Patent Description:
Aerodynamic stall causes a decrease in lift and an increase in drag coefficients for a wind turbine blade. The onset of stall is signalled by a sharp change in a wind turbine's performance evident by degradation in output power versus expect power.

<CIT> discloses a stall sensor for a wind turbine. The stall sensor comprises detector means adapted to measure vibration of a rotor blade of the wind turbine and to output a vibration signal representative of the vibration of the rotor blade, conversion means connected to the detector means and adapted to determine a noise figure representative of a spectral signal content within a frequency band of the vibration signal received from the detector means and arbiter means connected to the conversion means and adapted to signal a presence or an absence of stall based on the noise figure received from the conversion means.

<CIT> discloses a system and a method that utilizes wind turbine models to maintain continuous operation of a wind turbine without transitions to a detrimental stalled mode. Stall being determined if any increase in wind speed reduces the thrust on the rotor or if a decrease in the rotor speed causes a decrease in the aerodynamic torque produced by the rotor. In this document, a rotor stall margin is defined based upon wind speed and output power.

A first aspect of the invention provides a method of measuring a stall condition of a rotor of a wind turbine, the method comprising measuring a power parameter indicative of a power generated by the rotor; measuring a thrust parameter indicative of a thrust force generated by the rotor; obtaining a stall parameter on the basis of the power parameter and the thrust parameter; obtaining the stall parameter comprising dividing the power parameter and the thrust parameter; and comparing the stall parameter with a threshold to determine a binary stall condition of the rotor, estimating of whether or not the rotor has stalled.

The method of the first aspect of the invention has the advantage that it may use readily available sensors, not requiring additional hardware in the form of a dedicated stall sensor.

Another advantage is that by using both a power parameter and a thrust parameter to obtain the stall parameter, the reliability of the measurement may be improved compared with a method which only analyses power or thrust.

The power parameter may be based on a sum of an electrical power generated by the rotor and a power loss parameter.

The method may further comprise obtaining a wind speed measurement and normalising the stall parameter on the basis of the wind speed measurement. A wind speed measurement may also be an estimated wind speed obtained from measured values,
The power parameter may be a power coefficient, and the thrust parameter may be a thrust coefficient.

The method may further comprise determining an operating point of the rotor; obtaining a theoretical power parameter and a theoretical thrust parameter based on the operating point of the rotor; and obtaining the stall parameter on the basis of the theoretical power parameter and the theoretical thrust parameter.

The theoretical power parameter and theoretical thrust parameter may be obtained by inputting the operating point of the rotor into a look up table.

The operating point of the rotor may be based on a tip speed ratio and/or a blade pitch angle of the rotor.

The method may further comprising obtaining a time series of measurements of a performance parameter indicative of a performance of the rotor, each measurement in the time series being associated with an operating point of the rotor; analysing the time series of measurements to obtain a statistical measure of a sensitivity of the performance parameter with respect to the operating point of the rotor; and comparing the statistical measure with a threshold to determine the stall condition of the rotor.

The performance parameter may be indicative of a power or a thrust generated by the rotor.

The statistical measure may be a standard deviation, variance, root mean square or sum of squares.

The stall condition may indicate that the rotor is stalled when the statistical measure exceeds the threshold.

The method may be repeated over time to obtain a time series of measurements of the stall condition of the rotor; and the time series of measurements of the stall condition of the rotor may be analysed by a statistical change detection algorithm to confirm a change in the stall condition.

A further aspect of the invention provides a method of determining a stall condition of a rotor of a wind turbine, the method comprising obtaining a time series of measurements of the wind turbine; and analysing the time series of measurements by a statistical change detection algorithm to determine whether the rotor has stalled.

The use of a statistical change detection algorithm provides a more reliable indication of a change in the stall condition than a single stall condition measurement.

The statistical change detection algorithm may include a calculation of a cumulative sum, which may be compared with a threshold to confirm the change in the stall condition and/or to determine whether the rotor has stalled.

The statistical change detection algorithm may be a leaky bucket algorithm or a CUSUM algorithm.

