Patent Description:
Modern wind turbines are used for supplying electricity to the grid. A wind turbine generally includes a tower with a nacelle supported on top of the tower. A wind turbine rotor comprising a hub and a plurality of wind turbine blades may be rotatably mounted to the nacelle.

The wind turbine blades may be set in motion by wind. The hub of the wind turbine may be operatively coupled with a rotor of a generator. As the hub and blades rotate, the kinetic energy of the wind is converted to kinetic mechanical energy of the wind turbine rotor and ultimately to electrical energy or power in the generator. The generator may typically be arranged inside the nacelle.

The wind turbine rotor may be coupled directly to the generator rotor in so-called direct drive wind turbines. Or the wind turbine rotor may include a main rotor shaft (a so-called "low speed shaft" which leads to a gearbox. A high-speed shaft of the gearbox may then drive the generator. Regardless of the topology of the wind turbine, the electrical power output of the generator may be fed to an electric grid. The connection of the generator to the grid may include e.g. a converter, transformer, medium voltage line and other.

A wind turbine controller may be configured to determine suitable actuator setpoints for the wind turbine based on the prevailing circumstances. The actuator setpoints for modern variable speed wind turbines include e.g. the generator torque and the pitch angle of the blades. Through control of the pitch angle(s) of the blade(s) and the generator torque, the speed of the rotor may be controlled, as well as the electrical power output, aerodynamic thrust and further mechanical loads. The purpose of the control system is generally to maximize electrical power output while at the same time keeping loads in the wind turbine at an acceptable level.

As mentioned before, the actuator setpoints of torque and pitch (but also other actuators such as yaw) may be changed in accordance with circumstances. important input for the determination of the actuator setpoints include e.g. wind speed, and wind direction. The wind speed may be measured directly or indirectly e.g. through the use of a (generator) rotor speed sensor.

Wind turbines may also comprise load sensors on or in the blades for measuring loads on the blades caused by for example the wind and/or the weight of the blades. Too high loads on the blades can e.g. damage the blades and/or cause undesirable rotational speeds of the rotor which may damage other components of the wind turbine. The blade load sensors permit detecting high loads and make it possible to react, by e.g. acting on the pitch systems in such a way that loads on the blades may be reduced. These adjustments on the blades through the pitch systems may extend the life of the wind turbine and/or reduce the cost of producing power.

It is therefore important for the safe and efficient operation of a wind turbine that the load measurements as retrieved from load sensors are reliable, i.e. they correctly indicate the actual loads at any given time. Load sensors for measuring loads on wind turbines, and particularly wind turbine blades may include resistive strain gauges, fiber optic strain gauges or any other known strain sensing system. The performance of these kinds of sensors may degrade over time, under the influence of continuous loading, and under the influence of temperature changes, and from being exposed to different sorts of ambient conditions and environments.

Different blade loads may be defined for a wind turbine blade, namely edgewise loads, spanwise loads and flapwise loads. A spanwise direction refers to a direction along a longitudinal axis of the blade, extending from the blade root towards the blade tip. The edgewise direction refers to a direction along the chord of a section of the wind turbine blade, i.e. extending from leading edge to trailing edge. The flapwise direction is perpendicular to both the edgewise and the spanwise direction.

With respect to a wind turbine rotor, loads may be decomposed as in-plane loads (loads that are tangential to the rotor plane) and out-of-plane loads (loads that are perpendicular to the rotor plane). The rotor plane may herein be defined as a plane perpendicular to the rotor rotational axis and passing through a center of the blades at the blade root.

Blade load sensors can be calibrated in order to preserve their accuracy when taking measurements of the loads on the blades. Calibration normally comprises establishing correspondence between indications generated by the blade load sensors and values of reference according to calibration patterns (i.e. particular conditions for calibration). If the calibration process produces some inconsistency, suitable adjustments may be undertaken on the load sensors to improve their accuracy.

It is known that blade load sensors can be manually calibrated in a factory by e.g. statically pulling the blades to obtain particular conditions for calibration. This manual calibration is normally performed before mounting the blades on the wind turbine. However, over time, load sensors may need to be recalibrated.

<CIT> discloses a method of calibrating one or more load sensors of a blade of a wind turbine, wherein the wind turbine comprises: a main generator; a power electronic converter connected with the main generator; a rotor operationally connected with the main generator and carrying the blade. And the method being comprises: acting on the power electronic converter to operate the main generator as motor to set the blade in at least one predetermined condition; measuring loads in the predetermined condition using the load sensors of the blade; and calibrating the blade load sensors taking into account the measured loads.

Such a method can be carried out particularly after interruption of normal operation of a wind turbine.

