Patent Description:
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 or by 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 contains and protects e.g. the gearbox (if present) and the generator and, depending on the wind turbine, further components such as a power converter, and auxiliary systems.

A tendency in the wind turbine field is to increase the hub height and to increase the length of the blades in order to capture more energy of the wind. With an increased length of the blades, they become more flexible as well.

The loads on the wind turbine blades should generally be high enough to produce as much as energy as possible, but not as high to overcome deflection or strain limits for the blades. If the wind turbine blades deflect or bend in excess, e.g. due to wind gusts, they could hit the wind turbine tower and even break. These risks increase with the length of the blades. And also to reduce direct material cost, the tendency is to build the blades as light (and therefore flexible) as possible.

It is known to sense a tip of a wind turbine blade so that a blade tip clearance, i.e. a distance between the blade tip and the wind turbine tower, may be calculated and adjusted to avoid breaking of the blade or to avoid the blade hitting the tower. However, already existing blade tip sensing methods and devices may not be reliable enough or may require complicated and expensive equipment. For instance, installing instrumentation on the blade tips may be necessary. In some cases, information collected about the blade tip position may be obtained too late for avoiding blade damage.

<CIT> is a relevant example of prior art in this field.

Examples of the present disclosure provide devices and methods for determining deflection of (a tip of) a rotor blade of a wind turbine that use light and that at least partially resolve the aforementioned problems.

In a first aspect of the present disclosure, a wind turbine comprising a tower, a nacelle on top of the tower, a rotor hub with one or more rotor blades rotatably mounted to the nacelle and a first light emitting and collecting device mounted to the nacelle is provided. The first light emitting and collecting device is configured to emit light in a direction within a substantially vertical plane and to collect a reflection of the emitted light by one of the rotor blades; and the first light emitting and collecting device is mounted to the nacelle at a horizontal distance of the tower such that a tip of a blade is detected before or when it exceeds a blade deflection limit.

In accordance with this aspect, a wind turbine is provided with a simple device that, by emitting light and collecting reflections of the previously emitted light in a direction within a substantially vertical plane, may be able to avoid excessive blade deflection, and thus blade breaking and/or blade impact with the wind turbine tower. Throughout this disclosure, a light emitting and collecting device may be understood as a set of at least a light source element and a light receiving element. These light emitter and light sensor may be located together, e.g. inside a case or apparatus that contains them such as a laser distance meter; or may be separate elements, e.g. the light sensor may be a camera and the light emitter may be a laser or a light-emitting diode (LED). If the light emitter and the light sensor belong to a same apparatus, they may be controlled in a dependent manner, i.e. they may share a control processing unit (CPU) and a memory. If the light emitter and the light sensor are separate elements, they may not share a CPU and a memory. In this case, an additional device such as a controller placed in the nacelle may gather data from both elements and operate with this data. In some examples the emitter and sensor may be interlinked and able to communicate with each other, if needed, to control e.g. the emission of light at specific times and/or frequencies or perform function checks.

A blade deflection limit or blade deflection threshold may be understood as a deflection or strain limit over which the risk of a wind turbine blade break, or the risk of collision with the wind turbine tower or in general blade damage is higher than acceptable.

Herein, horizontal distance may be understood as a distance measured along a length of the nacelle. If the light emitting and collecting device is mounted to the bottom of the nacelle, a horizontal distance may refer to the distance between this device and the wind turbine tower along a length of the nacelle. The suitable distance depends on the tip clearance by tower geometry. If the light emitting and collecting device is mounted to the top of the nacelle, a horizontal distance may refer to the distance, along a length of the nacelle, between this device and a tower measured along an axis between the rotational axis of the nacelle and the rotor hub.

Vertical direction may be understood as a direction substantially parallel to the wind turbine tower in the absence of movement of the tower, e.g. in the absence of tower nodding. A vertical plane may be understood as a plane including such vertical direction.

