Method to individually optimize respective pitch angles of a plurality of blades in a wind turbine

A method to individually optimize respective pitch angles of blades in a wind turbine. The method allows supplying (102) a modulation signal to modulate a pitch angle of the individual blade being optimized. The method further allows filtering (104) output power of the turbine subject to the modulating frequency to extract a power signal in a frequency spectrum including the modulating frequency. The extracted power signal and a demodulation signal are mixed (106) to generate a signal including a product of the extracted power signal and the demodulation signal. The generated signal is filtered (108) to remove the modulating frequency and extract a gradient signal effective to search a pitch angle estimate conducive to optimize output power of the turbine. The gradient signal may be integrated (110) to cause the gradient signal to converge towards an optimal pitch angle for the individual blade being optimized.

FIELD OF THE INVENTION

The present invention is related to wind turbines for generating electric power, and, more particularly, to adaptive control techniques for individually optimizing respective pitch angles of a plurality of blades in a wind turbine.

BACKGROUND OF THE INVENTION

Renewable energy has become a major focus for energy and environment sustainability. Wind is an example of an appropriate energy source for utility-level power generation. The power generation for wind turbines may be substantially affected by the aerodynamic characteristics of wind-turbine interaction, such as blade pitch angle. Individual blades in a wind turbine may have different optimum pitch angle settings due to variations that may occur during the manufacturing and/or the installation of the individual blades. It is known that upon turbine commissioning, each blade may be manually calibrated to remove this potential error, but this generally entails burdensome and/or time-consuming tasks. Additionally, over time it is possible that the pitch angle setting obtained from such calibration may no longer be optimal due to various conditions that may affect the blades, such as dirt, ice, wear, etc.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments of the present invention. However, those skilled in the art will understand that embodiments of the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, to avoid pedantic and unnecessary description, well known methods, procedures, and components have not been described in detail.

Furthermore, various operations may be described as multiple discrete steps performed in a manner that is helpful for understanding embodiments of the present invention. However, the order of description should not be construed s to imply that these operations need be performed in the order they are presented, nor that they are even order dependent. Repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.

It is known to adjust the pitch of the blades of a wind turbine in response to changing environmental conditions. However, such adjustments change the pitch of all of the blades of the turbine at the same time and by the same amount. The present inventor has thus recognized that it would be desirable to provide reliable and cost-effective adaptive control techniques arranged to individually optimize respective pitch angles of a plurality of blades in a wind turbine, since optimization for the group of blades most often misses the optimization of any given one of the individual blades due to variations between blades. If an individual blade optimization technique can be automated, it can be applied to the turbine as often as changing conditions dictate to ensure that the turbine operates at its highest efficiency. To accomplish this improvement, the inventor induces a modulation with a known signature (e.g. frequency or other pattern) to the pitch angle of one blade and then monitors the change in power produced by the turbine to extract information regarding the effect of the modulation. The inventor has recognized that the signature of a blade pitch modulation is reflected in the change in the power output signal of the turbine, and that the magnitude of the reflected signature will change depending upon how far the blade is from its optimal pitch angle. When a blade is at a non-optimal pitch, the modulation will produce a variation in the power output signal, and that variation will have a slope relative to the modulation that reflects whether the blade has too much pitch angle or too little pitch angle. Furthermore, an integral of that variation will become smaller and will approach zero as the actual pitch angle of the one blade approaches the optimal pitch angle for that specific blade. The extracted slope information provides information for adjusting the pitch angle to a more optimal position and for recognizing an optimal position for that blade once it is obtained. This method can be applied to each blade individually and sequentially to achieve an overall optimization for the wind turbine. Alternatively, because the modulation frequency is recognizable in the power output signal, it can be applied simultaneously to multiple blades by modulating the pitch angle of each respective blade at a distinct modulating signature, thereby providing an individual optimal pitch solution for each specific blade.

FIG. 1is a perspective side view of an example wind turbine10that may benefit from aspects of the present invention. In this example embodiment, wind turbine10is a horizontal axis wind turbine. Alternatively, wind turbine10may be a vertical axis wind turbine. Wind turbine10may include a tower12that may extend from a supporting surface15, a nacelle16coupled to tower12, and a rotor18coupled to nacelle16. Rotor18includes a rotatable hub20and a plurality of rotor blades22coupled to hub20. In this example embodiment, rotor18has three rotor blades22. It will be appreciated that rotor18may have any number of rotor blades22that enables wind turbine10to function as described herein.

