The present invention relates to the technical field of renewable energies, more specifically to the generation of electricity using wind power.
The object of the invention is directed at a wind turbine control method which allows efficient management of its performance in anomalous situations such as those of misalignment.
Nowadays the use of renewable energies for generating electricity is common, wind power being one of the most efficient among them. Wind power allows electricity to be generated on the basis of wind by means of wind turbines. Said wind turbines basically comprise a tower, a nacelle containing the electric generator, a rotor further comprising at least two blades, and a power train which transmits power from the rotor to the electric generator. The power train can comprise a gearbox which connects a low-speed shaft connected to the rotor with a high-speed shaft connected to the electric generator.
In multi-megawatt wind turbines, there is a market trend towards bigger rotors, which provide energy at a lower cost. In said configurations the control system has a growing importance. Said system maximises energy production while it limits the mechanical loads produced by the wind. To do this, the control system acts on the blade pitch angle—pitch angle—and on the torque demanded from the generator.
On the one part, the pitch angle is controlled by means of a set of actuators which make the blade rotate about its longitudinal axis. Said actuation varies the aerodynamic torque, either to obtain the maximum possible power of the wind in certain meteorological conditions, or to limit the mechanical loads produced on the wind turbine.
On the other part, the control system modulates the torque demanded to the generator from the converter. Torque modulation is also carried out with the dual objective of obtaining the maximum possible power of the wind in given meteorological conditions, and to limit the mechanical loads produced on the wind turbine.
Due to the three-dimensional and stochastic nature of wind—throughout the area swept by the rotor, wind is a non-uniform vector in space and random—the loads experienced by each blade and consequently by the wind turbine, are variable in time. One example of this variability is observed regarding height with respect to ground surface, producing the phenomenon known as wind shear. Another example is the variability in the direction of the wind, which makes it necessary to consider the actuation of a system that orients the nacelle to maintain the rotor correctly oriented. This is the yaw system.
The yaw system does not act continuously. It only orients the rotor towards the direction of the wind when a system which comprises a windvane detects that the misalignment exceeds a certain threshold value for a determined time. In the course of time during which the rotor remains disoriented, situations can arise wherein the misalignment provokes at least one blade to stall, producing high aerodynamic loads on the rotor, and losing control capacity by means of blade pitch regulation. The present invention is conceived in order to overcome this problem.
In the current state of the art, the usual is as follows:
In the speed regulation zone wherein the power generated is equal to the nominal power PN, transients in the rotor speed caused by gusts of wind can lead to overspeeds (for example, after a drop in wind with an associated reduction in blade pitch angle, there can be such a sudden increase in wind where there is not enough time to increase the blade pitch angle in consequence). In this case, the wind turbine's control system causes the machine to disconnect from the power grid. To prevent said overspeeds which produce the wind turbine stoppage, there is the possibility of limiting the blade pitch angle lower limit value reached in transient events. To do this, the control system of the state of the art uses a predetermined curve that determines a minimum threshold which is applied to the blade pitch set point depending on the average blade pitch angle (the average blade pitch angle is commonly used as an indicative signal of average wind speed or power). In this way, for a determined average blade pitch angle calculated in a time window, no blade pitch excursions beneath a certain blade pitch value are permitted.
There are cases in which the blade pitch angle lower limit value (instead of being predetermined for each average wind speed or power or average blade pitch) is varied taking into account the modification in the aerodynamic efficiency of the blade due, for example, to the deposition of ice or dirt on it. Examples of these control techniques can be found in the following patent documents:
Hence patent document U.S. Pat. No. 8,096,761 describes a control method which, in the presence of ice, modifies the value of the blade pitch angle lower limit β min. This patent does not specify how the presence of ice is identified, and mentions only an estimation of the loss of aerodynamic efficiency. Meanwhile U.S. Pat. No. 4,656,362 presents a control method which modifies the value of the blade pitch angle lower limit β min using a value related to aerodynamic performance.
At present individual blade pitch control techniques use sensors to measure the loads on the blades, on the basis of which the loads on the fixed axis are estimated.