In a further aspect, a method for operating a wind turbine having a first light emitting and collecting device mounted to a nacelle is provided. The method comprises emitting light above a hub of the wind turbine and receiving the light when reflected by a blade of the wind turbine. The method further comprises, if a level of blade deflection based on the received reflected light is above a threshold, reducing blade loading of the blade before the blade reaches a vertically downward position.

Emitting light above a hub may be understood as emitting light between <NUM> o'clock and <NUM> o'clock, wherein <NUM> o'clock and <NUM> o'clock refer to a position of a wind turbine blade (e.g. <NUM> o'clock refers to the blade pointing downwards).

According to the invention, a method for monitoring deflection of a rotor blade of a wind turbine, wherein a first light emitting and collecting device is mounted to the nacelle of the wind turbine is provided. The method comprises emitting a light sheet; collecting reflections of the emitted light; and determining deflection of the rotor blade by determining a time during which the blade reflects the emitted light sheet.

In accordance with the invention, blade deflection may not be only detected, but the time of interference of the blade with the light sheet may be used to better adjust the mechanisms of blade load reduction. The operation of the wind turbine blade may then be optimized by achieving an adequate compromise between blade load reduction and keeping an energy production as high as possible.

Herein, a "light sheet" may be understood as a beam of light having a higher degree of divergence, i.e. which is not collimated. Conversely, herein the term "light beam" may be understood as a beam of light which has a lower degree of divergence, i.e. it is substantially collimated. Therefore, the difference between a light beam and a light sheet as used throughout this disclosure may be divergence or collimation.

As a visual example, a set of light rays substantially parallel among them and having a substantially constant width along the length of the set of rays (i.e. the rays are substantially collimated or have substantially no divergence) may be considered a light beam, but a set of light rays whose width increases with the distance from the source of emission (i.e. the rays are not collimated and show divergence) may be considered a light sheet. Therefore, a light beam may be visualized as a set of light rays traveling substantially in one direction and a light sheet may be visualized as a light plane, in the sense that the light rays of the light sheet may be visualized as traveling in more than one direction within a plane due to divergence of the light rays.

Thus, herein a light beam may for example be emitted from a light source with an angle of less than <NUM>º and a light sheet may for example be emitted from a light source with an angle equal to or greater than <NUM>º.

Each example is provided by way of explanation of the invention, not as a limitation of the invention.

<FIG> illustrates a perspective view of one example of a wind turbine <NUM>. As shown, the wind turbine <NUM> includes a tower <NUM> extending from a support surface <NUM>, a nacelle <NUM> mounted on the tower <NUM>, and a rotor <NUM> coupled to the nacelle <NUM>. The rotor <NUM> includes a rotatable hub <NUM> and at least one rotor blade <NUM> coupled to and extending outwardly from the hub <NUM>. For example, in the illustrated embodiment, the rotor <NUM> includes three rotor blades <NUM>. However, in an alternative embodiment, the rotor <NUM> may include more or less than three rotor blades <NUM>. Each rotor blade <NUM> may be spaced about the hub <NUM> to facilitate rotating the rotor <NUM> to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. For instance, the hub <NUM> may be rotatably coupled to an electric generator <NUM> (<FIG>) positioned within the nacelle <NUM> to permit electrical energy to be produced.

<FIG> illustrates a simplified, internal view of one example of the nacelle <NUM> of the wind turbine <NUM> of the <FIG>. As shown, the generator <NUM> may be disposed within the nacelle <NUM>. In general, the generator <NUM> may be coupled to the rotor <NUM> of the wind turbine <NUM> for generating electrical power from the rotational energy generated by the rotor <NUM>. For example, the rotor <NUM> may include a main rotor shaft <NUM> coupled to the hub <NUM> for rotation therewith. The generator <NUM> may then be coupled to the rotor shaft <NUM> such that rotation of the rotor shaft <NUM> drives the generator <NUM>. For instance, in the illustrated embodiment, the generator <NUM> includes a generator shaft <NUM> rotatably coupled to the rotor shaft <NUM> through a gearbox <NUM>.

