Controlling a wind turbine using a modified power reference

Abstract

The invention relates to controlling a wind turbine to address varying drive train loads. This is obtained by determining a correction signal to be set as a control signal. A signal indicative of a speed of a generator and a signal indicative of a requested output power are received. The signal indicative of the speed is filtered to isolate frequencies in a selected disturbance band to generate a disturbance signal. The disturbance signal is phase shifted and combined with the requested output power to obtain the correction signal.

Description

TECHNICAL FIELD

[0001] The present invention relates to the control of wind turbines, and in particular, the present invention relates to controlling a wind turbine to address varying drive train loads. BACKGROUND

[0002] Conventional horizontal axis WTGs typically have a rotor comprising three blades equally spaced around a central hub. When driven by the wind, the blades rotate around a horizontal axis coinciding with the central hub to extract energy, and in doing so cover a large disc-like "swept area".

[0003] The wind conditions are typically not uniform across the swept area of the rotor, meaning that each blade will experience varying loads as it completes one full rotation. As such, for a given wind condition, there will be a particular orientation of the rotor at which the rotor experiences the maximum load, and a corresponding orientation at which the load on the rotor is minimal. For a rotor with three blades, the rotor will transition between the maximum and minimum load positions three times per full rotation of the blades, resulting in a cyclic load on the rotor at a frequency of three times the rotor frequency. This is referred to as "3P disturbance" or "3P load", where the rotor frequency is "1P". It will be appreciated that for a rotor with a different number of blades, this value scales accordingly, such that for example a rotor with two blades experiences a 2P load. Furthermore, particular wind conditions can promote other periodic loads.

[0004] This periodic disturbance will also be reflected in a cyclic load in the drive train of the WTG, which promotes fatigue, reducing the operational lifetime of the components of the drive train.

[0005] It is against this background that the present invention has been designed.

[0006] SUMMARY

[0007] The disturbance on the rotor propagates to the drive train to induce drive train load variations, which in turn results in fatigue exposure of the drive train. It is an object of the present invention to provide a solution to reduce the fatigue exposure of the drive train due to load variations on the rotor.

[0008] One aspect of the present invention provides a method of controlling a wind turbine, the wind turbine comprising a rotor having one or more blades and a generator, the rotor being connected to the generator by a drive train, the method comprising:

[0009] receiving a signal indicative of a speed of the generator (11) and receiving a signal indicative of a requested output power;

[0010] filtering the signal indicative of the speed to isolate frequencies in a selected disturbance frequency band to generate a disturbance signal;

[0011] phase shifting the phase of the disturbance signal to match the phase of the torque variation of the rotor at the frequency of the selected disturbance frequency band to generate a phase shifted signal;

[0012] generating a corrected signal as a combination of the phase shifted signal and the requested output power;

[0013] setting the corrected signal as a control signal for the wind turbine.

[0014] In a wind turbine, wind drives a rotor and thereby generates a rotor torque, which is transmitted via a drive train to a generator. The generator is electromagnetically coupled to the drive train to extract rotational energy, and this coupling generates a generator induced counter torque on the drive train. The present invention corrects the torque generated by the generator at the frequency of the selected disturbance frequency band, thereby reducing the torque variation between the torque disturbance of the rotor and the generator induced counter torque. The torque variation between the rotor side and the generator side of the drive train can lead to fatigue exposure, and by reducing the torque variation, the fatigue exposure is also reduced.

[0015] The steps of the method result in forming a corrected signal, which is combined with a signal indicative of a requested output power, such that the control signal will comprise a component oscillating in phase with the torque disturbance on the rotor. In this way, the control signal will comprise a component following the torque disturbance. The disturbance in the torque on the rotor will thus (at least partly) be transmitted through the drive train and into the electrical system of the generator and converter, and risk generating power disturbances. The torque disturbance can be injected into the power grid or handled by the power system of the turbine, e.g. by a storage system arranged to absorb disturbances. The present invention thereby reduces fatigue exposure on the drive train at the expense of increased output power disturbances. The method does not dampen the drive train resonance oscillations, but is an alternative way of handling the drive train oscillations. Embodiments of the present invention can be implemented in combination with or as an alternative to traditional drive train damping routines.

[0016] In a further aspect, the present invention relates to a wind turbine control system arranged to perform the method according to the first aspect, and to a computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of the first aspect.