As wind (schematically represented by arrow24) strikes each rotor blade22, lift forces are induced on each rotor blade22and rotation of rotor18about a rotation axis14is induced as each rotor blade22is accelerated. A pitch angle of rotor blades22, i.e., an angle that determines a perspective of each rotor blade22with respect to the direction of wind24, may be controlled in response to a pitch control system30(FIG. 2) embodying aspects of the present invention. For example, increasing pitch angle of a given rotor blade may decrease a blade surface area (e.g., lowers the airfoil angle of attack and thus may lower the lift force exerted on the blade) of the given rotor blade. Conversely, decreasing a pitch angle of a given blade may increase the blade surface area (e.g., may increase the airfoil angle of attack and thus increases the lift force in the absence of stall) of the blade. A pitch angle of rotor blades may be adjusted about a pitch axis28for each blade22. In one preferred embodiment, the pitch angles of rotor blades may be controlled individually.

FIG. 2is a block diagram of a pitch control system30embodying aspects of the present invention. In one example embodiment, pitch control system30may be effective to perform a method to individually optimize respective pitch angles of a plurality of blades in a wind turbine. A modulator32may be coupled to supply to an adder50a modulation signal34having a predefined modulating frequency (e.g., frequency f1) to modulate at the modulating frequency a pitch angle of a respective one of the plurality of blades being presently optimized. In one example embodiment, modulation signal34may be a sinusoid. It will be appreciated, however, that any periodic signal (triangular wave, square wave, etc.), or even random signals may be effectively utilized to modulate the pitch angle of the blade being optimized. As a practical consideration, in one example embodiment the frequency of the modulation frequency may be selected to be proximate to a time constant of the rotor. For example, presuming the time constant of the rotor is 8 seconds, then the modulating frequency may be chosen to be approximately ⅛=0.125 Hz. It will be appreciated that numerical examples that may be described throughout this disclosure should be construed in an example sense and not in a limiting sense since inventive aspects of the present invention are in no way restricted to any specific numerical example.

In one example embodiment, a high-pass filter36(labeled HPF) may be coupled to filter output power of wind turbine10subject to the modulating frequency to extract a power signal (e.g., free of a DC component) in a frequency spectrum comprising at least the modulating frequency. For example, high-pass filter36may be set to pass at least the modulating frequency f1.

In one example embodiment, a mixer38may be coupled to high-pass filter36to mix the power signal extracted from high-pass filter36and a demodulation signal42(e.g., as may be obtained from a demodulator40) oscillating at the modulating frequency (e.g., at frequency f1) to generate a signal comprising a product of the extracted power signal and the demodulation signal.

In one example embodiment, a difference in a phase angle (e.g., a lead or lag) of the extracted power signal relative to a phase angle of the demodulating signal may be estimated or otherwise measured to appropriately compensate for any such phase angle difference. This would allow providing a substantially in-phase relationship between the extracted power signal and the demodulation signal. That is, the phase of demodulation signal42should be substantially aligned (e.g., to be in phase or 180 degrees out-of-phase) relative to the phase of the filtered power signal extracted by high-pass filter36. The phase angle shift (e.g., lead or lag) which may be experienced by the pitch signal may be measured using any appropriate technique, such as measuring the lead/lag from the peak (or through) of the pitch signal to the peak (or through) of the filtered power signal.

In one example embodiment, a low-pass filter44(labeled LPF) may be coupled to filter the signal generated by mixer38to remove at least the modulating frequency and extract a gradient signal effective to search a pitch angle estimate conducive to optimize output power of the turbine. As a practical consideration, in one example embodiment, low-pass filter44may be set to have a cutoff frequency at approximately half the modulating frequency f1. In one example embodiment, an integrator46may be coupled to integrate the gradient signal to cause the gradient signal to converge towards a value indicative of an optimal pitch angle for the respective one of the plurality of blades being presently optimized. In one example embodiment, a gain48(labeled K) may be applied to the output from integrator46prior to being combined in adder50with modulation signal34. Adjusting gain48allows varying how fast the pitch may be adapted. One should start with a relatively low gain to prevent instability. The gain may be gradually increased to accommodate a faster convergence.