It should be appreciated that the rotor shaft <NUM>, gearbox <NUM>, and generator <NUM> may generally be supported within the nacelle <NUM> by a support frame or bedplate <NUM> positioned atop the wind turbine tower <NUM>.

The nacelle <NUM> is rotatably coupled to the tower <NUM> through the yaw system <NUM> in such a way that the nacelle <NUM> is able to rotate about a yaw axis YA. The yaw system <NUM> comprises a yaw bearing having two bearing components configured to rotate with respect to the other. The tower <NUM> is coupled to one of the bearing components and the bedplate or support frame <NUM> of the nacelle <NUM> is coupled to the other bearing component. The yaw system <NUM> comprises an annular gear <NUM> and a plurality of yaw drives <NUM> with a motor <NUM>, a gearbox <NUM> and a pinion <NUM> for meshing with the annular gear <NUM> for rotating one of the bearing components with respect to the other.

Blades <NUM> are coupled to the hub <NUM> with a pitch bearing <NUM> in between the blade <NUM> and the hub <NUM>. The pitch bearing <NUM> comprises an inner ring and an outer ring. A wind turbine blade may be attached either at the inner bearing ring or at the outer bearing ring, whereas the hub is connected at the other. A blade <NUM> may perform a relative rotational movement with respect to the hub <NUM> when a pitch system <NUM> is actuated. The inner bearing ring may therefore perform a rotational movement with respect to the outer bearing ring. The pitch system <NUM> of <FIG> comprises a pinion <NUM> that meshes with an annular gear <NUM> provided on the inner bearing ring to set the wind turbine blade into rotation around a pitch axis PA.

<FIG> schematically illustrates a front view of an example of a wind turbine including a light emitting and collecting device. Similarly to the indicated with respect to <FIG>, the wind turbine <NUM> comprises a tower <NUM>, a nacelle <NUM> on top of the tower <NUM> and a rotor hub <NUM> with at least one rotor blade <NUM> rotatably mounted to the nacelle <NUM>. The wind turbine <NUM> also comprises a first light emitting and collecting device (LECD) <NUM> mounted to the nacelle <NUM>, e.g. to a top <NUM> or a bottom <NUM> of the nacelle <NUM> in <FIG>. In particular, the wind turbine <NUM> of <FIG> includes three blades <NUM> and two LECDs, a first LECD <NUM> mounted to the bottom <NUM> of the nacelle <NUM> and an additional LECD <NUM> mounted to the top <NUM> of the nacelle <NUM>.

In general, there may be any number of LECDs mounted to any side of the nacelle <NUM>. A nacelle <NUM> may have a top side <NUM>, a bottom side <NUM>, a front or upstream side <NUM>, a back or downstream side <NUM> and two lateral sides <NUM>, <NUM>. The lateral sides <NUM>, <NUM> may be thus substantially parallel to a length of the nacelle <NUM> and may extend from the front <NUM> of the nacelle <NUM> to its back <NUM> without being the top <NUM> and the bottom <NUM> of the nacelle <NUM>. In the example of <FIG>, any number of LECDs may be mounted to the bottom <NUM> of the nacelle <NUM> and any number of LECDs may be mounted to the top <NUM> of the nacelle <NUM>. In these or some other examples, one or more LECDs may be mounted to any of the lateral sides <NUM>, 313of the nacelle <NUM>. This may be particularly useful when an LECD emits light above a hub <NUM>, e.g. upwards, in order to detect excessive blade deflection before the blade <NUM> passes in front of the tower <NUM>. The mounting at lateral sides of the nacelle <NUM> may be an option available for all the examples throughout this disclosure.

The first LECD <NUM> is configured to emit light in a direction within a substantially vertical plane and to collect a reflection of the emitted light by one of the rotor blades. In some examples, the first LECD <NUM> is configured to emit light in a substantially vertical direction, e.g. upwards or downwards. As shown in <FIG>, the first LECD <NUM> is emitting light downwards and it may collect the reflected light coming up which has been reflected on the ground <NUM>. In some examples the ground <NUM> may correspond to the support surface <NUM> of <FIG>. However, in circumstances, the blades may deform to such an extent that at least a tip of the blades reaches a position in which the light beam from LECD <NUM> to the ground is intercepted and reflected.