The stall condition may be a binary estimation of whether or not the rotor has stalled.

A further aspect of the invention provides a method of controlling a wind turbine, the wind turbine comprising a rotor with one or more wind turbine blades, the method comprising: determining a stall condition of the rotor by a method according to the preceding aspect of the invention, and controlling the wind turbine in accordance with the stall condition.

Controlling the wind turbine in accordance with the stall condition may comprise adjusting a pitch angle of the wind turbine blades, adjusting a generator torque of the wind turbine, or adjusting any other part of the wind turbine.

Controlling the wind turbine in accordance with the stall condition may comprise adjusting a pitch angle of the wind turbine blades in response to a change in the stall condition.

A further aspect of the invention provides a computer program product comprising software code adapted to determine a stall condition of a rotor of a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of any preceding aspect.

Aspects of the invention may be implemented in a wind turbine stall measurement system configured to determine a stall condition of a rotor of a wind turbine.

<FIG> shows a general view of a wind turbine <NUM> having a main tower <NUM> extending upwardly from a foundation <NUM> and supporting a nacelle <NUM>. A rotor <NUM> is rotatably mounted on the nacelle <NUM>. The rotor <NUM> has blades <NUM> extending radially outwardly from it and wind incident on the wind turbine <NUM> may rotate the blades <NUM> and thereby rotate the rotor <NUM>. The rotor <NUM> may transfer rotational movement to a generator housed within the nacelle <NUM>, which may generate electricity. The electricity generated may then be transferred to a grid or other power supply network outside the wind turbine <NUM>. The wind turbine may also comprise various sensors, which are not shown, such as anemometers, and power meters, as well as encoders for measuring the rotational speed of the rotor <NUM>.

Ice, sand, or other debris may build up on the wind turbine blades <NUM> and the aerodynamic characteristics of the blade may therefore change. Accordingly, the torque transferred to the generator and the amount of electricity produced may be reduced. The lift and/ or drag curves of the blade may also change, and the blade may stall unexpectedly.

The aerodynamic performance of the blades may also be reduced or otherwise changed due to abrasion of the leading edge or other damage to the wind turbine blade such airborne debris.

An example of a degraded wind turbine blade is shown in <FIG>. The wind turbine blade <NUM> extends from a root end <NUM> to a tip end <NUM>, having a leading edge <NUM> extending therebetween, the leading edge <NUM> being arranged to face into the wind as the blade moves, and a trailing edge <NUM> which is arranged to face out of the wind. The wind turbine blade <NUM> also has two major aerodynamic surfaces: a pressure surface <NUM> and a suction surface <NUM>.

In <FIG>, ice I can also be seen, which has built up on the pressure surface <NUM> of the wind turbine blade <NUM> near the leading edge <NUM>. It will, however, be understood that ice and other debris may build up anywhere on the wind turbine blade such as at the trailing edge <NUM> or may be localised at the root end <NUM> or at the tip end <NUM>.

In order to obtain data over a range of wind turbine blade degradation states, wind turbine blades may be simulated or modelled with a range of different debris conditions and may be categorised according to their reduction in power coefficient. Data pertaining to a wind turbine blade having a particular degradation state may therefore not be indicative of data for a specific wind turbine blade having a specific debris build up pattern, but may be a heuristic generally indicating expected wind turbine blade characteristics.

<FIG> shows a flowchart illustrating a method of controlling the rotor <NUM>. The system may be initialised at step <NUM>. The initialisation may comprise obtaining necessary data, which may be data related to power coefficients expected from wind turbine blades at a range of pitch angles and degradation states and/or measuring a tip speed ratio of the blade.

The stall condition of the rotor is measured at step <NUM>, to determine whether the rotor is stalled or not stalled. Initially, the rotor is not stalled. Various methods of measuring the stall condition at step <NUM> are described below.

Once stall is detected, the pitch angle may be increased at step <NUM> in response to the change in the stall condition of the rotor.

