatively, the wind turbine  10  may be a vertical-axis wind turbine. In the example, the wind turbine  10  includes a tower  15  that extends from a support system  14  on a ground  12 , a nacelle  16  mounted on tower  15 , and a rotor  18  that is coupled to nacelle  16 . The rotor  18  includes a rotatable hub  20  and at least one rotor blade  22  coupled to and extending outward from the hub  20 . In the example, the rotor  18  has three rotor blades  22 . In an alternative embodiment, the rotor  18  includes more or less than three rotor blades  22 . The tower  15  may be fabricated from tubular steel to define a cavity (not shown in  FIG.  1   ) between a support system  14  and the nacelle  16 . In an alternative embodiment, the tower  15  is any suitable type of a tower having any suitable height. According to an alternative, the tower can be a hybrid tower comprising a portion made of concrete and a tubular steel portion. Also, the tower can be a partial or full lattice tower. 
     The rotor blades  22  are spaced about the hub  20  to facilitate rotating the rotor  18  to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. The rotor blades  22  are mated to the hub  20  by coupling a blade root region  24  to the hub  20  at a plurality of load transfer regions  26 . The load transfer regions  26  may have a hub load transfer region and a blade load transfer region (both not shown in  FIG.  1   ). Loads induced to the rotor blades  22  are transferred to the hub  20  via the load transfer regions  26 . 
     In examples, the rotor blades  22  may have a length ranging from about 15 meters (m) to about 90 m or more. Rotor blades  22  may have any suitable length that enables the wind turbine  10  to function as described herein. For example, non-limiting examples of blade lengths include 20 m or less, 37 m, 48.7 m, 50.2 m, 52.2 m or a length that is greater than 91 m. As wind strikes the rotor blades  22  from a wind direction  28 , the rotor  18  is rotated about a rotor axis  30 . As the rotor blades  22  are rotated and subjected to centrifugal forces, the rotor blades  22  are also subjected to various forces and moments. As such, the rotor blades  22  may deflect and/or rotate from a neutral, or non-deflected, position to a deflected position. 
     Moreover, a pitch angle of the rotor blades  22 , e.g. an angle that determines an orientation of the rotor blades  22  with respect to the wind direction, may be changed by a pitch system  32  to control the load and power generated by the wind turbine  10  by adjusting an angular position of at least one rotor blade  22  relative to wind vectors. Pitch axes  34  of rotor blades  22  are shown. During operation of the wind turbine  10 , the pitch system  32  may particularly change a pitch angle of the rotor blades  22  such that the angle of attack of (portions of) the rotor blades are reduced, which facilitates reducing a rotational speed and/or facilitates a stall of the rotor  18 . 
     In the example, a blade pitch of each rotor blade  22  is controlled individually by a wind turbine controller  36  or by a pitch control system  80 . Alternatively, the blade pitch for all rotor blades  22  may be controlled simultaneously by said control systems. 
     Further, in the example, as the wind direction  28  changes, a yaw direction of the nacelle  16  may be rotated about a yaw axis  38  to position the rotor blades  22  with respect to wind direction  28 . 
     In the example, the wind turbine controller  36  is shown as being centralized within the nacelle  16 , however, the wind turbine controller  36  may be a distributed control system throughout the wind turbine  10 , on the support system  14 , within a wind farm, and/or at a remote-control center. The wind turbine controller  36  includes one or more processors  40  configured to perform the steps and/or methods described herein, see also  FIG.  3   . Further, many of the other components described herein include one or more processors. 
       FIG.  2    is an enlarged sectional view of a portion of the wind turbine  10 . In the example, the wind turbine  10  includes the nacelle  16  and the rotor  18  that is rotatably coupled to the nacelle  16 . More specifically, the hub  20  of the rotor  18  is rotatably coupled to an electric generator  42  positioned within the nacelle  16  by the main shaft  44 , a gearbox  46 , a high-speed shaft  48 , and a coupling  50 . In the example, the main shaft  44  is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle  16 . A rotation of the main shaft  44  drives the gearbox  46  that subsequently drives the high-speed shaft  48  by translating the relatively slow rotational movement of the rotor  18  and of the main shaft  44  into a relatively fast rotational movement of the high-speed shaft  48 . The latter is connected to the generator  42  for generating electrical energy with the help of a coupling  50 . Furthermore, a transformer  90  and/or suitable electronics, switches, and/or inverters may be arranged in the nacelle  16  in order to transform electrical energy generated by the generator  42  having a voltage between e.g. 400V to 1000 V into electrical energy having medium voltage (e.g. 10-35 KV). Offshore wind turbines may have for example generator voltages between 650 V and 3500 V, and transformer voltages may for instance be between 30 kV and 70 kV. Said electrical energy is conducted via power cables from the nacelle  16  into the tower  15 . 