In some examples, the ground <NUM> and/or particles in the air, e.g. in dusty or foggy conditions, may absorb part of the emitted and/or reflected light, thereby complicating the detection of one or more reflections of the emitted light. In order to facilitate detection of light reflected by the ground <NUM>, the emitted light may be split, e.g. in two portions, such that each portion may be detected by different light sensors. In an example, the first LECD <NUM> may include two light receiving elements, e.g. a main light sensor and a secondary light sensor, for this purpose. In another example, a secondary light receiving element may be provided as an element not belonging to the first LECD <NUM>. In both examples, the secondary sensor may be used to indicate a fault or confirm the correct functioning of the light emitter and/or main light sensor.

<FIG> schematically illustrates a side view of the wind turbine of <FIG>. As shown in <FIG>, the first LECD <NUM> is mounted to the nacelle <NUM> at a horizontal distance <NUM> of the tower <NUM> such that a tip <NUM> of a blade <NUM> is detected before or when it exceeds a blade deflection limit. The detection of a tip <NUM> of a blade <NUM> may thus warn that the blade <NUM> may be too close to the tower <NUM> such that the blade <NUM> may hit the tower <NUM>. Therefore, precautionary actions may be taken to avoid such a collision, or in general avoid blade <NUM> damage.

The first LECD <NUM> may be configured to emit a substantially collimated light beam <NUM>. The first LECD <NUM> may include one or more lenses to obtain a collimated light beam <NUM>. The first LECD <NUM> may also be configured to measure a distance within a substantially vertical plane distance <NUM> between the first LECD <NUM> and a position of a reflection. In some examples, the first LECD <NUM> may be configured to obtain a position of the place where one or more reflections occur. A distance within a substantially vertical plane may be a substantially vertical distance <NUM> in some examples. In some other examples, a distance within a substantially vertical plane may be a substantially horizontal distance. Still in some other examples, a distance within a substantially vertical plane may be a distance within the substantially vertical plane which has an angle (greater than <NUM> °) with respect to any of a vertical direction or a horizontal direction.

In <FIG>, light may be reflected on the ground <NUM>. The measurements of a vertical distance <NUM> may be registered and recorded on a memory of the first LECD <NUM>. In an example, the first LECD <NUM> may be a laser distance meter.

<FIG> schematically depicts an example of an evolution of a measured vertical distance <NUM>, i.e. the distance representing a distance between the LECD and the point of reflection in this particular example, over time. The following thus may apply when a distance between the LECD and the position of a reflection of the emitted light is not a vertical distance. A vertical distance <NUM> (VD) may be measured indirectly. For instance, the first LECD <NUM> may quantify the time that an emitted collimated light beam <NUM> takes to hit the ground <NUM> and then be reflected and be detected by the first LECD <NUM>. This time value may then be converted into a vertical distance <NUM> value.

Each of the lines <NUM>, <NUM>, <NUM> represent a plurality of measured vertical distances <NUM>. The case of line <NUM> represents the case where no interference between the light beam <NUM> and a blade <NUM> has been detected. This is why the measured vertical distance <NUM> gives a substantially constant value over time.

However, the other two lines <NUM>, <NUM> illustrate cases where blade <NUM> interference with the light beam <NUM> has been detected. Such an interference causes a reduction in the measured vertical distance <NUM>. Thus, three interferences are shown in <FIG>. Lines <NUM>, <NUM>, <NUM> may correspond to the evolution of the blades in cases <NUM>, <NUM>, <NUM> of <FIG> respectively. In the case of deflection <NUM>, the blades do not interfere with the light beam <NUM> whereas interference takes places for cases <NUM>, <NUM>.