[0017] The computer program product can be provided on a computer readable storage medium or downloadable from a communication network. The computer program product comprises instructions which, when loaded onto a data processing system in the form of a controller, cause the data processing system to execute the instructions.

[0018] Generally, the controller can be a unit or a collection of functional units including one or more processors, input / output interfaces, and a memory capable of storing instructions that can be executed by the processor(s).

[0019] Generally, the various aspects of the application can be combined and coupled in any way possible within the scope of the application. These and other aspects, features, and / or advantages of the application will become apparent to those of ordinary skill in the art upon examination of the following embodiments described herein in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0020] For a more complete understanding of the present application, it will be described, by way of example only, with reference to the following drawings in which like reference numerals represent similar elements and in which:

[0021] Figure 1 is a schematic illustration of a wind turbine;

[0022] Figure 2 is a schematic illustration of a rotor connected to a generator by a drive train;

[0023] Figure 3 is a schematic illustration of a functional computing element of an embodiment of the application; and

[0024] Figures 4 to 6 shows graphs of analog signals obtained with and without a signal processing module according to an embodiment of the application. DETAILED DESCRIPTION

[0025] Before embodiments of the technology are described in further detail, for the sake of providing a context for the present application, reference is made to Figure 1 A single WTG 1 that can be controlled according to embodiments of the application is described. It should be understood that Figure 1 The WTG 1 of is referred to herein only as an example, and embodiments of the application can be implemented into many different types of wind turbine systems and power plant architectures.

[0026] The illustrated WTG 1 is a three-bladed upwind horizontal axis wind turbine (HAWT), which is the most common turbine type in use. The WTG 1 includes a rotor 2 having three blades 3 extending radially from and equally spaced about a central hub 4, which is supported at the front of a nacelle 5 by its hub 4. It should be noted that while three blades are common, different numbers of blades can be used in alternative embodiments. The nacelle 5, in turn, is mounted at the top of a tower 6, which is fixed to a foundation (not shown) embedded in the ground.

[0027] The nacelle 5 contains a generator 7 driven by the rotor 2 to produce electrical energy. The generator 7 is connected to a power converter 8, which converts the output of the generator 7 to a form suitable for transmission to the power grid. The power converter 8 is connected to a transformer 9, which steps up the voltage of the power converter 8 output to a level suitable for transmission to the power grid. The transformer 9 is connected to a power grid 10, which is the electrical power distribution network to which the WTG 1 is connected. Figure 1The swept area is indicated by the dashed circle in the middle (not shown in the figure). Thus, the WTG 1 is able to generate power from the wind flow passing through the swept area of the rotor 2, thereby causing the blades 3 to rotate.

[0028] In this respect, the blades 3 cover a circular swept area in the middle, indicated by the dashed circle encompassing the tips of the blades 3. The swept area of a conventional grid-connected WTG typically has a diameter exceeding 80 meters, and thus, the wind conditions are not uniform throughout the swept area. As mentioned above, the spatial variation of the wind field within the swept area causes peaks and dips in the wind-induced load on the rotor 2 as it rotates, thereby risking disturbances to the rotor. Figure 1

[0029] Figure 2 The rotor 2 and the generator 20 are schematically illustrated, where the rotor is connected to the generator through a drive train 21, 22 and 23. The drive train is schematically illustrated by a low speed shaft 22 mechanically coupled to the rotor, a gearbox 21 and a high speed shaft 23 electromagnetically coupled to the generator 20. The generator output is typically connected to a grid, schematically illustrated by the line 24, via a converter, a transformer and additional electrical equipment (not shown). Although a gearbox is illustrated, it should be noted that many turbines operate without a gearbox, and embodiments of the present invention also cover turbines without a gearbox.

[0030] As mentioned above, the torque disturbance at the rotor 25 is coupled to the generator via the drive train, and will generate a disturbance at the generator torque 26. By amending the generator torque 26 with a control command of the generator, where the amendment is in phase with the rotor torque disturbance, the drive train fatigue exposure is reduced.

[0031] Figure 3 The general elements according to embodiments of the present invention are illustrated, which achieve an increase in torsional stiffness of the drive train by operating the generator in a way that the generator torque is relaxed at the disturbance band of the rotor torque disturbance.

