Patent Description:
Wind turbines are generally regarded as an environmentally safe and a desirable source of renewable energy. In summary, a wind turbine harnesses the kinetic energy of wind and transforms this kinetic energy into electrical energy. Thus, electrical power can be generated with virtually zero emissions, unlike existing natural gas-fired or coal-fired power generation technologies. To maximize the efficacy of power generation and to simplify connection to a power grid, several wind turbines are often located in proximity to one another in what are generally referred to in the pertinent art as "wind farms. " Advantageously, these wind farms are located in regions having relatively strong winds, such as, for example, at offshore locations.

At offshore locations, in order to better access the prevailing winds around the year and to limit visibility from the shore, it is desirable to install wind farms at increasing distances from the shore, and consequently deeper water depths. The typical foundation structure for an offshore wind turbine installation comprises a monopile upon which a tower is secured. A monopile is essentially a long cylindrical caisson, assembled in sections on-shore and driven to the required soil penetration depth at the offshore installation site.

In order to keep wind energy economically competitive with traditional and other renewable energy sources the cost of energy (COE) must be low. Today's offshore wind turbines rely on sophisticated load control systems to assure optimal operation and achieve low COE. Structural and fatigue loads are key factors in turbine design and the management of these loads could create a significant decrease in turbine cost by reducing required materials, lessening scheduled and unscheduled maintenance, and improving overall turbine reliability.

Load management systems of offshore wind turbines typically rely on the knowledge of the modal characteristics of the wind turbine. The modal characteristics of offshore wind turbines vary widely between design and actual values, and can even change drastically, on a daily basis, based on sea and wind conditions (e.g., sea-bed conditions, scour formation, marine growth, tidal variations, etc.). Structural parameters are needed for optimum site and turbine-specific tuning of turbine controls. For commissioning of large wind farms with several wind turbines, it is not practical to manually identify these structural parameters. In particular, turbine-specific structural parameters are needed for frequency-avoider closed loop algorithms. There is usually a deviation between the as-designed and actual values of these system parameters due to variability in site-conditions, manufacturing tolerances, etc. In addition to spatial variability, there tends to be temporal evolution of these structural parameters over time due to changes in sea-bed conditions, scour formation, marine growth, tidal variations, etc. Accordingly, it is important to identify/mitigate response to these changes in a continuous manner to ensure expected system performance and reliability.

Relevant background art is disclosed in for instance<NPL>, <NPL> and <NPL>.

Therefore, there is a need to design an automated procedure to identify the modal characteristics of an offshore wind turbine in a continuous manner to ensure optimum site and turbine-specific tuning of turbine controls.

Briefly, in accordance with one aspect of the present technique, provided is an automated method to determine modal characteristics of a wind turbine in a continuous manner, according to claim <NUM>.

In accordance with yet another aspect, provided is a wind turbine. The wind turbine includes a tower, a monopile upon which the tower is secured, one or more sensors arranged on at least one of the tower and the monopile for producing one or more sensor data signals and a turbine control system including a processor for processing the one or more sensor data signals to determine modal characteristics of the wind turbine in a continuous manner. The processor includes an algorithm configured to perform the automated method of claim <NUM>.

Other objects and advantages of the present disclosure will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.

It is noted that the drawings as presented herein are not necessarily to scale. The drawings are intended to depict only typical aspects of the disclosed embodiments, and therefore should not be considered as limiting the scope of the disclosure.

Here and throughout the specification and claims, range limitations may be combined and/or interchanged. Such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.

As used herein, the terms "processor" and "computer" and related terms, e.g., "processing device" and "computing device", are not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. In the embodiments described herein, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM), and a computer-readable non-volatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, additional input channels may be utilized, but are not limited to, computer peripherals associated with an operator interface such as a mouse and a keyboard. Alternatively, other computer peripherals may also be used that may include, for example, but not be limited to, a scanner. Furthermore, in the exemplary embodiment, additional output channels may be utilized, but are not be limited to, an operator interface monitor.

Further, as used herein, the terms "software" and "firmware" are interchangeable and include any computer program stored in memory for execution by personal computers, workstations, clients and servers.

As used herein, the term "non-transitory computer-readable media" is intended to be representative of any tangible computer-based device implemented in any method or technology for short-term and long-term storage of information, such as, computer-readable instructions, data structures, program modules and sub-modules, or other data in any device. Therefore, the methods described herein may be encoded as executable instructions embodied in a tangible, non-transitory, computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. Moreover, as used herein, the term "non-transitory computer-readable media" includes all tangible, computer-readable media, including, without limitation, non-transitory computer storage devices, including, without limitation, volatile and nonvolatile media, and removable and non-removable media such as a firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal.