In addition, the variation (drop) in vertical distance <NUM> may be correlated with blade deflection (e.g. with a distance <NUM> between the blade <NUM> and the tower <NUM>, see <FIG>), and used for deciding which measures of load reduction to take. For example, different measures of load reduction may be implemented for the cases of lines <NUM> and <NUM>. In some examples, even if the blade reflects the light no measures are necessary. In other examples, operational changes such as pitching or reducing rotor speed may be made. Also, actions may be taken immediately after detecting an interference or may be taken after several blade passes and/or blade interferences, e.g. depending on how large the deflection of a blade <NUM> may be.

For instance, in some examples, reducing blade loading may be performed after having monitored multiple passes of the same blade <NUM>. In some of these examples, statistical evaluation of the interference of a blade <NUM> with the light emitted by the first LECD <NUM> over time may be performed to decide whether to react or not, and if reacting, how. In some other examples, a single pass, e.g. a single interference event of a blade <NUM> with the emitted light, may be used for deciding whether to react, and in which way. These options may be combined, e.g. one of them may be used during a certain period of time and the other one may be used during a later or at least partially overlapping period of time.

If blade deflection is acceptable, e.g. if one or more blades <NUM> are not detected during a period of time, power of operation may be increased in some examples. In some of these examples, power may be increased beyond the nameplate rating.

Mounting a first LECD <NUM> configured to emit a light beam <NUM> and collect its reflection may be an easy and not expensive way of detecting blade <NUM> deflection. In addition, it may be a convenient way as compared to prior art systems which employ specific equipment on a blade tip <NUM> or on the tower <NUM>.

In some examples, the blades may include a paint or coating at least in a section including the blade tip to ensure effective reflection of the light emitted by the LECD.

<FIG> also illustrate an LECD <NUM> mounted to the top <NUM> of the nacelle <NUM>. In this case, as the LECD <NUM> emits light above a hub <NUM>, e.g. upwards, a distance between the LECD <NUM> and the place where a reflection occurs, e.g. a vertical distance, may only be measured when a blade <NUM> interferes with the light beam <NUM>. Such a measurement may trigger preventive actions, which may be adapted depending on the magnitude of the measured vertical distance.

The values of measured distance, e.g. an upwards vertical distance, may help to anticipate a possible collision between a blade <NUM> and the tower <NUM> and reduce a blade load in order to avoid it.

<FIG> schematically illustrates a front view of another example of a wind turbine including a light emitting and collecting device. In the example of <FIG>, the first LECD <NUM> is configured to emit a light sheet <NUM>. In an example, a light sheet <NUM> may be generated by a cylinder lens diverging a light beam. In another example, a light sheet <NUM> may be generated by a rotating or vibrating mirror which rapidly deflects a light beam, covering a certain angle in a substantially negligible time. A light sheet <NUM> may, in some examples, be emitted in a substantially vertical direction. Other directions of emission may be used in some other examples.

The first LECD <NUM> or a processor coupled to the first LECD may also be configured to measure an intensity of the received light. , the first LECD may emit light <NUM> and receive light, wherein the received light may partially include reflections of the emitted light <NUM>. If a blade <NUM> does not interfere with the light sheet <NUM>, the received intensity does not change. However, if a blade <NUM> intersects with the light sheet <NUM>, it may increase light reflection and scattering, and thus increase the received light by the first LECD <NUM>. Thus, the first LECD <NUM> may detect that a blade <NUM> is interfering with the light sheet <NUM> and that preventive actions may need to be implemented to avoid an excessive blade deflection.

Herein, a change in light intensity may be understood as a variation, e.g. an increase, of an amount of light with respect to a previously received amount of light.

In some examples, a modulation technique may be used to discriminate between blade <NUM> intersection and other undesired detections which may give rise to false positives, e.g. birds or flying plastic bags. In this case, intensity may be time averaged and pulse frequency filtered. In some examples, color filtering, or in general wavelength filtering, may be used to help in this regard. Still in some examples, a rotor position and/or a rotor speed may allow to discern between intensity variations detected and discard undesired detections.