It will be appreciated by those skilled in the art that the foregoing concept of utilizing a gradient signal effective to search a pitch angle estimate conducive to optimize output power of the turbine is one example application of a self-optimizing control strategy based on Extremum Seeking Control (ESC). As will be appreciated by those skilled in the art, this is a self-optimizing control strategy effective to search for unknown and/or time-varying input parameters (e.g., pitch angle setting) for optimizing a performance index (e.g., power output of the wind turbine). ESC can be considered as a dynamic realization of the gradient search through the usage of a modulating (e.g., dithering) signal. For readers desirous of general background information regarding ESC, reference is made to Chapter 2 (Introduction to Extremum Seeking) of textbook titledStochastic Averaging and Stochastic Extremum Seeking, Communications and Control Engineering, by S. J. Liu and M. Krstic, ©Springer-Verlag London 2012, which is herein incorporated by reference.

FIG. 3is a flow chart100of one example embodiment of a method to individually optimize respective pitch angles of a plurality of blades in a wind turbine. Subsequent to a start step101, step102allows supplying a modulation signal34(FIG. 2) having a predefined modulating frequency (e.g., frequency f1) to modulate at the modulating frequency a pitch angle of a respective one of the plurality of blades being presently optimized. Step104allows filtering (e.g., high-pass filtering) output power of the wind turbine subject to the modulating frequency to extract a power signal in a frequency spectrum comprising at least the modulating frequency.

Step106allows mixing the extracted power signal and a demodulation signal42(FIG. 2) oscillating at the modulating frequency (e.g., frequency f1) to generate a signal comprising a product of the extracted power signal and the demodulation signal. As noted above, the signals being mixed, e.g., the high-pass filtered power signal and the demodulation signal, should have a substantially in-phase relationship between one another. Step108allows filtering (e.g., low-pass filtering) the generated signal to remove at least the modulating frequency and extract a gradient signal effective to search a pitch angle estimate conducive to optimize output power of the turbine. Step110allows integrating the gradient signal to cause the gradient signal to converge towards a value indicative of an optimal pitch angle for the respective one of the plurality of blades being presently optimized.

In one example embodiment, step112allows comparing the value of the integrated gradient signal relative to a predefined threshold value (Th). For example, further iterations may be needed to reach a zero or pre-determined near-zero value for the gradient signal and thus establish the optimal pitch angle for the respective one of the plurality of blades. The threshold value (Th) may be a numerical indication of how closely one may want to approach such zero value.

If the gradient signal is higher than threshold value (Th), step114allows adding a present value of the integrated gradient signal (or other defined amount) to the modulation signal to modulate an adjusted pitch angle at the modulating frequency and then iteratively performing steps104through112to meet the threshold value for the gradient signal and establish the optimal pitch angle for the respective one of the plurality of blades. Step116allows checking whether there is a further blade of the wind turbine that should be optimized next. If yes, then steps102through112may be performed next to establish the optimal pitch angle for the further blade. If there is no further blade that needs pitch angle optimization, then one proceeds to return step118. Accordingly, flow chart100is one example embodiment of an adaptive control technique to sequentially optimize the individual pitch angles of a plurality of blades in a wind turbine.

FIG. 4is a flow chart200of another example embodiment of a method to individually optimize respective pitch angles of a plurality of blades in a wind turbine. Subsequent to a start step201, step202allows concurrently supplying a plurality of modulating signals each having a different modulating frequency to individually modulate at a respective different modulating frequency each respective pitch angle of the plurality of blades. In an example case of a wind turbine with three blades, such as blades B1, B2and B3, the different modulating frequencies may be f1, f2 and f3. For example, modulating frequency f1 may be used to modulate the pitch angle of blade B1; modulating frequency f2 may be used to modulate the pitch angle of blade B2; and modulating frequency f3 may be used to modulate the pitch angle of blade B3. As elaborated in greater detail below, output power of the wind turbine subject to the different modulating frequencies may be processed to extract respective gradient signals effective to search respective pitch angle estimates conducive to optimize output power of the turbine.