In a first aspect, the present invention relates to a control method for wind turbines that are in circumstances such as those described above. More specifically, the control method described herein becomes especially useful when it is determined that there is a misalignment of the wind turbine with respect to incident wind, which can cause non-optimal functioning in terms of energy capture as well as causes anomalies in the wind turbine and its components. A second aspect of the present invention relates to a wind turbine blade pitch control system adapted to carry out the method related to the first aspect of the present invention.
Consequently and in a preferred embodiment of the wind turbine control method with blade pitch control system, action is carried out on the different control systems thereof when it is determined that there is a misalignment of the nacelle with respect to the direction of incident wind; to be able to carry out the following steps of the preferred embodiment of the method described herein it allows the value of the blade pitch angle β to be adapted on the basis of the value of the wind turbine misalignment φ, this value of the blade pitch angle β allows, by means of a set point sent to the blade actuators, a reduction in the loads associated for example, to an excessive turbulence of the wind direction (there are gust effects of wind direction which are harmful).
The control method of the present invention, carries out a series of data gathering and on the basis thereof proceeds to carry out a calculation of the lower limit of the blade pitch angle βMIN on the basis of the value of the wind turbine misalignment φ. To do this, the control method proceeds to obtain the value of at least one indicative signal of the wind speed incident on the wind turbine or of an average of same, for example, a signal related to the average angle of the blade pitch angle and a value indicative of the misalignment of the wind turbine φ.
To obtain a value of the lower limit of the blade pitch angle βMIN on the basis of said value indicative of the misalignment of the wind turbine φ, initially use is made of some calculations that allow a correlation to be obtained which defines the blade pitch angle lower limit value βMIN which marks a stalling threshold for each value of the indicative signal of wind speed. Said correlation is modelled in the form of a table and is implemented in the control system of the wind turbine in order to have the correlation between the lower limit of the blade pitch angle βMIN which marks a stalling threshold and each value of the signal indicative of wind speed λ. This makes it possible to obtain for each value of the signal indicative of wind speed A the lower limit value of the blade pitch angle βMIN to avoid operation in the zone of aerodynamic stalling.
Additionally, the method described herein presents an additional term −ΔβMIN to the lower limit value of the blade pitch angle βMIN obtained on the basis of the comparison of the signal indicative of wind speed with a curve or table comprising a correlation that defines the blade pitch angle lower limit value βMIN which marks a stalling threshold for each value of the signal indicative of wind speed. The addition of the aforesaid additional term −ΔβMIN is cancelled in the event that it is determined that the wind direction is maintained, after a time has passed since there has been a rapid variation in the value indicative of misalignment as a consequence of a rapid change in wind direction.
Alternatively, different time constants are applied in a filter for the estimation of a filtered value of the blade tip speed ratio λ, used as a signal indicative of wind speed for obtaining the lower limit of the blade pitch angle βMIN.
The wind turbine for which the control method of the invention is intended comprises a series of blades, and control system of blade pitch angle β such as the one observed in
In this way, and making use of the control system mentioned above or of a similar one, one proceeds to calculate:
To subsequently proceed to modify at least one initial set point of blade pitch angle if the latter is lower than the lower limit value of the blade pitch angle βMIN, calculated on the basis of the value indicative of misalignment φ, in such a way that a blade pitch final set point is greater than or equal to the lower limit value of the blade pitch angle βMIN to subsequently act on at least one of the blades of the wind turbine based on the blade pitch angle final set point value.
Furthermore, the control method comprises calculating the value of the lower limit of the blade pitch angle βMIN using the value indicative of misalignment φ.
In this way, the value of the lower limit of the blade pitch angle βMIN is adapted to the conditions of wind turbine orientation, to prevent excessive stalling and/or loads.
To be able to calculate the value of the lower limit of the blade pitch angle βMIN one proceeds to make a comparison of a signal indicative of wind speed with a curve or table comprising a correlation between the blade pitch angle lower limit value βMIN and the signal indicative of wind speed which defines the blade pitch angle lower limit value βMIN which determines an aerodynamic stalling threshold for each value of the signal indicative of wind speed. The data that gives rise to the table or curve can be obtained by means of simulation of the points related to the power coefficient Cp for each blade pitch angle β at different blade tip speeds.