[0032] A signal indicative of the speed ω G ,30 of the generator and a signal indicative of the requested output power P ref ,31 are received. In one embodiment, the signal indicative of the speed of the generator is a rotational speed measurement of the generator measured by a detector at the high speed shaft or by a detector in the generator.

[0033] As a pre-processing signal filter optional before applying a harmonic filter (H res ), a drive train notch filter NF 32 can be applied to remove the resonance frequency of the drive train. This removes the drive train frequency component, thereby removing the frequency component that is not related to the rotor disturbance.

[0034] The pre-processed signal is filtered by a harmonic filter (H res ​) is filtered. A resonant filter is implemented to isolate frequencies in a selected disturbance band to generate a disturbance signal 33.

[0035] The disturbance signal 33 is further filtered by one or more filters H n The disturbance signal is further filtered. The disturbance signal is phase shifted such that the resulting signal phase matches the phase of the rotor torque variation at frequencies in the selected disturbance band to generate a phase shifted signal 34.

[0036] The phase shifted signal 34 is optionally modified with a controller gain g and combined with a requested output power P ref to generate a modified signal 36, P* ref The modified signal is a combination of the phase shifted signal and the requested output power. This can be an addition or a superposition of the two input signals to the combiner 35.

[0037] The modified signal is set as a control signal for the wind turbine. In one embodiment, the modified signal is forwarded as a power reference for a converter controller.

[0038] Further embodiments will now be discussed Figure 3 with reference to the accompanying drawings.

[0039] The present invention takes as input a signal indicative of the speed of the generator. In Figure 3 embodiments, this is the generator speed ω G Typically, the signal can also be based on a rotor speed signal or a combination of a rotor speed signal and a generator speed signal. Furthermore, in yet another embodiment, the signal indicative of the speed of the generator can be based on a rotational load signal of the rotor axis.

[0040] Figure 3 Fig. illustrates a signal processing module 300 implemented according to an embodiment of the present invention.

[0041] Figure 3 Fig. illustrates that the requested output power can be a requested power reference. Typically, the signal indicative of the requested output power can be a requested output power or a requested generator torque (e.g. in the form of a power reference or a torque reference) where the torque reference relates to a target generator torque. The signal indicative of the requested output power is modified and the modified signal is set as a control signal for the wind turbine. The control signal will typically be a modified version of the signal indicative of the requested output signal, thus in embodiments, the control signal comprises a power reference or a torque reference for controlling the output power of the wind turbine.

[0042] As mentioned, an optional notch filter NF can be applied before filtering the signal to isolate frequencies in the selected disturbance frequency band. The input signal is filtered using a drivetrain notch filter, which removes the drivetrain's resonant frequency. The drivetrain notch filter is a band-stop filter with a narrow stop band centered on the drivetrain frequency to remove this frequency band from the signal, as the drivetrain is an important noise source in the measurement. By removing the drivetrain resonance, the corrected signal can more clearly be directed at the disturbance signal components.

[0043] The notch filter NF can also be configured to remove other frequencies corresponding to other known noise sources.

[0044] A harmonic filter (H res ) can be used to isolate frequencies in the selected disturbance frequency band to generate the disturbance signal. Such a filter is also often referred to as a band-pass filter and can be implemented in any suitable way.

[0045] In a general embodiment, the selected disturbance frequency band is chosen to match the frequency band of the torque variations of the rotor movement. In a general method, the frequency characteristics of the torque variations can be determined. However, the harmonic filter can specifically be implemented to isolate frequencies in the disturbance frequency band from a frequency band around an order of the rotor frequency. That is, to isolate signal components at the nP frequency, n being an integer. In one embodiment, the harmonic filter can be implemented as a band-pass filter centered on a selected order of the rotor frequency, with a bandwidth of a few percent, such as between 5% and 10%, many suitable band-pass filters directly provide such bandwidths. In an important embodiment, the target order of the rotor frequency is 3, such that the harmonic filter is implemented to isolate signal components in a frequency band centered on the 3P frequency. A three-bladed turbine will experience 3P loading in use due to the interaction between the blades and the spatially varying wind field within the swept area of the rotor 2.