     In some examples, the wind turbine  10  may include one or more shaft sensors  51 . The shaft sensors may be configured to monitor at least one of torque loads acting on the main shaft  44  and/or the high-speed shaft  48 , and a rotational speed of the shaft  44 ,  48 . In some examples, the wind turbine  10  may include one or more generator sensors  53 . The generator sensors may be configured to monitor at least one of a rotational speed of the generator  42  and a generator torque. Shaft sensors  51  and/or generator sensors  53  may include, for instance, one or more torque sensors (e.g., strain gauges or pressure sensors), optical sensors, accelerometers, magnetic sensors, speed sensors and Micro-Inertial Measurement Units (MIMUs). 
     The gearbox  46 , generator  42  and transformer  90  may be supported by a main support structure frame of the nacelle  16 , optionally embodied as a main frame  52 . The gearbox  46  may include a gearbox housing that is connected to the main frame  52  by one or more torque arms  103 . In the example, the nacelle  16  also includes a main forward support bearing  60  and a main aft support bearing  62 . Furthermore, the generator  42  can be mounted to the main frame  52  by decoupling support means  54 , in particular in order to prevent vibrations of the generator  42  to be introduced into the main frame  52  and thereby causing a noise emission source. 
     Optionally, the main frame  52  is configured to carry the entire load caused by the weight of the rotor  18  and components of the nacelle  16  and by the wind and rotational loads, and furthermore, to introduce these loads into the tower  15  of the wind turbine  10 . The rotor shaft  44 , generator  42 , gearbox  46 , high speed shaft  48 , coupling  50 , and any associated fastening, support, and/or securing device including, but not limited to, support  52 , and forward support bearing  60  and aft support bearing  62 , are sometimes referred to as a drive train  64 . 
     In some examples, the wind turbine may be a direct drive wind turbine without gearbox  46 . Generator  42  operates at the same rotational speed as the rotor  18  in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox  46  for providing a similar amount of power than a wind turbine with a gearbox. 
     The nacelle  16  may also include a yaw drive mechanism  56  that may be used to rotate the nacelle  16  and thereby also the rotor  18  about the yaw axis  38  to control the perspective of the rotor blades  22  with respect to the wind direction  28 . 
     For positioning the nacelle  16  appropriately with respect to the wind direction  28 , the nacelle  16  may also include at least one meteorological measurement system which may include a wind vane and an anemometer. The meteorological measurement system  58  can provide information to the wind turbine controller  36  that may include wind direction  28  and/or wind speed. 
     In the example, the pitch system  32  is at least partially arranged as a pitch assembly  66  in the hub  20 . The pitch assembly  66  includes one or more pitch drive systems  68  and at least one sensor  70 . Each pitch drive system  68  is coupled to a respective rotor blade  22  (shown in  FIG.  1   ) for modulating the pitch angle of a rotor blade  22  along the pitch axis  34 . Only one of three pitch drive systems  68  is shown in  FIG.  2   . 
     In the example, the pitch assembly  66  includes at least one pitch bearing  72  coupled to hub  20  and to a respective rotor blade  22  (shown in  FIG.  1   ) for rotating the respective rotor blade  22  about the pitch axis  34 . The pitch drive system  68  includes a pitch drive motor  74 , a pitch drive gearbox  76 , and a pitch drive pinion  78 . The pitch drive motor  74  is coupled to the pitch drive gearbox  76  such that the pitch drive motor  74  imparts mechanical force to the pitch drive gearbox  76 . The pitch drive gearbox  76  is coupled to the pitch drive pinion  78  such that the pitch drive pinion  78  is rotated by the pitch drive gearbox  76 . The pitch bearing  72  is coupled to pitch drive pinion  78  such that the rotation of the pitch drive pinion  78  causes a rotation of the pitch bearing  72 . 
     Pitch drive system  68  is coupled to the wind turbine controller  36  for adjusting the pitch angle of a rotor blade  22  upon receipt of one or more signals from the wind turbine controller  36 . In the example, the pitch drive motor  74  is any suitable motor driven by electrical power and/or a hydraulic system that enables pitch assembly  66  to function as described herein. Alternatively, the pitch assembly  66  may include any suitable structure, configuration, arrangement, and/or components such as, but not limited to, hydraulic cylinders, springs, and/or servomechanisms. In certain embodiments, the pitch drive motor  74  is driven by energy extracted from a rotational inertia of hub  20  and/or a stored energy source (not shown) that supplies energy to components of the wind turbine  10 . 