Furthermore, as used herein, the term "real-time" refers to at least one of the time of occurrence of the associated events, the time of measurement and collection of predetermined data, the time to process the data, and the time of a system response to the events and the environment. In the embodiments described herein, these activities and events occur substantially instantaneously.

Although exemplary embodiments of the present disclosure will be described generally in the context of an offshore wind turbine, for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present disclosure may be applied to any wind turbine structure, such as land-based wind turbines, and is not intended to be limiting to offshore structures.

Turning now to the drawings, <FIG> illustrates an offshore wind turbine system <NUM> in accordance with an exemplary embodiment of the present technique. The wind turbine system <NUM> includes a wind turbine generator <NUM> comprising a rotor <NUM> having multiple blades <NUM>. Various electrical and mechanical components of the wind turbine generator <NUM>, such as the drive train, electrical generator, etc. are housed in a nacelle <NUM>. The rotor <NUM> and the nacelle <NUM> are mounted atop a tower <NUM> that exposes the blades <NUM> to the wind. The blades <NUM> transform the kinetic energy of the wind into a rotation motion of a shaft that drives a generator (not shown) to produce electrical power.

The tower <NUM>, the nacelle <NUM> and the rotor <NUM> are mounted on a foundation structure <NUM>. In the illustrated embodiment, the foundation structure <NUM> includes a monopile or central caisson <NUM> upon which the tower <NUM> is secured. The monopile <NUM> is a cylindrical column, extending from the tower <NUM> to a depth 'di' below a water level <NUM>. The monopile <NUM> is driven into the soil to a depth 'd<NUM>' below a soil surface <NUM>, also referred to as mud line. In an embodiment, the monopile <NUM> may be driven, for example <NUM>-<NUM> meters below the mud line <NUM>. The monopile <NUM> is configured to support the dead weight of the tower <NUM>, nacelle <NUM> and rotor <NUM>, axial (vertical direction) loads resulting. In an embodiment, the foundation structure <NUM> may include one or more mooring lines (not shown) coupling the monopile <NUM> to an anchor (not shown) or the like.

One or more tower acceleration sensors <NUM> are arranged on the tower <NUM>, such as near the top of the tower, or at any other location on the tower. Other acceleration sensors may also be arranged at other locations on the tower <NUM> and/or at other locations on the wind turbine system <NUM> for measuring lateral and vertical vibrations. Each of the one or more acceleration sensors <NUM> includes a motion sensor for measuring acceleration in one or more dimensions. For example, the acceleration sensors <NUM> may be tri-axial or biaxial, measuring lateral and longitudinal vibrations in the time domain. As alluded to, other process variables besides vibration, such as displacement, velocity, temperature, strain and/or pressure, may also be similarly sensed at various turbine locations in a similar manner.

Vibrations in the wind turbine structural components, including the foundation structure <NUM>, tower <NUM>, the nacelle <NUM> and the rotor <NUM> components, may considerably reduce the life of the components and/or lead to early fatigue failures. For example, the monopile <NUM> should be desirably designed such that the overall natural vibration frequency of the foundation structure <NUM> is outside the frequency range of excitation due to the rotor operation and the hydrodynamic wave loading. Accordingly, a control system <NUM> is provided that may receive input from the one or more acceleration sensors <NUM> and includes one or more processors, such as microcontrollers, which provide signals in response to the received sensor data to control a variable pitch blade drive and/or other components of the wind turbine system <NUM>. The acceleration sensors <NUM> are arranged to communicate with the control system <NUM> via wired and/or wireless means.

Loads management and turbine operability strategies based on frequency avoidance rely on that frequency being known to controller. In many turbine control systems for onshore wind turbines, the vibrations are typically measured with respect to a stationary reference point using accelerometers arranged at critical locations on the components of interest. These types of turbine control systems are not trying to identify modal characteristics, but to modify the behavior of the turbine without explicitly computing modal frequencies. In other instances, turbine control systems for both onshore and offshore wind turbines rely on the knowledge of modal characteristics of the design as compared to an actual value and do not consider varying operating conditions in a continuous manner. Typically, data is collected during turbine commissioning and processed by a field engineer through a Power Spectral Density-based tool. Structural modes/frequencies are identified during this process and input to a real-time turbine control system manually during commissioning and/or maintenance. As previously indicated, while for some applications this may suffice, control of offshore wind turbines requires the identification of modal characteristics in a continuous manner to ensure expected system performance and reliability in light of varying operating conditions. In many instances, the natural frequencies of the offshore wind turbine may vary widely between design and actual values, and certain modes may vary daily.