At the new operating point, it may be determined again whether the rotor is stalled or not stalled at step <NUM>. The determination made may be substantially similar to the detection of stall at step <NUM>. If stall is detected, then the pitch angle may be increased again at <NUM>, and if attached flow is detected (indicating that the rotor is not stalled) then the process may move to step <NUM>.

At step <NUM> debris removal may be determined. This may be by detecting a power coefficient of the blade or by other sensors, such as frost sensors. If it is determined that a sufficient amount of debris has been removed, then the pitch angle may be reduced at step <NUM>. Otherwise, the wind turbine blade may be maintained at its current pitch angle for a longer time.

The reduction of the pitch angle at step <NUM> may be a straightforward reduction in pitch angle or may involve the implementation of a more complex wind turbine blade control scheme.

<FIG> shows a schematic control diagram showing certain features of a wind turbine <NUM> which may be used within the method described above. The wind turbine may comprise a control system <NUM>. The control system <NUM> may have a memory, which may store data pertaining to power coefficients and pitch angles of the blade at various states of degradation and may store instructions for carrying out the control methods. The control system <NUM> may also comprise a processor for carrying out the method.

The control system <NUM> may control a wind turbine blade actuator <NUM>, which may be a motor arranged to alter a pitch angle of a wind turbine blade <NUM>. The wind turbine blade <NUM> may provide rotational movement to a wind turbine generator <NUM>, which may generate electricity. The amount of electricity generated may therefore provide a measure of the torque from the wind turbine blade <NUM>. The wind turbine generator <NUM> may supply electrical power to an electrical grid and the control system <NUM> may determine the power output by the wind turbine blade by measuring the power output to the grid by the wind turbine generator <NUM> and compensating for any power losses within the wind turbine <NUM>.

The wind turbine <NUM> may also have wind sensors <NUM>, which may measure wind speed and/or wind direction in order to determine a tip speed ratio of the wind turbine blade and the wind speed may also be used in determining the power coefficient of the wind turbine blade <NUM>. The wind sensors <NUM> may provide such data to the control system <NUM>.

<FIG> show various methods of measuring a stall condition of the rotor <NUM> of a wind turbine, which may be used to determine whether the rotor is stalled or not stalled at step <NUM> and/or step <NUM>. The methods use readily available sensors, thus not requiring additional hardware.

The control system <NUM> may have a memory and a data processing system. The memory stores a computer program product comprising software code adapted to determine a stall condition of the rotor when executed on the data processing system. Thus, the control system <NUM> provides a wind turbine stall measurement system configured to measure a stall condition using the method of any of <FIG>.

The computer program product may also be adapted to perform the control method of <FIG>.

The methods of <FIG> measure a power parameter indicative of a power generated by the rotor <NUM>; measure a thrust parameter indicative of a thrust force generated by the rotor; obtain a stall parameter γ1 or γ2 on the basis of the power parameter and the thrust parameter; and compare the stall parameter γ1 or γ2 with a threshold to determine the stall condition of the rotor.

In the case of <FIG>, a stall parameter γ1 is calculated in accordance with Equation (<NUM>): <MAT> where: Powerloss is the electrical power generated by the rotor <NUM> but lost as heat within the generator <NUM> and any other part of the wind turbine (such as the drive train including any gearbox); Powergrid is the electrical power delivered to the grid by the generator <NUM>; ThrustForce is the thrust force generated by the rotor <NUM>, and WindSpeed is the wind speed measured by the wind sensors <NUM>.

The electrical power (Powergrid) delivered to the grid by the generator <NUM> may be measured by measuring voltages and currents in the output from the generator <NUM>.

The power loss parameter (Powerloss) may be obtained by estimation, for instance based on known design parameters of the drive train, and a power loss model which estimates the power lost to heat based on the rotor speed and the friction in the drive train. Alternatively the power loss parameter (Powerloss) may be measured more directly, for instance by measuring the temperature of the lubricating oil of the drive train.

The power loss parameter (Powerloss) is particularly important for cases where there is a high degradation in performance such that Powergrid is so low that it is comparable with Powerloss.