     The pitch assembly  66  may also include one or more pitch control systems  80  for controlling the pitch drive system  68  according to control signals from the wind turbine controller  36 , in case of specific prioritized situations and/or during rotor  18  overspeed. In the example, the pitch assembly  66  includes at least one pitch control system  80  communicatively coupled to a respective pitch drive system  68  for controlling pitch drive system  68  independently from the wind turbine controller  36 . In the example, the pitch control system  80  is coupled to the pitch drive system  68  and to a sensor  70 . During normal operation of the wind turbine  10 , the wind turbine controller  36  may control the pitch drive system  68  to adjust a pitch angle of rotor blades  22 . 
     According to an embodiment, a power generator  84 , for example comprising a battery and electric capacitors, is arranged at or within the hub  20  and is coupled to the sensor  70 , the pitch control system  80 , and to the pitch drive system  68  to provide a source of power to these components. In the example, the power generator  84  provides a continuing source of power to the pitch assembly  66  during operation of the wind turbine  10 . In an alternative embodiment, power generator  84  provides power to the pitch assembly  66  only during an electrical power loss event of the wind turbine  10 . The electrical power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine  10 , and/or failure of the wind turbine controller  36 . During the electrical power loss event, the power generator  84  operates to provide electrical power to the pitch assembly  66  such that pitch assembly  66  can operate during the electrical power loss event. 
     In the example, the pitch drive system  68 , the sensor  70 , the pitch control system  80 , cables, and the power generator  84  are each positioned in a cavity  86  defined by an inner surface  88  of hub  20 . In an alternative embodiment, said components are positioned with respect to an outer surface of hub  20  and may be coupled, directly or indirectly, to the outer surface. 
       FIG.  3    schematically illustrates an example of a wind turbine controller  36  or control system  36 . The controller  36  may be configured to perform one or more of the methods, steps, determinations and the like disclosed herein. As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. 
     A control system  36  may also include a memory  41 , e.g. one or more memory devices. A memory  41  may comprise memory element(s) including, but not limited to, a computer readable medium (e.g., random access memory (RAM)), a computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s)  41  may generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s)  40 , configure the controller  36  to perform, or trigger the performance of, various steps disclosed herein. A memory  41  may also be configured to store data, e.g. from measurements and/or calculations. 
     Additionally, the control system  36  may also include a communications module  43  to facilitate communications between the controller  36  and the various components of the wind turbine  10 . For instance, the communications module  43  may serve as an interface to permit the turbine controller  36  to transmit control signals to a pitch drive system  66  for controlling the pitch angle of the rotor blades  22 . The communications module  43  may be configured to communicatively connect the control system  36  with other elements of the wind turbine  10 . Connecting may be carried out via a wired connection and/or via a wireless connection, e.g. by using any suitable wireless communications protocol known in the art. Moreover, the communications module  43  may include a sensor interface  49 , e.g. one or more analog-to-digital converters, to permit signals transmitted from one or more sensors  51 ,  53 ,  58  to be converted into signals that can be understood and processed by the processors  40 . 
     A method  100  for determining a surface condition of one or more wind turbine blades  22  of a wind turbine  10  comprising a rotor  18  including a first wind turbine blade  221  and one or more additional wind turbine blades  222 ,  223  (with reference to the numbering used in  FIG.  6   ) is provided. The method is schematically shown in the flow chart of  FIG.  4   . The method comprises, at block  110 , rotating the wind turbine blade rotor  18  under the influence of a wind in predetermined rotation conditions. The predetermined rotation conditions include at least a predetermined pitch angle  252  of the one or more additional wind turbine blades  222 ,  223 . The method further comprises, at block  120 , determining a current value of one or more parameters of the wind turbine  10  when rotating in the predetermined rotation conditions. The method further comprises, at block  130 , comparing the current value of one or more parameters of the wind turbine  10  with one or more reference values  37  to determine the surface condition (and particularly an indication or state of roughness) of one or more of the wind turbine blades  221 ,  222 ,  223 . 
     Parameters may herein be understood to be characteristics that help define or describe the operation of the wind turbine in the predetermined rotation conditions. Parameters may herein include control settings (i.e. setpoints for actuators or elements such as a pitch angle, generator torque or other) and/or measured variables (e.g. rotor speed). It may be understood that “predetermined” refers to the fact that the rotation conditions are determined in advance, i.e. before rotation of the rotor  18  in certain conditions is started. “Predetermined” should not necessarily be interpreted as certain condition(s), e.g. certain parameter value(s), being kept constant during an entire period of time in which rotation of the rotor occurs. In some examples this could be the case (for example, one or more pitch angles  25  may be kept substantially constant while performing method  100 ), but in other examples this may not be the case (for example, a particular rotor speed may be set for rotation, but this speed may vary over time due to blade roughness). 