[0046] The disturbance signal 33 obtained after applying the harmonic filter (H res ) will be in phase with the generator speed. This signal is phase shifted to match the phase of the torque variations of the rotor at the frequencies of the selected disturbance frequency band. The phase of the torque variations is the same as the phase of the position of the rotor. In one embodiment, the phase shift is such that the disturbance signal becomes perpendicular to the torque variations. For a signal that is the generator speed, the phase shift should be around -90° with respect to the 3P loading. A phase shift of -90° can be obtained by one or more low-pass filters, depending on the specific implementation of the low-pass filter. Alternative phase shift filters can be used if desired.

[0047] In the embodiment shown in Figure 3 , the phase-shifted signal is based on the generator speed to be combined with the power reference. It can therefore be desirable to adjust the amplitude of the signal, i.e. to further correct the phase-shifted signal with a signal gain g.

[0048] In embodiments, the signal gain can be a fixed gain or a gain set in dependence of an input or a state of the turbine. In one embodiment, the signal gain comprises a term based on the wind speed or based on the operating point of the wind turbine. In particular, if the control signal is a power reference and the disturbance is related to torque, a smaller gain can be beneficial for low rotor speeds, since power is equal to torque times generator speed.

[0049] As mentioned previously, a reduction of the drive train oscillations can migrate to the grid in the form of increased grid power disturbances. In case of intolerable levels of grid power disturbances, the signal gain g can be used to reduce the modification of the power reference. By determining the electrical disturbances at a measurement point of the electrical connection of the wind turbine to the grid, e.g. at the point of common coupling, the signal gain can be based on the determined electrical disturbances. That is, for high electrical disturbances, the gain is set lower.

[0050] In embodiments, a limit can be set on the size of the modification signal. Such a limit can be implemented by a saturation filter. A saturation filter cuts off signals above a certain threshold. A simple cut-off can cause undesired cut-off effects in the signal, and in one embodiment, the phase-shifted signal is dynamically range scheduled instead of applying a saturation filter. In dynamic range scheduling, the amplitude of the signal is scaled so that the peak value substantially matches the cut-off level.

[0051] Modern wind turbines are operated by many computer controllers that determine an actuator output signal based on an input signal. Many turbine controllers are implemented as PID type feedback controllers. PID type controllers are well known and reliable controllers. Wind turbines are very complex machines that are operated in a complex, varying and often unpredictable environment. Therefore, it can be advantageous to apply more advanced control techniques of the multiple-input-multiple-output (MIMO) type. One example of such an advanced controller is a controller based on the so-called model predictive control (MPC) method.

[0052] Model predictive control (MPC) is a multivariable control algorithm that uses an optimization cost function J to calculate the best control settings over a receding prediction horizon.

[0053] In one embodiment, the signal indicating the requested output power can be determined using a model predictive control (MPC) routine.

[0054] The MPC controller repeatedly calculates a predicted operating trajectory (sometimes also referred to as a prediction horizon). The MPC routine is a receding horizon control routine, which optimizes the current time slot k, while taking future time slots into account by using the current time slot as a control value and then repeatedly optimizing again to optimize a finite time horizon. The operating trajectory comprises a time series of at least one variable, here a signal value indicative of the requested output power.

[0055] In one embodiment, a predicted operating trajectory of a predicted signal indicative of the requested output power is calculated using a model predictive control (MPC) routine, wherein the trajectory comprises a time series of values indicative of the requested output power, and wherein the trajectory comprises a first control value of the predicted signal. In this embodiment, the signal indicative of the requested output power (P ref ) is set based on the first control value of the predicted signal.

[0056] Figures 4 to 6 A plot of a simulated signal is shown. The simulation is based on a computer simulation of a wind turbine operating at a wind speed of 8 m / s under a normal turbulence model. A plot A illustrates a sensor plot in the time domain, while a plot B illustrates a corresponding plot in the frequency domain obtained by a Fourier transform.

[0057] Figure 4 A simulated plot of the generator speed ω G is illustrated. Figure 4 A illustrates two simulations, one simulation 40 obtained without applying the signal processing module 300, and one simulation 41 obtained with applying the signal processing module 300.

[0058] As can be seen from the plots, the generator speed has a distinct oscillation at the 3P frequency. While small changes can be seen between the simulations, the application of the signal processing module does not affect the frequency components of the generator speed signal, whereby the application of the signal processing module does not excite the system or destroy the stability of the system.