As shown in <FIG>, the thrust force <NUM> may be measured by obtaining flapwise bending moments <NUM> of the three blades <NUM>; obtaining pitch angles <NUM> of the three rotor blades; and at step <NUM> using the pitch angles <NUM> to project the flapwise bending moments <NUM> onto the axis of the rotor (which is out of the plane of the rotor) and summing the projected bending moments <NUM> to produce a thrust force <NUM> which is normalised at step <NUM> on the basis of the rotor speed <NUM> to produce a normalized thrust force <NUM>.

At any given radial position the load can be projected to an in plane and out of plane moment. The out of plane moment gives the force (knowing the moment arm) which can then be used to determine the acting thrust force on the rotor.

The flapwise bending moments <NUM> may be obtained by direct measurement, for instance using strain sensors on the blades or any other direct measurement technique.

The power <NUM> and power loss <NUM> are then divided by the normalised thrust force <NUM> at step <NUM> and divided by a wind speed measurement <NUM> at step <NUM> to produce the stall parameter γ1. Thus, the stall parameter γ1 is normalised on the basis of the wind speed measurement <NUM>.

The stall parameter γ1 may then be compared with a threshold to determine the stall condition. For instance, the stall condition may indicate that the rotor <NUM> is not stalled when the stall parameter γ1 exceeds the threshold, and/or the stall condition may indicate that the rotor <NUM> is stalled when the stall parameter γ1 is below the threshold.

The power parameter in Equation (<NUM>) is based on a sum (Powerloss + Powergrid) of an electrical power (Powergrid) generated by the rotor and a power loss parameter (Powerloss); and the thrust parameter (ThrustForce) in Equation (<NUM>) is based on a direct measurement of flapwise bending moments.

In a second method shown in <FIG>, the power parameter is a power coefficient and the thrust parameter is a thrust coefficient. Specifically, the stall parameter γ2 in the case of <FIG> is calculated in accordance with Equation (<NUM>): <MAT> where: Cpest is an estimated power coefficient based on a current operating point of the rotor; Ctest is an estimated thrust coefficient based on the current operating point of the rotor; Cptheo is a theoretical power coefficient based on the current operating point of the rotor; and Cttheol is a theoretical thrust coefficient based on the current operating point of the rotor.

The estimated coefficients Cpest and Ctest may be estimated online from the current operating point by inverting the well-known relations linking power and thrust to the wind speed, air density, tip-speed ratio and pitch angle.

For example, the estimated power coefficient Cpest may be obtained on the basis of equation (<NUM>): <MAT> where: Powerloss is the electrical power generated by the rotor <NUM> but lost as heat within the generator <NUM> and any other part of the wind turbine (such as the drive train including any gearbox); Powergrid is the electrical power delivered to the grid by the generator <NUM>; ρ is the air density; A is the swept area of the rotor <NUM>; and V is the wind speed.

Similarly, the estimated thrust coefficient Ctest may be obtained on the basis of Equation (<NUM>): <MAT> where: ThrustForce is the thrust force generated by the rotor <NUM>; ρ is the air density; A is the swept area of the rotor <NUM>; and V is the wind speed.

The ThrustForce parameter in equation (<NUM>) may be obtained in the same as the ThrustForce parameter in equation (<NUM>), using the method shown in <FIG> including normalisation based on the rotor speed.

Alternatively, rather than normalizing based on rotor speed, the ThrustForce parameter in equation (<NUM>) may be normalized versus the rated thrust force, i.e. the force acting on the rotor, in power curve conditions, when producing rated power at rated wind speed.

The theoretical coefficients Cptheo and Cttheo may be obtained by inputting the current tip-speed ratio and blade pitch angle into a look-up-table which is generated during blade design.

The stall parameter γ2 may then be compared with a threshold to determine the stall condition. For instance, the stall condition may indicate that the rotor <NUM> is not stalled when the stall parameter γ2 exceeds the threshold, and/or the stall condition may indicate that the rotor <NUM> is stalled when the stall parameter γ2 is below the threshold.