     Throughout this disclosure, a pitch angle of a wind turbine blade  22  may be understood as an angle  25  that may be measured, in cross-section, between a reference line  26  and a chord of the blade  27 , see  FIGS.  5 A,  5 B and  5 C . The reference line  26 , indicated as a dotted line in  FIGS.  5 A,  5 B and  5 C , may be substantially parallel to a rotor plane of a wind turbine  10 .  FIGS.  5 A,  5 B and  5 C  schematically illustrate a blade  22  in cross-section. The wind, see arrow “TW”, may blow from left to right in these figures. The wind turbine blade  22  rotates in the rotor  18  plane and moves, in this figure, in a downwards direction, resulting in an apparent wind flow, see arrow “AW” upwards. The apparent wind AW is composed of the wind caused by the rotation of the blade and the wind blowing against the blades  22  in an axial direction TW. 
     The right hand side of the profiles shown in  FIG.  5    may be understood to be the suction side of the blade, whereas the left hand side may be understood to be the pressure side of the blade. 
     In  FIG.  5 A , the blade is in a reference position for the pitch angle  25 . In the reference position, a chord  27  of the blade is substantially parallel to the reference line  26 . In  FIG.  5 A , a chord  27  of the blade and the reference line  26  overlap. The pitch angle  25  may therefore be 0° or a “default pitch angle”. The default pitch angle, or “reference position” may be a position that the wind turbine blade  22  will maintain over a range of low wind speeds, e.g. of sub-nominal wind speeds. 
     In  FIG.  5 B , the blade has been pitched away from the reference position. A pitch angle  25  in  FIG.  5 B  is therefore higher in  FIG.  5 B  than in  FIG.  5 A . In  FIG.  5 C , the pitch angle  25  has been further increased with respect to the reference position. Increasing the pitch angle  25  may generally slow down the wind turbine rotor i.e. the wind turbine blade is set in a position in which it is configured to generate less lift and more drag to reduce the aerodynamic torque of the wind turbine rotor. Pitching the blades  22  about 90° from the reference position may put the wind turbine in a feathered position and possibly stop it or at least greatly reduce its rotational speed. The feathered position of the blades is the position in which the blades may be placed when the wind turbine is parked. Similarly, reducing the pitch angle  25 , e.g. from a feathered position, may increase the rotational speed of the wind turbine rotor  18 . 
     Accordingly, pitching the blades  22  may be used for accelerating and slowing the rotation of the rotor  18 . Rotating the wind turbine rotor  18  under the influence of the wind in predetermined rotation conditions, e.g. at least with all the blades except one at a known pitch angle  25 , may help to determine whether one or more blades  22  have a roughened surface or not. For example, a first blade  221  may be positioned at a first pitch angle  251  higher than a second pitch angle  252  of the other blades  222 ,  223 , e.g. the other two blades. In this example, the predetermined rotation conditions would therefore include the predetermined first pitch angle  251  and the predetermined second angle  252 . When the rotor is rotated, the first blade  221  may tend to slow down the rotor  18  of the wind turbine  10  whereas the other blades, e.g. the second  222  and third  223  blades, may tend to accelerate the rotor  18 . A schematic illustration of such an example is depicted in  FIG.  6   . In the example of  FIG.  6   , the first wind turbine blade  221  has a higher pitch angle  251  than a pitch angle  252  of the second blade  222  and the third blade  223 . The second  222  and third  223  blades therefore tend to accelerate the rotation of the rotor  118 , and the first blade  221  tends to break this rotation, i.e. to slow down the rotation of the rotor  118 . 
     Determining one or more current values of one or more parameters and comparing one or more of the determined values to corresponding known reference values, e.g. with a first blade positioned at a predetermined first pitch angle  251  higher than a predetermined second pitch angle  252  at which the remaining blades are positioned, may allow checking whether the roughness of the blades  22  has increased after some time of operation of the wind turbine  10 . Determining a current value may in general include both direct measurements and indirect measurements. Instead of attaching sensors to the blades  22 , using drones, cameras or other ways which require additional equipment and/or operators for checking whether a blade has roughened during operation, method  100  may be used. This method may provide a faster, more convenient and less expensive check on blade roughness. 
     A parameter may have one or more values. If the values of the parameters have been determined in certain known reference conditions, e.g. for clean blades, and e.g. blade being positioned at a first pitch angle higher than a second pitch angle of the remaining blades, these values may be referred to as reference values. In some examples the reference values therefore correspond to a clean blade. A set of reference values  37  is schematically illustrated in  FIG.  7   . In the example of  FIG.  7   , a first pitch angle  251  has a reference value of about 70° and a second pitch angle  252  has a reference value of about 10°. The set of reference values  37  further comprises a plurality of values of a speed of rotation of the rotor  18  at a plurality of values of wind speed. Such a curve of data has been represented as rs=f(ws) in  FIG.  7 B . 