Disclosed is an exemplary method of determining modal characteristics of wind turbine system, applicable to both land-based and offshore wind turbines. The method includes the steps of: (i) reading all available signals; (ii) prefiltering the signals and dividing the signals into homogeneous segments; (iii) obtaining a frequency domain representation of each segment via a Power Spectral Density (PSD); (iv) assigning a probability to each peak in the PSD; (v) combining all probabilities from all peaks coming from all sensors; and (vi) computing the most likely peaks.

Referring more specifically to <FIG>, illustrated is a flowchart illustrating an exemplary automated method of determining modal characteristics of the wind turbine system of <FIG>, in accordance with one or more embodiments shown or described herein. More particularly illustrated is an automated method <NUM> in which sensor data signals <NUM> from the one or more sensors <NUM> are input into an algorithm <NUM> that provides for the identification of the modal characteristics of a wind turbine in a continuous manner. The algorithm <NUM> may be implemented as part of the microcontroller or other processor that is arranged local to or remote from for the wind turbine system <NUM>. As previously alluded to, the processor is inclusive of software and/or firmware stored in the processor memory and or non-transitory computer-readable media.

As an overview of the method, in an initial step of the method <NUM>, one or more sensor data signals <NUM> are input for prefiltering <NUM> and detection of operational condition <NUM>. During the step of prefiltering <NUM>, the sensor data signals <NUM> are divided into homogeneous segments as described presently. In addition, any portion of the one or more sensor data signals <NUM> that is not needed can be filtered out. It is additionally anticipated by this disclosure that other types of initial signal processing, in addition to filtering, may also be used, such as amplification and/or noise reduction. In a next step <NUM>, a frequency domain representation of each segment is computed via a Power Spectral Density (PSD). For each signal, the set of computed PSDs is used to determine the probability that a resonant peak exists versus noise in the data, in a peak detection step <NUM>. The probabilities from all resonant peaks coming from all sensors <NUM> are then combined and the most likely resonant peaks are computed during a post-processing step <NUM>. The likelihood that a particular structural mode of interest corresponds to that resonant peak can then be determined.

Referring now to <FIG>, during the operational condition detection in step <NUM>, multiple variables are considered to aid in segmenting the obtained sensor data signals <NUM>. The variables may include an internal variable defining the turbine operating mode or state <NUM>, such as standstill, startup, stop, below rated, above rated, or the like. In addition, the yaw angle <NUM>, wind turbulence <NUM>, wind speed <NUM>, rotor or generator speed <NUM> and tower/nacelle/hub/external temperature and humidity conditions <NUM> may be considered.

As previously stated, during this step, the sensor data signals <NUM> are divided into homogeneous segments as best illustrated in <FIG>. More particularly, illustrated graphically, for exemplary purposes, are plotted a generator speed signal <NUM>, an operational condition signal <NUM> and a rotor speed signal <NUM>. In the exemplary illustration, each homogeneous segment <NUM> is subdivided into an incremental time segments <NUM>, <NUM>, <NUM>. In an embodiment, incremental time segments <NUM>, <NUM> and <NUM> may be between <NUM>-<NUM> minutes in length (?), and preferably <NUM>-minutes in length. As further illustrated in <FIG>, the time segments <NUM>, <NUM>, <NUM> may be further subdivided if the obtained sensor data signals <NUM>, and more particularly, the generator speed signal <NUM>, the operational condition signal <NUM> and the rotor speed signal <NUM>, are not in a steady state regime. In the illustrated graph of <FIG>, the time segments <NUM>, <NUM>, <NUM> are further split into segments <NUM>, such as 1a, 1b, 3a, 3b, 3c, based on rotor speed and/or rotor speed variability. These data segments can then be collected / binned based on the operating condition, etc. described above. Subsequent to identification of the segments, an independent PSD of every segment is performed as described presently.

Referring now to <FIG>, the prefiltering of the data signals <NUM> provides for the selection of the incremental time segments <NUM>, <NUM>, <NUM> based on wind turbulence and operating conditions and removes trends. As illustrated, during the prefiltering, all recorded data is input, in a step <NUM>, and one or more valid incremental time segments, such as incremental time segments <NUM>, <NUM>, <NUM> are output, in a step <NUM>, whose power spectral density (PSD) is considered reliable. During the prefiltering of the obtained data signals <NUM>, frequencies and/or times which are not of interest may be excluded. The "filtered differential vibration signal" obtained during the step of prefiltering <NUM> is then sent for computing of a PSD.