The stall parameter γ2 in Equation (<NUM>) is mathematically equivalent to the stall parameter γ1 in Equation (<NUM>) multiplied by a factor which varies on the basis of the operating point of the rotor. In other words, the stall parameters γ1 and γ2 are related according Equation (<NUM>): <MAT>.

Thus Equation (<NUM>) is mathematically equivalent to comparing the stall parameter γ1 from Equation (<NUM>) to a threshold, and varying the threshold on the basis of the operating point of the rotor - for example varying the threshold on the basis of <MAT>.

<FIG> shows how Equation (<NUM>) may be implemented. Ctest and Cttheo are divided at step <NUM>; Cpest and Cptheo are divided at step <NUM>; and the outputs are divided at step <NUM> to generate the stall parameter γ2 which is then compared with a threshold.

The methods of <FIG> take advantage of the fact that during stall the power reduces and the thrust increases (or does not decrease). By using both a power parameter and a thrust parameter to obtain the stall parameter γ1 or γ2, the reliability of the measurement is improved compared with a method which only analyses the power or thrust.

In further methods shown in <FIG>, the stall condition of the rotor is determined by obtaining a time series of measurements of a performance parameter indicative of a performance of the rotor (for instance Cp, Ct or similar), each measurement in the time series being associated with an operating point of the rotor (for instance tip speed ratio, blade pitch angle or similar); analysing the time series of measurements to obtain a statistical measure of a sensitivity of the performance parameter with respect to the operating point of the rotor (for instance standard deviation, variance, root-mean-square, sum of squares etc.); and comparing the statistical measure with a threshold to determine the stall condition of the rotor.

The methods of <FIG> take advantage of the observation that when operating in deep stall, the fundamental sensitivities such as <MAT> or <MAT> increase significantly compared to operating outside deep stall.

In the case of <FIG>, a stall parameter γ3 is calculated according Equation (<NUM>): <MAT> where: Cp is a power coefficient based on a current operating point of the rotor; θ is a blade pitch angle; and std is a standard deviation function implemented in step <NUM> in <FIG>.

In the case of <FIG>, a stall parameter γ4 is calculated according Equation (<NUM>): <MAT> where: Cp is a power coefficient based on a current operating point of the rotor; λ is a tip speed ratio; and std is a standard deviation function implemented in step <NUM> in <FIG>.

In the case of <FIG>, a stall parameter γ5 is calculated according Equation (<NUM>): <MAT> where: Cp is a power coefficient based on a current operating point of the rotor; θ is a blade pitch angle; and rms is a root-mean-square function implemented in step <NUM> in <FIG>.

In the case of <FIG>, a stall parameter γ6 is calculated according Equation (<NUM>): <MAT> where: Cp is a power coefficient based on a current operating point of the rotor; λ is a tip speed ratio; and rms is a root-mean-square function implemented in step <NUM> in <FIG>.

In the case of <FIG>, a time series of measurements of Cp is obtained, each measurement in the time series being associated with a respective blade pitch angle (θ); and the time series is analysed to obtain a statistical measure of a sensitivity of Cp with respect to θ.

In the case of <FIG>, a time series of measurements of Cp is obtained, each measurement in the time series being associated with a respective tip speed ratio (λ); and the time series is analysed to obtain a statistical measure of a sensitivity of Cp with respect to λ.

The power coefficient Cp in Equations (<NUM>) to (<NUM>) may be the same as the parameter Cpest in Equation (<NUM>), and may be obtained on the basis of Equation (<NUM>).

The sensitivity <MAT> could be obtained by changing the pitch, and the sensitivity <MAT> could be obtained by correlating the wind speed (keeping the pitch constant) to variation of Cp.

In the case of <FIG>, the time series of measurements are measurements of Cp, but in other embodiments they may measure another performance parameter such as thrust coefficient Ct.

The stall parameter γ3, γ4, γ5 or γ6 may then be compared with a threshold to determine the stall condition. For instance the stall condition may indicate that the rotor <NUM> is stalled when the stall parameter γ3, γ4, γ5 or γ6 exceeds the threshold, and/or the stall condition may indicate that the rotor <NUM> is not stalled when the stall parameter γ3, γ4, γ5 or γ6 is below the threshold.