     In some examples, the predetermined rotation conditions may comprise a predetermined pitch angle  251  of the first blade  221 , and the parameters of the wind turbine may include a speed of rotation. I.e., a current value of a speed of a wind turbine rotor  18  may be determined and then compared to a reference value of wind turbine rotor speed. The determination may be performed at a certain wind speed and the comparison may accordingly be to a reference value of rotor speed at substantially the same wind speed. For instance, the first blade  221  may be positioned at the reference value of the first pitch angle  251  and the remaining additional blades  222 ,  223  may be positioned at the reference value of the second pitch angle  252 . The current wind speed may for example be determined by a meteorological measurement system  58 , e.g. a wind anemometer. The current value of rotor speed may for instance be determined by a shaft sensor  51  or a generator sensor  51 . The current value of the speed of the wind turbine rotor at a current wind speed may be compared to a reference value of the speed of the wind turbine rotor at a corresponding wind speed. 
       FIG.  8    schematically illustrates an example of a reference curve (dashed line  45 ) of speed of a wind turbine rotor  18 , measured e.g. in revolutions per minute (rpm), as a function of wind speed, measured e.g. in meters per second (m/s). The rotor speed data (circles) may be fitted according to a suitable equation, e.g. a linear equation, to obtained intermediate values of data. The dashed line  45  represents a fit for the rotor speed data.  FIG.  8    also illustrates an example of a value of a current rotor speed at a current wind speed, which has been labeled as  47 . As the current value of rotor speed  47  is in this example lower than a corresponding reference rotor speed  55 , it may be concluded that the blades are rougher than before. 
     By comparing the values of rotor speed it may be seen whether the current value is substantially the same as the reference one, which may be indicative of absence of irregularities on the blades  22 , or whether it is lower than the reference speed, which may indicate that the blades  22  have roughened. The operation of the wind turbine may be adjusted depending on the outcome of the comparison. 
     In some examples, the predetermined rotation conditions may comprise a predetermined pitch angle  251  of the first blade  22 , and the parameters of the wind turbine  10  that are measured or otherwise determined may include a tip speed ratio (TSR). I.e., a current value of a tip speed ratio may be determined and compared to a reference value of a TSR. The first blade  221  may be positioned at the reference value of the first pitch angle  251  and the remaining blades  222 ,  223  may be positioned at the reference value of the second pitch angle  252  before the determination is made. In some of these examples, a TSR may be determined by first measuring a current (e.g. angular) rotor speed (rs) and a current wind speed (ws) measured e.g. by a nacelle anemometer, and then calculating a current tip speed by taking into account a length of the blade  22  (L). This value may then be divided by the measured current wind speed (ws) for obtaining the TSR. The current speed of the wind turbine rotor  18  may e.g. be measured in revolutions per minute (rpm), in some examples. Other ways of determining a TSR may be possible. The determined current TSR may be compared to a reference TSR. 
     As a TSR is a ratio of speed of a blade tip to a wind speed. Determining a TSR and comparing it to a TSR reference value may be less limitative than determining a rotor speed and comparing it to a corresponding reference value. A slope of a rotor speed reference curve  45  may be related to the value of the reference TSR, in particular the TSR and the slope may be related by a constant of proportionality that includes the length of the blades.  FIG.  8    also illustrates that a slope of a possible current curve of rotor speed as a function of wind speed  57 , and therefore a current TSR value, may be less than a value of a slope of the reference curve  45 , and therefore of a reference TSR. 
     In some examples, the predetermined rotation conditions may comprise a predetermined rotor speed, and the parameters of the wind turbine may include a pitch angle  251  of the first blade  221  to maintain the predetermined rotor speed. In some of these examples, the remaining blades  222 ,  223  may be positioned at the reference value of the second pitch angle  252 , and optionally the first blade  221  may be positioned at the reference value of the first pitch angle  251 , before the determination is made. In other examples, such positioning may be omitted. The controller  36  may know that, with clean or semi-clean blades  22 , a certain speed of the rotor  18  may be attained with a selected configuration of pitch angles  251 ,  252 . During a blade roughness check, the first pitch angle  251  may be varied to reach and maintain a particular reference value of rotor speed. The pitch angle  251  needed to maintain a specific rotor speed may be indicative of whether the blades keep a regular surface or whether irregularities such as recesses and/or protrusions have appeared on the blade surface. 