It is also known that blade load sensors can be manually calibrated when the blades are mounted on the wind turbine by manually (i.e. mechanically) acting on the wind turbine to e.g. set the blade in a particular position (e.g. horizontal position) with a particular pitch angle. This manual calibration permits recalibrating the load sensors regularly. However, this type of calibration may take a long time and may be especially expensive for offshore wind turbines because operators need to go where the wind turbine is located.

It is also known to use automatic calibration of blade load sensors during operation of the wind turbine by recording several minutes of data (or indications or load measurements) from the blade load sensors. For example, data from the load sensors may be recorded e.g. when predetermined conditions for calibration are met during idle operation of the wind turbine at low winds. Some of said predetermined conditions may be obtained after several hours or days of idling operation of the wind turbine. Therefore, a drawback of this type of calibration may be that it can take a long time, depending on the wind conditions. Document <CIT> is another prior art example that determines the reliability of load sensors on a wind turbine.

The present disclosure provides examples of methods and systems for determining reliability or correct functioning of load sensors that resolve at least some of the aforementioned disadvantages.

In a first aspect, a method for determining reliability of one or more load sensors in a wind turbine is provided. The method comprises, measuring loads with the load sensors during operation of the wind turbine and determining in-plane moments with rotor rotational speed frequency of one or more blades based on the measured loads. The method then further comprises comparing the in-plane moments with rotor rotational speed frequency with theoretical in-plane moments due to a mass of the blades and determining that the load sensors have reduced reliability if the in-plane moments with rotor rotational speed frequency deviate from the theoretical in-plane moments by more than a first threshold value.

In accordance with this aspect, the reliability or correct functioning of load sensors can be determined during operation of the wind turbine, i.e. the operation of the wind turbine does not need to be interrupted or put in specific conditions in order to be able to determine whether the load sensors are correctly indicating the loads. The loads measured may be measured as in-plane moments or converted to in-plane moments. The in-plane moments at any given moment during operation will be a combination of aerodynamic loads and loads due to mass of the blades. The mass of the blades will however provide the same moment throughout every rotation of the blade. At a <NUM> o'clock position and at a <NUM> o'clock position, the mass of the blade will not contribute to a bending moment. But at a <NUM> o'clock position and a <NUM> o'clock position, the bending moment due to the mass of the blade will be at a maximum (in one direction, and in the opposite direction). The mass of the blade will have a well-defined contribution with 1p frequency, i.e. the variation of the moment due to the mass will have the same frequency as the rotor rotational speed. The rotor in this respect refers to the wind turbine rotor, not to the generator rotor. The generator rotor may have the same rotational speed in the case of a direct drive wind turbine, but may have a very different speed in the case of a wind turbine with gearbox.

The terms "1p frequency" and "rotor rotational speed frequency" may be used interchangeably throughout this disclosure.

By selecting the in-plane moments with the rotor rotational speed frequency and comparing the selected in-plane moments with theoretical in-plane moments due to a mass of the blade, a deviation between the two indicates a possible malfunctioning of the load sensors. Once such a possible malfunctioning is detected, different actions can be taken to reduce the risk of a malfunctioning load sensor.

In these figures, the same reference signs have been used to designate matching elements.

<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.

The energy produced by the generator may be delivered to a converter which adapts the output electrical power of the generator to the requirements of the power grid. The electrical machine may comprise electrical phases, e.g. three electrical phases. The converter may be arranged inside the nacelle or inside the tower or externally.

<FIG> schematically illustrates an example of a method for determining reliability of one or more load sensors in a wind turbine. The method comprises, at block <NUM>, measuring loads with the load sensors during operation of the wind turbine. At block <NUM>, in-plane moments based on the measured loads are determined. Then, at block <NUM>, the in-plane moments with 1p frequency are selected. At block <NUM>, the selected in-plane moments with theoretical in-plane moments due to a mass of the blade are compared. Then, at block <NUM>, the determination may be made that the load sensors have reduced reliability if the selected in-plane moments deviate from the theoretical in-plane moments by more than a first threshold value.

Various examples of the steps of the method, or blocks in <FIG>, will be explained with reference to <FIG>.

In some examples, the measuring loads at block <NUM> may comprise measuring flapwise and edgewise moments. Standard wind turbine blades may incorporate suitable load sensors. The load sensors may be strain gauges. Depending on where and how the load sensors are mounted, strains may be measured in different directions. Sensors arranged in the hub, or any other (indirect) blade support may be used to determine loads as well.

Edgewise and flapwise loads may be used in control of wind turbines. <FIG> illustrates measurement of flapwise and edgewise moments for three blades of a wind turbine throughout a number of full rotations.

The edgewise and flapwise moments may be converted to in-plane moments and out-of-plane moments based on a pitch angle of the blade at block <NUM>. Only the in-plane moments need to be considered in the present method, since the loads of rotor wind turbine blades will be in-plane loads, not out-of-plane loads. The result for the three blades may be seen at <FIG>.