Next, as best illustrated in <FIG>, the incremental time segments <NUM>, <NUM>, <NUM> corresponding to an operating condition are input, in a step <NUM> to compute a PSD in step <NUM>, and output the PSD and corresponding frequency vector, in a step <NUM>. According to the invention, the PSD is computed by using Welch's averaged modified periodogram method of spectral estimation, also noted herein as the pWelch technique. By default, there is a <NUM>% overlap, Hann window and frequency resolution of <NUM>.

Referring now to <FIG>, in the peak detection step <NUM>, resonant peaks in a particular signal are identified, according to a set of user-specified criteria. A potential resonant peak is given a likelihood value depending on how many of these user-specified criteria it fulfils. To provide such, the power spectral density whose peaks need to be identified (frequency vector for said power spectral density and time series of the speed of the turbine) are input, in a step <NUM>, and a list of detected resonant peaks (value, frequency and likelihood) are output, in a step <NUM>.

In the post-processing step <NUM>, as illustrated in <FIG>, the peaks from all the sensors <NUM> (<FIG>) are merged. More particularly, a list of peaks from all sensors (PCH, generator speed, other accelerometers/strain gauges at the tower, or the like) are input in a step <NUM>, and a unique list of resonant peaks according to likelihood are output, in a step <NUM>. With respect to the determination of the likelihood of the identified peaks, the determination may be made based on boolean (criterion is fulfiled or not) vs quantitative metrics. If boolean, an assignment of likelihood may use <NUM>/<NUM> whether a particular criterion is satisfied (min height, theoretical model, min distance between peaks, etc.). If quantitative, an assignment of likelihood may be based on the peak height, how close it is to an nP component or to a theoretical and/or operating conditions. Then, all criteria all added up and ranked accordingly. The likelihoods may be updated based on expected theoretical modes: type of sensor, operating condition and closeness to an expected mode. A determination is then made as to the likelihood that a particular structural mode of interest corresponds to the resonant peak.

As an example, during the post-processing step <NUM>, and with reference to <FIG>, if the process is started for a new turbine that was just installed, the process would start from the assumption the frequency of these modes (modal frequency) in a priority sense (would likely come from a design model of the wind turbine for example), as illustrated theoretical values <NUM>. As the obtained sensor data signals <NUM> are processed by the algorithm <NUM>, it may be determined that the frequencies of the modes for that particular turbine are different than what is expected and the theoretical values <NUM> would be replaced with the values <NUM> that are computed via the algorithm <NUM>. In an embodiment, a best guess at what the modal frequencies are is stored, so that a likelihood that a modal frequency has changed or shifted can be determined. As an example, initially the theoretical values may be used, and based upon confidence that the values have changed from their theoretical values, the values provided by the post-processing step <NUM> are used.

The above described technique enables the automated identification of modal characteristics of a wind turbine in either an offshore location or onshore location based on measurements from different sensors and operating conditions of the turbine. The method provides for the automated identification of modal characteristics in light of deviations between the as-designed and actual values of turbine-specific structural parameters due to temporal evolution of site-conditions and manufacturing tolerances. The method further provides for the identification and mitigation of a response to the changes in modal characteristics in a continuous manner to ensure expected system performance and reliability.

Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

Claim 1:
An automated method to determine modal characteristics of a wind turbine in a continuous manner, the automated method comprising:
reading one or more sensor data signals (<NUM>), wherein the one or more sensor data signals include vibration signals from one or more structural components of the wind turbine, wherein the one or more structural components of the wind turbine include a foundation structure (<NUM>), a tower (<NUM>), a nacelle (<NUM>) and a rotor component (<NUM>);
prefiltering the one or more sensor data signals (<NUM>) to divide the one or more sensor data signals into a plurality of time segments (<NUM>, <NUM>, <NUM>);
obtaining a frequency domain representation of each of the plurality of time segments (<NUM>, <NUM>, <NUM>) by computing a Power Spectral Density (PSD) of each of the plurality of time segments using Welch's averaged modified periodogram method of spectral estimation and a Hann window function to determine the probability that a resonant peak exists:
identifying one or more resonant peaks in each of the plurality of time segments (<NUM>, <NUM>, <NUM>);
combining all assigned probabilities; and
determining the likelihood of the one or more resonant peaks as an indicator of the modal characteristics of the wind turbine.