     For example, if a required current value of the first pitch angle  251  is substantially the same as a corresponding reference value of the first pitch angle  251 , it may be concluded that the blades remain clean or semi-clean. However, if the value of the first pitch angle  251  necessary for keeping a substantially constant rotor speed is lower than a corresponding reference value, this may mean that the blade surface of one or more of the blades has been modified and negatively affects the wind turbine performance. According to the explanations given with respect to  FIG.  5 A- 6   , a current value of the first pitch angle  251  lower than a reference value of the first pitch angle may mean that, for reaching and maintaining certain rotor speed, rotation of the rotor  18  has to be favored. I.e., if a blade, in particular the additional blade  222  or  223 , is now rough, it may be necessary to decrease the first pitch angle  251  for increasing the speed of rotation and therefore be able to reach the reference rotational speed. 
     A blade  222 ,  223  which has a second pitch angle  252  as a reference pitch angle  25  may likewise be used for performing a roughness check. In some examples, a pitch angle  252 ,  253  of one of the remaining blades  222 ,  223  may be varied for keeping a substantially constant speed of rotation, whereas a first pitch angle  251  of the first blade  221  and the second pitch angle  252  of the other blades may be kept substantially constant. 
     In some examples, the predetermined rotation conditions may include a pitch angle  251  of the first blade  221  and a predetermined rotor speed, and the parameters of the wind turbine may include a generator torque to maintain the predetermined rotor speed. I.e., a value of a torque provided by a wind turbine generator  42  for keeping a known speed, e.g. a certain reference speed, of a wind turbine rotor  18  may be determined and then compared to a reference value of torque. The first blade  221  may be positioned at the reference value of the first pitch angle  251  and the remaining blades  222 ,  223  may be positioned at the reference value of the second pitch angle  252  before the determination is made. For example, it may be known that, with clean or semi-clean blades, a certain speed of the rotor  18  may be attained with a selected configuration of pitch angles  251 ,  252 . To maintain that rotor speed, a certain value of generator torque may be required. During the roughness check, torque may be varied to reach and maintain that predetermined rotor speed. If a required current value of torque is substantially the same as a corresponding reference torque value, it may be concluded that the blades remain clean or semi-clean. However, if the torque necessary for keeping a substantially constant rotor speed is lower than a reference value of torque, this may mean that one or more of the blades have a surface with increased roughness. 
     In some examples, the predetermined rotation conditions may include idling of the wind turbine  10 . I.e., in some examples, the method may further comprise starting to idle the wind turbine rotor  18  before determining a current value of one or more parameters. Herein, idle or idling may refer to the fact that the wind turbine blades  22  are (slowly) rotating but no energy is produced, namely because the generator  42  is not connected to the grid. An idling rotor  18  may facilitate observing variations in blade roughness and measuring relevant parameters. The rotor  18  may for instance be set to idle before determining a current value of a rotational rotor speed, a tip speed ratio, or one or more pitch angles  25 . A power converter and the grid may be used to vary generator torque in some examples, whereas other power sources may be used in other examples. For example, one or more auxiliary or additional power sources may be used for using the wind turbine generator  42  as a motor. If determination of current values is done during idling, the reference values may be determined during idling as well to make a direct comparison more meaningful. 
     Regarding the values of the second pitch angle  252 , and optionally also of the first pitch angle  251 , in predetermined rotation conditions, a pitch angle  251  of the first blade  221  may be higher than the pitch angle  252 ,  253  of the other blades  222 ,  223 . These predetermined values, as well as the reference values for the second pitch angle  252 , and optionally of the first pitch angle  251 , may be chosen such that the effect of roughness in one or more parameters to be determined may be maximized, or at least increased, with respect to other values of the pitch angle  25 . 
     In some examples, a predetermined and/or reference value of the second pitch angle  252  may be near a stall position. I.e., the second pitch angle  252  may be less than a pitch angle above which stall takes place, but close to this angle. The effects of a rough blade surface may be measured easier at such pitch angles. In some examples, a predetermined and/or reference value of the second pitch angle  252  may be between 0° and 30°, and more in particular between 5° and 15°. In some of these examples, a reference value of the second pitch angle  252  may be about 10°. 
     A predetermined and/or reference value of the first pitch angle  251  may be chosen to optimize the measurement conditions. For example, the first pitch angle may be selected to adapt the speed of rotation of the rotor  18 . If the rotor turns too fast, the wind turbine blade may be damaged due to the overspeed. The first pitch angle  251  may also be chosen such that the determined values may be distinguished from an accuracy of the measurement, e.g. such that a variation in the parameter of interest due to blade roughness is increased with respect to other possible values of the first pitch angle  251 . In some examples, a predetermined and/or reference value of the first pitch angle  251  may be between 45° and 90°, and more in particular between 60° and 80°. In some of these examples, a reference value of the first pitch angle  252  may be about 70°. 