A schematic representation of the evolution of an intensity received by an LECD (e.g. the first LECD <NUM> of <FIG>) with time, over which the rotor <NUM> and blades <NUM> rotate, may be depicted in <FIG> illustrates a case a) wherein there is no intersection, i.e. interference, between a blade <NUM> and the light sheet <NUM>. In such a case, a substantially constant value of intensity of received light may be measured. Line <NUM> depicts a plurality of unfiltered measurements whereas line <NUM> depicts a plurality of high pass filtered measurements. A high pass filter may be used to filter out the standard intensity measurements. By applying a high pass filter, the deviations with respect to the normal or default situation are registered and highlighted.

<FIG> also illustrates a case b) wherein a blade <NUM> has intersected with the light sheet <NUM>, therefore causing a variation in the measurements of light intensity received as explained above. In case b) three blades <NUM> - light sheet <NUM> interferences have been detected.

The duration of a different value of measured light intensity and/or the magnitude of the change in intensity may be correlated with the distance 407between a blade <NUM> and the tower, and used to trigger actions to prevent blade <NUM> damage, e.g. collision of the blade <NUM> with the tower <NUM>. Thus, an LECD or a processor coupled to the LECD may be configured to measure a time during which the emitted light is reflected by the blade <NUM>.

As previously commented, a wind turbine <NUM> may further comprise an additional LECD <NUM>. The additional LECD may be mounted to the top <NUM> or the bottom <NUM> of the nacelle <NUM> in some examples. Like the first LECD <NUM>, the additional LECD may be configured to emit light in a direction within a substantially vertical plane, e.g. in a substantially vertical direction <NUM>, and to collect a reflection of the emitted light by one of the rotor blades. In some examples, the first <NUM> and the additional <NUM> LECDs may be mounted to a same side of the nacelle <NUM>. For instance, <FIG>, which schematically illustrates a side view of the wind turbine of <FIG>, shows a first <NUM> and an additional <NUM> LECDs mounted to a bottom <NUM> of the nacelle <NUM>. In these or other examples, e.g. in <FIG>, the first <NUM> and the additional <NUM> LECDs may be mounted to opposite sides of the nacelle <NUM>. <FIG> shows two additional LECDs <NUM>, one mounted to the same side (bottom <NUM>) and the other one mounted to the opposite side (top <NUM>) than the first LECD <NUM>.

When more than one LECD is mounted to a nacelle <NUM>, an LECD may emit a light beam <NUM> or a light sheet <NUM> independently of what the other LECDs emit. , all the LECDs of a set of LECDs may emit a light beam <NUM>, may emit a light sheet <NUM>, or some of them may emit a light beam <NUM> and some of them, different from the ones that may emit a light beam <NUM>, may emit a light sheet <NUM>.

Any of the blades <NUM> of a wind turbine as disclosed herein may include reflective paint at their tips <NUM>. The reflective paint may help to increase the signal to noise (SNR) ratio of the light reflected by the blade tip <NUM>.

When exposed to wind, a nacelle <NUM> and/or a wind turbine tower <NUM> may nod, i.e. oscillate in a forward-backward direction. This nodding may deviate light emitted by an LECD from a substantially vertical plane in the absence of loads on the wind turbine <NUM>. Therefore, false alarms or indications that a blade <NUM> is overly deflected when the blade <NUM> is actually behaving acceptably may arise. The opposite may happen too, i.e. blade deflection may be excessive, but such a danger may be missed. This may be particularly relevant when an LECD may be emitting a light sheet <NUM>.

A possible solution to this issue is depicted in <FIG> schematically illustrates a side view of wind turbine including a light emitting and collecting device according to another example. The wind turbine <NUM> may comprise a first positioning sensor <NUM> (drawn as a triangle) attached to a bottom of the nacelle <NUM> and configured to detect a nodding of the nacelle <NUM> and/or the tower <NUM>. The first LECD <NUM> may be configured to adjust the direction of the emitted light for keeping a direction of the emitted light within a substantially vertical plane. In some examples the first LECD <NUM> may be configured to adjust the direction of the emitted light for keeping a substantially vertical direction of the emitted light.