[0059] Figure 5 A simulated plot of the main bearing drive torque, i.e. the drive torque of the rotor at the main bearing, is illustrated. The simulated plot 50 illustrates the drive torque without applying the signal processing module 300, while the simulated plot 51 illustrates the drive torque with applying the signal processing module 300. As can be seen, the 3P component of the drive torque has been significantly reduced. This is confirmed in the frequency plot B, where the frequency component in the 3P region is significantly reduced between the frequency signals of the respective signals obtained with applying 510 and without applying 500 the signal processing module 300. Figure 5

[0060] The torque variations in the drive torque cause fatigue exposure, and with a reduction of these variations, the fatigue exposure is also reduced. ​

[0061] Figure 6 Analog plots of the power reference generated by the signal processing modules not applied 60 and applied 61 are again illustrated. With respect to Figure 3 , this corresponds to P ref , and P* ref * applied with the module. The corresponding frequency plots are shown as 600 and 610.

[0062] It can be seen that Figure 5 the disturbance in the torque on the rotor shown is at least partially transmitted through the drive train and into the electrical system of the generator, as reflected in the oscillating power reference.

[0063] The skilled person will understand that modifications can be made to the specific embodiments described above without departing from the inventive concept as defined in the claims.

Claims

1. A method for controlling a wind turbine (1), the wind turbine (1) comprising a rotor (2) having one or more blades (3) and a generator (11), the rotor (2) being connected to the generator (11) via a drive system (14), the method comprising: Receives a signal indicating the speed of the generator (11) and a signal indicating the requested output power; The signal indicating the speed of the generator (11) is filtered to isolate frequencies in a selected interference band to generate an interference signal; The phase of the interference signal is phase-shifted to match the phase of the rotor torque change at the selected interference frequency band, thereby generating a phase-shifted signal. Generate a correction signal that is a combination of the phase shift signal and the requested output power; The correction signal is set as the control signal for the wind turbine.

2. The method according to claim 1, wherein, The signal indicating the requested output power is either the requested output power or the requested generator torque.

3. The method according to claim 1 or 2, wherein, The control signals include a power reference or a torque reference for controlling the output power of the wind turbine.

4. The method according to claim 1 or 2, wherein, Before filtering the signal to isolate frequencies in a selected interference band, the signal is filtered using a drive system notch filter, which removes the resonant frequency of the drive system.

5. The method according to claim 1 or 2, wherein, The selected interference frequency band is the frequency band near the rotor frequency order.

6. The method according to claim 1 or 2, wherein, The phase-shifted signal is also corrected using signal gain.

7. The method according to claim 6, wherein, The signal gain includes a wind speed-based term.

8. The method according to claim 6, wherein, The signal gain includes a term based on the operating point of the wind turbine.

9. The method of claim 6, further comprising determining electrical interference at a measurement point where the wind turbine is electrically connected to the power grid, wherein the signal gain includes an item based on the determined electrical interference.

10. The method according to claim 1 or 2, wherein, The phase shift signal is dynamically range-scheduled.

11. The method according to claim 1 or 2, wherein, The signal indicating the speed of the generator is based on the generator speed signal, the rotor speed signal, or a combination of the rotor speed signal and the generator speed signal.

12. The method according to claim 1 or 2, wherein, The signal indicating the generator speed is based on the rotational load signal of the rotor shaft.

13. The method according to claim 1 or 2, further comprising: The Model Predictive Control (MPC) routine is used to calculate a predicted operating trajectory of a predicted signal indicating the requested output power, wherein the trajectory includes time-series values ​​indicating the requested output power, and wherein the trajectory includes a predicted first control value. Based on the predicted first control value, a signal indicating the requested output power is set.

14. A computer program product comprising software code adapted to control a wind turbine when executed on a data processing system, the computer program product being adapted to perform the method of any one of claims 1-13.

15. A wind turbine control system, the wind turbine control system comprising a signal processing module (300), the signal processing module being configured to: Receive a signal indicating the speed of the generator (11); The signal indicating the speed of the generator (11) is filtered to isolate frequencies in a selected interference band to generate an interference signal; The phase of the interference signal is phase-shifted to match the phase of the rotor torque change at the selected interference frequency band, thereby generating a phase-shifted signal. The control system is also adapted to receive a signal indicating a requested output power and generate a correction signal as a combination of the phase shift signal and the requested output power; and to set the correction signal as a control signal for the wind turbine.

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