     In some examples, the pitch angle  251  of the first blade  221  may be in a range of 45° to 90°, specifically in a range of 60° to 80°, and the pitch angle  252  of the other blades  222 ,  223  may be in a range of 0° to 30°, specifically in a range of 5° to 15°. 
     For most implementations, it may be sufficient to determine blade roughness in general i.e. without differentiating between the individual blades. In most occasions a build-up of matter or erosion may be assumed to occur at similar rates for all blades of a wind turbine rotor. 
     It may happen however that the different blades roughen at different rates. In view of this, in some examples, the method may further comprise repeating the steps of rotating, determining and comparing with one or more, e.g. all, of the additional blades  222 ,  223  acting as the first blade. Repeating the determination of current values of one or more parameters and comparing the determined current values to corresponding reference values for each rotor configuration, i.e. a configuration in which a particular blade  221  is positioned at a particular first pitch angle  251  and the remaining blades  222 ,  223  are positioned at a particular second pitch angle  252 , may help to differentiate which blade  22  is more irregular in case they have experienced a different evolution in roughness. 
     For the sake of illustration, let us assume that the first blade  221  is rougher than the remaining blades  222 ,  223 , and that a current value of the first pitch angle for keeping a speed of the rotor substantially constant is to be compared with a reference value of the first pitch angle. In such a situation, when a current value of the first pitch angle is determined for each rotor configuration, it may be seen that the current value in the rotor configuration in which the pitch angle of the first blade  221  is varied is different than a current value in the other rotor configurations in which a pitch angle of the remaining blades is varied. 
     It is noted that examples of the methods may be carried out regardless of the predetermined rotation conditions selected, at a wind speed prevailing at the moment. I.e. waiting for specific wind conditions to perform these methods may be avoided. 
     In some examples, any of the above methods may be performed as part of a method of operating a wind turbine  10 . Such a method of operating a wind turbine  10  may comprise operating the wind turbine with default control settings. The method may further comprise carrying out the above method  100 . The method may further comprise adjusting the operation of the wind turbine if the current values of the parameters differ from the reference value by, or in more than, a predetermined threshold. 
     In some examples, the wind turbine  10  may be operated with default control settings upon starting of operation after installation and commissioning. Suitable control settings may be based on prototype testing, simulation and other. Thee reference values to be used in the methods for determining roughness of the blades may have been determined during installation or commissioning of a wind turbine after all the blades have been installed. Additionally or alternatively, the reference values may have been determined after an occurrence of a certain event subsequent to the start of operation of the wind turbine in other examples, e.g. after the blades  22  have been cleaned, replaced or repaired. For example, if a wind turbine has already been operating for a while, its blades may be cleaned and then reference values may be determined for the cleaned blades. It may also be possible that one or more of the wind turbine blades may be replaced and then reference values may be determined for the new rotor  18 . Replaced blades may be considered clean blades. Cleaned blades may be considered semi-clean blades. 
     In some examples, the method of operating may further comprise determining a reference curve  45  of a speed of a wind turbine rotor as a function of wind speed, e.g. for clean or semi-clean blades, a first blade  221  positioned at the reference value of the first pitch angle  251  and the remaining blades  222 ,  223  positioned at the reference value of the second pitch angle  252 . For example, a curve  45  similar to the one schematically illustrated in  FIG.  7    may be obtained. 
     Values of a wind speed and of a speed of a rotor  18  may be determined during a certain period and then averaged. In some examples they may be measured during one, five, ten or more minutes. In some examples, they may be measured during a time period in which a wind speed is, or may be deemed to be, substantially constant. In some of these examples, a wind speed may be considered to be substantially constant during a time period if the values of wind speed stay within an interval defined by the average value of wind speed plus and minus 10% of the average value of wind speed, i.e. within an interval [0.9·average wind speed, 1.1·average wind speed]. This explanation may be applicable to any value which is directly measured, e.g. any suitable reference value as well as any suitable current value. For instance, a current value of a parameter of a set of parameters may be determined as an average value of a plurality of values measured during a period of time, e.g. during ten minutes. 
     When the blades  22  are clean, they should all perform equally well. Accordingly, having a set of reference values  37  for a single rotor configuration may be enough. Therefore, it may be indifferent which clean blade has the higher reference pitch angle  251  as the obtained reference values may be the same regardless of this. However, determining more than one set of reference values  37 , e.g. determining N sets of reference values, N being equal to the number of blades  22  of the wind turbine  10 , is not precluded and could be performed. This may be performed for instance if semi-clean blades are to be used for determining reference values. 