To this end, in some examples, the first LECD <NUM> may be rotatably mounted (to the nacelle <NUM>) in such a way that light is emitted in a direction within a substantially vertical plane, and more in particular in a substantially vertical direction, when the nacelle oscillates. The movement of the first LECD <NUM> may be similar to that of a pendulum. In some of these examples, the first LECD <NUM> may be rotatably mounted to the nacelle <NUM> such that the LECD points substantially vertically downwards at all times, again like a pendulum. One or more LECDs may behave this way, e.g. one or more additional LECDs may be rotatable as just indicated.

The first positioning sensor <NUM> may be placed close to a back or downstream side <NUM> of the nacelle <NUM>, as in <FIG>. In other examples, it may be placed closest to a front or upstream side <NUM> of the nacelle <NUM>. The positioning sensor <NUM> may measure a distance to the ground <NUM> or to any other reference. A reference value for this distance may be easily known by the positioning sensor <NUM>. If the positioning sensor <NUM> measures a distance which differs from the reference value, this may be indicating nacelle <NUM> and/or tower <NUM> nodding.

In some examples, the variation in the values of distances measured by one or more positioning sensors (e.g. a first sensor <NUM> and an additional sensor <NUM> as in <FIG>) may be used to rotate an LECD such that a substantially vertical direction of the emitted light may be kept.

In any of these examples or in some other examples, any values obtained by an inclinometer placed in the nacelle <NUM> may be used to adjust the direction of the emitted light. Measurements of thrust, rotor imbalance or blade <NUM> or main (rotor) shaft bending loads may be also used in this regard.

In some other examples, instead of rotating an LECD, the variation in the values of distances measured by one or more positioning sensors <NUM>, <NUM> may be used to modify the information collected by the LECD <NUM>.

In an example, a controller (not shown) in the nacelle <NUM> may gather information from an LECD <NUM> and a positioning device <NUM>, <NUM> and may perform the necessary corrections in the information. In another example, the measurements of the positioning sensor <NUM>, <NUM> may be sent to the LECD <NUM> such that the LECD makes the appropriate corrections.

<FIG> schematically illustrates an example of a method for operating a wind turbine <NUM>. In an aspect, a method for operating a wind turbine having a first light emitting and collecting device <NUM> mounted to a nacelle <NUM> is provided. The first LECD <NUM> may be mounted to a top of the nacelle <NUM> in some examples.

The method comprises emitting light above a hub of the wind turbine at block <NUM>. The method further comprises, at block <NUM> receiving the light when reflected by a blade <NUM> of the wind turbine <NUM>; i.e. receiving at least a portion of the previously emitted light which has been reflected by a blade <NUM>. The method further comprises, if a level of blade deflection based on the received reflected light is above a threshold (block <NUM>), reducing blade loading of the blade <NUM> before the blade <NUM> reaches a vertically downward position at block <NUM>.

In some examples, the method may further include determining a level of blade deflection before testing whether a level of blade deflection based on the received reflected light is above a threshold at block <NUM>.

In some examples, reducing blade loading may include at least one of: reducing a rotor speed, pitching of one or more of the blades, and activating aerodynamic actuators if provided on the blade. Actuators on blades may include one or more of flaps, spoilers and pitching tips. Pitching of the blades may be individual (for a single blade) or collective (for all blades). Reducing a rotor speed may include changing a generator torque and/or mechanically braking in a case of emergency or severe disturbance. In some cases, reducing a rotor speed may include stopping operation of a wind turbine.

In some examples, the light may be emitted substantially in a direction within a substantially vertical plane, e.g. in a vertical direction. This may include emitting the light vertically downwards or vertically upwards as has been illustrated in previous examples. For instance, in one example, emitting light (block <NUM>) comprises emitting light (vertically) upwards from the nacelle <NUM>. In other examples, the light may deviate with respect to a vertical plane.

In some examples, emitting light (block <NUM>) includes emitting a light sheet <NUM> and the level of blade deflection is determined based at least in part on a time that one of the blades <NUM> interferes with the light sheet <NUM>. The explanation concerning <FIG> may apply.