At block <NUM>, the selecting the in-plane moments with rotor rotational speed frequency may comprise filtering the determined in-plane moments using a peak filter. A peak filter is a frequency filter that passes a narrow band of frequencies and is configured to stop all other frequencies. A peak filter in this respect is, in essence, a very narrow band pass filter. <FIG> schematically illustrates measured loads in the frequency domain and how a peak filter may select the in-plane moments of 1p frequency. The result of the filter may be seen in <FIG>, in which for three blades of a wind turbine, in-plane modes with 1p frequency are shown during a number of complete revolutions of the wind turbine rotor.

For the comparison between the theoretical in-plane moments due to the mass of the blade may be determined based on an azimuthal position of the rotor blade.

At block <NUM>, the determination may be made that the load sensors have reduced reliability if the selected in-plane moments deviate from the theoretical in-plane moments by more than a first threshold value.

In some examples, the method may further comprise generating a first warning if the load sensors have reduced reliability. A first threshold may be defined which indicates a malfunctioning. If the threshold is passed, different actions may be taken. Maintenance may be planned to substitute or recalibrate load sensors. Or the operation of the wind turbine may be downrated, i.e. loads on the wind turbine may consciously be reduced at the expense of electrical power generation, because the measurements of the load sensors are not as reliable as they should be. In other examples, the wind turbine operation may be stopped if the load sensors are determined to be unreliable.

In some examples, the method may further comprise determining whether the selected in-plane moments deviate from the theoretical in-plane moments by more than a second threshold value, the second threshold value being higher than the first threshold value. In these examples, a first threshold may generate a warning signal (operators are made aware of a potential problem and operation may continue, as normal or with some changes), and the passing of a second threshold indicates a more serious warning (e.g. interruption of the operation).

In some examples, the threshold value may be a percentage of an absolute value of a theoretical in-plane moment. In some examples, the comparing the selected in-plane moments with theoretical in-plane moments due to a mass of the blade comprises determining a Root Mean Square value of the selected in-plane moments. These examples will be discussed with reference to <FIG> and <FIG>.

<FIG> illustrates a comparison of in-plane moments due to mass of a wind turbine blade with in-plane moments of 1p frequency. A bandwidth may be defined around the theoretical moment due to mass of a blade as indicated with dotted lines. The bandwidth may be defined as a percentage of the theoretical moment due to a mass of a blade. The bandwidth might also be of a given fixed value. As long as the moment of 1p frequency derived from the actual measurements stays within the bandwidth (as in <FIG>), the load sensors may be regarded as functioning correctly. As mentioned before, more than one bandwidth might be defined in examples.

<FIG> illustrates an example in which the load sensors are determined less reliable or unreliable. It may be seen that in various occasions, the loads derived from the measurements go outside the defined bandwidth.

<FIG> schematically illustrate an alternative example. In the example of <FIG>, another comparison between the measured and theoretical moments is shown. For the 1p in-plane moments, Root Mean Square (RMS) of deviations with respect to the theoretical moments, if indeed only mass of the blade is measured, should be roughly constant. Actual measurements will inevitably oscillate to some extent, because of e.g. inevitable vibrations of the blades. Root Mean Square (RSM) of differences between the theoretical and measured in-plane 1p loads may be used for a comparison with a threshold. In the example of <FIG>, the RMS values stay well within a defined bandwidth, i.e. deviation is below a given threshold. In <FIG> on the other hand, it may be seen that for all three blades, RMS exceeds a given threshold. A malfunctioning of different load sensors for the different blades may be concluded.

Also when comparing RMS to determine a deviation from a theoretical curve or theoretical loads, more than one threshold, or more than one bandwidth may be defined.

During operation of the wind turbine, both the examples of <FIG> and <FIG> may be used at the same time, or one of the examples may be selected. In some examples, the comparison according to <FIG> may be used particularly for slow rotations of the wind turbine, or measurements after an interruption of operation and the comparison according to <FIG> may be used for steady state operation above a minimum rotor speed.

In a further aspect, of the present disclosure a wind turbine system is provided. The system comprises a wind turbine <NUM> including a wind turbine rotor <NUM> with a plurality of blades <NUM>. The system includes a plurality of load sensors for measuring loads on the blades, and a control system. The control system may be configured to receive signals from the load sensors during operation, determine in-plane moments on one or more of the blades, select the in-plane moments with 1p frequency for the one or more blades and compare the selected in-plane moments with theoretical in-plane moments due to a mass of the one or more blades.

The control system may further be configured to generate a warning signal if the selected in-plane moments deviate from the theoretical in-plane moments by more than a predefined threshold.