     In some examples, adjusting the operation of the wind turbine  10  may include one or more of changing control settings, outputting a status message and triggering a corrective action. In some examples, a message may indicate whether a particular blade, or all the blades in general, are as clean as before or are rougher. A level of roughness may be indicated. For instance, a variation of a current value of a parameter with respect to a corresponding reference value of the parameter may be linked to a certain degree of roughness. Different thresholds may be created, either with regard to absolute values (e.g. parameter X has reached value Y) or to relative values (e.g. parameter X has changed more than Y %). Depending on how much a current value differs from a reference value, different messages may be configured and/or different actions may be taken. A corrective action may be triggered if blade roughness, e.g. blades roughness above a predetermined threshold, is detected. A corrective action may aim at decreasing blade roughness and/or, as roughness may have decreased power output, to increase power production of the wind turbine. A corrective action may include one or more of repairing, replacing, cleaning, defrosting, pitching and varying a TSR of one or more blades. Other corrective actions may be possible. A status message may also recommend a particular corrective action. 
     In examples, control settings may be changed to continue operation of the wind turbine taking into account eh actual state of the wind turbine blades. E.g. PID settings, and/or generator torque control and/or aerodynamic actuator settings may be changed after the finding of a different surface roughness of the blades. 
     In some examples, the method may be performed at regular predetermined intervals, e.g. daily, once per week or once per month. In other examples, the method may be performed after a manual indication or request, e.g. by an operator. In some examples, the method may be triggered after the occurrence of certain environmental conditions, e.g. rain, a storm or snow. 
     According to a further aspect, a controller  36  for a wind turbine  10  is provided. As explained with respect to  FIG.  3   , the controller  36  comprises a communications module  43 , a processor  40  and a memory  41 . The memory  41  comprising instructions that, when executed by the processor  40 , cause the processor to the methods disclosed herein. A wind turbine  10  comprising such a controller  36  may also be provided. 
     In a further aspect of the disclosure, a method  200  for controlling a wind turbine  10  is provided. The method is shown in the flow chart of  FIG.  9   . Aspects and explanations with respect to method  100  may be combined and applied to method  200  and vice versa. 
     The method  200  comprises, at block  210 , determining one or more reference values of one or more wind turbine parameters  35  with a first blade  221  positioned at a reference value of a first pitch angle  251  and the remaining blades  222 ,  223  positioned at a reference value of a second pitch angle  252  for an idling wind turbine rotor  18  before the wind turbine starts  10  to operate. Determining one or more reference values may comprise in some examples measuring a reference curve of a speed of a wind turbine rotor as a function of wind speed. Determination of reference values may be performed after all the wind turbine blades  22  have been installed on the wind turbine  10  during wind turbine installation or commissioning. In some examples, the reference values of the first pitch angle  251  and the second pitch angles  252  may be between 60° and 80°, and between 5° and 20°, respectively. For example, a reference value may be set about 70° for first pitch angle  251  and a reference value may be set about 10° for the second pitch angle. 
     The method further comprises, at blocks  220  and  230 , starting wind turbine operation; and after a period of time, starting to idle the wind turbine rotor  18  and positioning the first blade  221  at the reference value of the first pitch angle  251  and the remaining blades  222 ,  223  at the reference value of the second pitch angle  252 . 
     The method further comprises, at block  240 , determining one or more current values of one or more wind turbine parameters and comparing them to the corresponding reference values. In some examples, one or more reference values and one or more current values are obtained at least for one of a rotor speed and a tip speed ratio. 
     The method further comprises, at block  250 , adapting the wind turbine operation based on the comparison. Adapting the operation of the wind turbine may comprise one or more of outputting a status message and, if blade roughness is detected, triggering a corrective action or an operative change as described before. A corrective action may include one or more of repairing, replacing, cleaning, defrosting, pitching and varying a tip speed ratio of one or more blades. 
     In some examples, after a corrective action is implemented, steps  230  and  240  may be performed again for checking whether the corrective action has been successful. For instance, if blade roughness due to the presence of ice on one or more blades  22  is detected, the wind turbine  10  may be stopped. The blades may be de-iced, and then the rotor  18  may be idled and the blades  22  pitched to suitable pitch angles (step  230 ). One or more current values of parameters may be determined and compared to corresponding reference values (step  240 ). This may allow to verify whether the de-icing has been successful or not. If successful, the wind turbine  10  may be re-started. 
     This written description uses examples to disclose the teaching, including the preferred embodiments, and also to enable any person skilled in the art to put the teaching into practice, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspects, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments and techniques in accordance with principles of this application. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.