Once a stall parameter γ1- γ6 is chosen from the list above, its values are compared to a threshold. In some embodiments of the invention, when the stall parameter crosses the threshold, the pitch angle of the wind turbine blades is immediately adjusted in step <NUM> or step <NUM> in response to the change in the stall condition.

In another embodiment of the invention, shown in <FIG>, any potential change in the stall condition of the rotor is confirmed by a statistical change detection algorithm before the pitch angle of the wind turbine blades is changed. Thus the stall condition is determined by obtaining a time series of measurements of the wind turbine; and analysing the time series of measurements by a statistical change detection algorithm to determine whether the rotor has stalled. The use of a statistical change detection algorithm provides a more reliable indication of a change in the stall condition than a single stall condition measurement.

In the example of <FIG>, once a first threshold (Threshold_1) is exceeded (in the case of γ3, γ4, γ5 or γ6) or dropped below (in the case of γ1 or γ2) a "leaky bucket" counter counts up, down otherwise. Once a second threshold (Threshold_2) on the counter state is exceeded, deep stall is detected.

The stall parameter is compared with a first threshold (Threshold_1) at step <NUM>. If Threshold_1 is exceeded by the stall parameter (in the case of γ3, γ4, γ5 or γ6) or the stall parameter is less than Threshold_1 (in the case of γ1 or γ2) then the step <NUM> outputs a logical "true" which indicates that the rotor has potentially stalled. Otherwise the step <NUM> outputs a logical "false".

A counter is incremented at step <NUM> when the step <NUM> outputs a logical "true", and decremented at step <NUM> when the step <NUM> outputs a logical "false".

A Min/Max function <NUM> places a maximum limit (Max) on the counter, and a minimum value of zero. The counter is initialised as zero. So if there is no potential stall for the first sample, then the counter is decremented to -<NUM> but the Min/Max function <NUM> constrains the counter so that it remains at zero. This situation remains for each sample until a potential stall is detected, and the counter is incremented to <NUM>.

The counter is divided by Max at step <NUM>, and the output <NUM> is compared with a second threshold (Threshold_2) at step <NUM>. For example, if Max=<NUM> and Threshold_2 is <NUM>, then the output <NUM> is <NUM> and Threshold_2 is not exceeded.

As an example, if a potential stall is detected for the next eight samples, then the output <NUM> remains below Threshold_2 until the counter has reached <NUM> on the ninth sample, the output <NUM> has increased to <NUM>, and Threshold_2 is exceeded so the threshold comparison step <NUM> outputs an indication that the rotor is stalled, confirming a change in the stall condition from "not stalled" to "stalled".

As soon as the counter is decremented to <NUM>, the output <NUM> no longer exceeds Threshold_2 so the threshold comparison step <NUM> outputs an indication that the rotor is not stalled.

Thus, the stall condition output by the threshold comparison step <NUM> provides a binary estimation of whether or not the rotor has stalled, which is more reliable than the potential stall condition output by the threshold comparison step <NUM>.

The process can be fine-tuned by choosing appropriate values for the various parameters (Max, the size of increment/decrement at steps <NUM> and <NUM>, Threshold_1 and Threshold_2).

The process of <FIG> implements a leaky bucket algorithm, which includes a calculation of a cumulative sum (in the form of a counter) which is compared with a threshold. Other statistical change detection algorithms may be used which involve the calculation of a cumulative sum, for instance a CUSUM algorithm.

Claim 1:
A method of measuring a stall condition of a rotor of a wind turbine (<NUM>), the method comprising:
measuring a power parameter (<NUM>) indicative of a power generated by the rotor (<NUM>);
measuring a thrust parameter (<NUM>) indicative of a thrust force generated by the rotor (<NUM>);
obtaining a stall parameter (γ) on the basis of the power parameter and the thrust parameter, obtaining the stall parameter comprising dividing the power parameter and the thrust parameter; and
comparing (<NUM>) the stall parameter with a threshold to determine a binary stall condition of the rotor, estimating of whether or not the rotor has stalled.