In some examples, each of the blades may comprise strain gauges. Suitable strain gauges may include resistive foil strain gauges. The resistive strain gauges may be attached to the blade with a suitable glue, e.g. epoxy based glue. Other types of strain gauges and sensors may also be used such as e.g. piezoresistors, capacitive strain gauges, or fiber optics to measure strain along an optical fiber.

In some examples, the strain gauges may be mounted such as to measure edgewise and flapwise loads.

In some examples, the load sensors may be mounted in or near a root portion of the blades. In examples, measurements from load sensors on the blades that are not at the root of the blade may be extrapolated to indicate moments at the root of a blade. In examples, the sensors may be mounted at a suitable location on the hub, instead of the blade.

In some examples, the control system may be at a remote location from the wind turbine. The control system may be part of a SCADA system of a wind farm, or may be at a remote operating centre. In some examples, the wind turbine controller itself may incorporate the functions for determining a potential malfunction or loss of reliability of the load sensor.

In a further aspect, with reference to <FIG>, a method for on-line determination of correct functioning of load sensors mounted on a wind turbine blade is provided. On-line, as used herein shall mean that the method can be carried out during standard operation of the wind turbine, i.e. without the need of interrupting the operation of the wind turbine and without the need to recreate a specific load or rotational speed scenario.

The method may comprise, at block <NUM>, measuring edgewise and flapwise strains in the wind turbine blade. At block <NUM>, edgewise and flapwise bending moments in the wind turbine blade based on the measured strains can be determined. At block <NUM>, the edgewise and flapwise bending moments may be converted to measured in-plane moments and measured out-of-plane moments on the wind turbine blade. A peak filter may be applied, at block <NUM>, to determine the measured in-plane moments with 1p frequency. At block <NUM>, an azimuth angle of the rotor blade is determined, and at block <NUM>, based on the azimuth angle, a theoretical variation of a moment due to the mass of the rotor blade can be determined. At block <NUM>, the theoretical variation of the moment due to the mass of the rotor blade may be compared with the in-plane moment with 1p frequency.

In some examples, the load sensors may be determined to function correctly, if the measured in-plane moments with 1p frequency substantially corresponds to the theoretical variation of the moment due to the mass of the rotor blade. Whether or not the measured in-plane moments correspond substantially to the theoretical variation along a rotor rotation may be determined in a variety of manners. One or more thresholds may be used. And the thresholds may be defined in different manners.

In some examples, the method may comprise generating a warning if the measured in-plane moments with 1p frequency deviate from the theoretical variation of the moment due to the mass of the rotor blade by more than a first threshold. Such a warning signal may be sent to an operator at a remote operating center, and/or may be sent to a wind turbine controller or SCADA system to implement a predefined action for such a warning signal.

In some examples, the method may further comprise downrating the wind turbine if the warning is generated.

In some examples, the edgewise and the flapwise bending moments in the wind turbine blade may be determined at a sensor location in the blade, and these may be converted to edgewise and flapwise bending moments at a root of the blade.

The order shown of method steps in <FIG> and <FIG> are not to be regarded as necessarily sequential. In particular, the determination of azimuth angles does not need to occur at any specific moment of time in comparison to the measurement of the loads. The measured loads and azimuth angle only need to be correlated in time to make a meaningful comparison. Also, the methods may be continuously carried out during operation of the wind turbine. Steps may be carried out at a frequency of more than <NUM>, specifically at a frequency of higher than <NUM>.

Examples of the methods disclosed herein may be implemented with hardware, software, firmware and combinations thereof.

Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the disclosure herein may be implemented as electronic hardware, computer software, or combinations of both.

The various illustrative logical blocks, modules, and circuits described in connection with the disclosure herein may be implemented or performed with one or more general-purpose processors, a digital signal processor (DSP), cloud computing architecture, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein.

The present disclosure also related to computing systems adapted to carry out any of the methods disclosed herein.

The present disclosure also relates to a computer program or computer program product comprising instructions (code), which when executed, performs any of the methods disclosed herein.

The computer program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the processes. The carrier may be any entity or device capable of carrying the computer program.

If implemented in software/firmware, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. A storage media may be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD/DVD or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code means in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. For example, if the software/firmware is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers.

This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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.

Claim 1:
A method for determining reliability of one or more load sensors in a wind turbine, comprising:
measuring (<NUM>) loads with the load sensors during operation of the wind turbine;
determining (<NUM>, <NUM>) in-plane moments with rotor rotational speed frequency of one or more blades based on the measured loads;
comparing (<NUM>) the in-plane moments with rotor rotational speed frequency with theoretical in-plane moments due to a mass of the blades;
determining (<NUM>) that the load sensors have reduced reliability if the in-plane moments with rotor rotational speed frequency deviate from the theoretical in-plane moments by more than a first threshold value.