In some examples, emitting light (block <NUM>) comprises emitting a substantially collimated light beam <NUM>.

In some examples, the emitted light is in a region of the electromagnetic spectrum different than the visible region of the electromagnetic spectrum. This may reduce the visual impact for people and animals.

Also, a wavelength of the emitted light may be selected such that, after filtering the light received by a light sensor, the light corresponding to one or more reflections of the emitted light may be easily detected. In some examples, the emitted light is in a region of the electromagnetic spectrum different than the light emitted by the sun. This may be particularly helpful to avoid blinding of a light sensor by the sun.

In some examples, the first LECD <NUM> is configured to measure a distance <NUM> between the first LECD <NUM> and a position of a reflection. The explanation concerning <FIG> may apply.

<FIG> schematically illustrates an example of a method of monitoring a deflection of a rotor blade of a wind turbine.

A first light emitting and collecting device <NUM> is mounted to the nacelle <NUM> of the wind turbine <NUM>, e.g. to a top <NUM> or a bottom <NUM> of the nacelle <NUM>. The method comprises emitting, at block <NUM>, a light sheet <NUM>. The light sheet <NUM> may be emitted in a direction within a substantially vertical plane in some examples. In some of these examples, the light sheet <NUM> may be emitted in a substantially vertical direction, e.g. upwards or downwards. The method furthermore comprises collecting reflections of the emitted light at block <NUM>; and determining deflection of the rotor blade, at block <NUM>, by determining a time during which the blade <NUM> reflects the emitted light sheet (block <NUM>). The explanation concerning <FIG> may apply.

A speed of rotation of the rotor <NUM> may be used besides the time of interference with the light sheet <NUM> in order to determine deflection of the rotor blade <NUM>. In some examples, more than one light sheet <NUM> may be emitted. This may enable obtaining multiple measurements regarding the portions of a blade <NUM> which may be interfering with the light sheets, e.g. positions of interference along a length of the blade <NUM>. Therefore, the accuracy of blade deflection calculation may be increased e.g. if the rotor <NUM> does not maintain a substantially constant speed of rotation.

In some examples, the wind turbine may further comprise a positioning sensor <NUM>, <NUM> attached to the nacelle <NUM> (e.g. as illustrated with reference to <FIG>) and the method may further comprise determining a position and/or orientation of the nacelle <NUM>. The position and/or orientation of the nacelle <NUM> may be taken into account when determining blade deflection.

In some examples, the first light emitting and collecting device <NUM> may be rotatably mounted to the nacelle <NUM> in such a way that light is emitted within a substantially vertical plane, and more in particular in a substantially vertical direction when the nacelle <NUM> oscillates.

In some examples, the wind turbine <NUM> may comprise one or more additional light emitting and collecting devices, e.g. an additional LECD <NUM>, mounted the nacelle <NUM>, e.g. to a top <NUM> or a bottom <NUM> of the nacelle <NUM>, and determining deflection of the rotor blade <NUM> includes collecting reflections of the emitted light of the first <NUM> and additional <NUM> light emitting and collecting devices. The first <NUM> and additional <NUM> light emitting and collecting devices may e.g. be mounted on the same side of the nacelle <NUM> or on opposite sides.

In some examples, determining a time during which the blade <NUM> reflects the emitted light sheet <NUM>, may include measuring a light intensity of light received by the first light emitting and collecting device <NUM>. Particularly if the light is emitted above a hub <NUM>, e.g. upwards, the intensity of light received may be a good measure for determining interference of a blade <NUM> with the light sheet <NUM>.

Claim 1:
A method for operating a wind turbine having a first light emitting and collecting device mounted to a nacelle, the method comprising:
emitting light above a hub of the wind turbine, wherein the emitting light comprises emitting a light sheet;
receiving the light when reflected by a blade of the wind turbine; and
if a level of blade deflection based on the received reflected light is above a threshold, reducing blade loading of the blade before the blade reaches a vertically downward position;
characterized in that the level of blade deflection is determined based at least in part on a time that the blade interferes with the light sheet.