The processing of output power of the wind turbine subject to the different modulating frequencies may involve the following: Step204allows filtering (e.g., high-pass filtering) the output power of the wind turbine subject to the different modulating frequencies to extract respective power signals in a frequency spectrum comprising at least the different modulating frequencies. Step206allows mixing each of the extracted power signals with a corresponding demodulation signal oscillating at a respective one of the different modulating frequencies to generate respective signals each comprising a product of a respective one of the extracted power signals and the corresponding demodulation signal. For example, the filtered power signal oscillating at frequency f1 would be mixed with a demodulating signal oscillating at frequency f1; the filtered power signal oscillating at frequency f2 would be mixed with a demodulating signal oscillating at frequency f2; and the filtered power signal oscillating at frequency f3 would be mixed with a demodulating signal oscillating at frequency f3. As noted above, the respective signals being mixed with one another, e.g., the extracted power signal and the demodulation signal should have a substantially in-phase relationship between one another.

Step208allows filtering (e.g., low-pass filtering) the respective generated signals to remove at least each different modulating frequency and extract respective gradient signals effective to search respective pitch angle estimates conducive to optimize output power of the turbine. Step210allows integrating the respective gradient signals to cause each gradient signal to converge towards respective values indicative of respective optimal pitch angles for each of the plurality of blades being concurrently optimized.

In one example embodiment, step212allows comparing the respective values of the gradient signals relative to a predefined threshold value (Th). For example, further iterations may be needed to reach a zero value for the respective gradient signals and thus establish the optimal pitch angles for blades B1, B2and B3. The threshold value (Th) may be a numerical indication of how closely one may want to approach such zero value. If the respective values of the gradient signals are higher than threshold value (Th), step214allows adding a respective present value of each integrated gradient signal to the modulation signals to modulate at each different modulating frequency respective present values of pitch angles for the plurality of blades and then iteratively performing steps204through212to meet the threshold value for the gradient signals and establish the respective optimal pitch angles for each of the plurality of blades prior to return step216. Accordingly, flow chart200is one example embodiment of a technique to contemporaneously optimize the individual pitch angles of a plurality of blades in a wind turbine.

In one example embodiment one may supply a modulating signal to modulate the pitch angle of a blade being optimized. One may process output power of the wind turbine to extract a gradient signal responsive to the modulating signal. The gradient signal may be integrated and the pitch angle may be changed in response to a value of the integrated gradient signal towards a value indicative of an optimal pitch angle for the blade being optimized. One may add the value of the integrated gradient signal to the modulation signal to modulate the changed pitch angle of the blade. Further iterations may be performed to reach or approximate a target value (e.g., zero value) for the gradient signal and thus establish the optimal pitch angle for the blade.

FIG. 5is a plot of an example pitch angle signal50of an individual blade where the pitch is subject to a modulation signal (e.g., dithering) as a function of time. In this example, the pitch angle of the individual blade contains an initial pitch angle error of approximately three degrees, which is gradually reduced over time to practically zero.

FIG. 6shows respective plots of an example modulating signal52and an example demodulating signal54, which exhibit relatively close phase alignment relative to one another.

FIG. 7is a plot of a high-pass filtered power signal55, as may be extracted from HPF36.

FIG. 8shows respective plots of an example mixer output signal56, as may be generated by mixer38; and an example gradient signal58as may be extracted from LPF44.

FIG. 9is a plot of an integrated signal60, as may be integrated by integrator46starting in this example at t=100 sec.

FIG. 10is a plot of example output power62of the wind turbine, which is gradually increased as the pitch angle of the individual blade is optimized.

It will be appreciated that aspects of an example inventive system—as may be used to individually optimize respective pitch angles of a plurality of blades in a wind turbine—and methods disclosed herein may be implemented by any appropriate processor system using any appropriate programming language or programming technique. The system can take the form of any appropriate circuitry, such as may involve a hardware embodiment, a software embodiment or an embodiment comprising both hardware and software elements. In one embodiment, the system may be implemented by way of software and hardware (e.g., processor, sensors, etc), which may include but is not limited to firmware, resident software, microcode, etc.

Furthermore, parts of the processor system can take the form of a computer program product accessible from a processor-usable or processor-readable medium providing program code for use by or in connection with a processor or any instruction execution system. Examples of processor-readable media may include non-transitory tangible processor-readable media, such as a semiconductor or solid-state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk—read only memory (CD-ROM), compact disk—read/write (CD-RAN) and DVD.

While various embodiments of the present invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.