In one possible embodiment a blade tip speed ratio λ is used as a signal indicative of wind, although in other possible embodiments the signal indicative of wind speed can be taken on the basis of instantaneous wind speed data, average wind speed or on the basis of data related to the average power or average blade pitch angle. However, the use of the blade tip speed ratio λ makes it possible to take into account not only the effects of wind speed on the rotor, but also the effects of the rotation of the rotor itself, as the influence of the speed of rotation of the rotor on the profile lift force (measured through the power coefficient Cp) is substantial. The blade tip speed ratio λ is determined on the basis of wind speed measurements—which can be taken by means of wind data capture means such as anemometers—and speed of rotation of the rotor and is calculated by means of the following formula, as the quotient between linear speed of the blade tip and wind speed, according to the formula:
Said curves do not take into account lift losses associated to a potential deposition of particles on the surface of the blade which alter the geometry of the aerodynamic profiles nor other effects which vary the lift of the blades, such as misalignment and its corresponding value φ.
The maximum points of the curves which relate the power coefficient [Cp] with the blade pitch angle β for different values of the blade tip speed ratio λ define pairs of points β−λ which are used to characterise a curve (βMIN−λ), which can be appreciated in
As the table implemented in the wind turbine control system comprises a limited number of pairs of points, for those measurements of blade tip speed ratio values λ which do not correspond to any of the points on the table, a process of interpolation is carried out between at least two of them using conventional interpolation techniques, such as for example a linear interpolation.
Given that the signals related to the measurements of wind speed and speed of rotation of the rotor necessary for obtaining the blade tip speed ratio λ can have noise and produce undesirable effects such as fluctuations; the method described herein envisages applying at least one filter F1 to any signal which requires it, as in this case to the signals related to the blade tip speed ratio λ so as to therefore carry out a filtration process and smooth said signal in time and for said fluctuations in the measurements not to be reflected in the lower limit value of the blade pitch angle βMIN.
Said filters may be of any type that allows the required result to be obtained, such as a low pass filter which allows the lower frequencies to pass and attenuates those higher frequencies and which moreover presents a configurable time constant, or a filter based on a moving average which can be calculated with a configurable number of points.
In one preferred embodiment, the method comprises adding an additional term of the blade pitch angle lower limit value ΔβMIN (which can be predetermined or dependent on the misalignment value φ) to the lower limit value of the blade pitch angle βMIN obtained from comparing the signal indicative of the blade tip speed ratio λ with the predetermined curve or table which defines the blade pitch angle lower limit value βMIN at which the blade does not stall, as illustrated in
According to an embodiment such as the one shown in
Also, in one possible embodiment, it is possible to modify the calculation of the lower limit value of the blade pitch angle βMIN only if it is determined that the misalignment value φ is above a predefined misalignment threshold value, by means of a comparison of the misalignment value φ of the wind turbine with the threshold.
Thus, for example, in one possible embodiment in which it is determined or known that there is no misalignment or that the misalignment is below the predefined threshold value, the calculation of the lower limit value of the blade pitch angle βMIN is carried out on the basis of a signal indicative of wind speed, preferably the blade tip speed ratio λ, and a predetermined curve which defines the blade pitch angle lower limit value βMIN which marks the stalling threshold for each value of blade tip speed ratio λ, without carrying out the sum of any additional term or modification of the filtration time constant τ. However, when the misalignment is significant, the lift losses or variation in actual Cp is relevant and it is advisable to protect against new changes in wind direction; meaning that if it is determined that the misalignment exceeds the misalignment threshold value then said calculation of the lower limit value of the blade pitch angle βMIN is modified, either by means of the sum of an additional term of the blade pitch angle lower limit value ΔβMIN (which can be predetermined or dependent on the misalignment value) or by means of modifying the filtration time constant τ in the filter F1.
When the wind turbine is operating in the nominal production zone according to
In one embodiment, when the wind turbine is operating in said zone, if the misalignment value of the wind turbine exceeds the predefined threshold value, the lower limit value of the blade pitch angle βMIN is calculated by means of adding a predetermined value ΔβMIN to the lower limit value of the blade pitch angle βMIN initially obtained from comparing the signal indicative of wind speed with the predetermined curve or table which defines the blade pitch angle lower limit value βMIN at which the blade does not stall for each value of the signal indicative of wind speed, as can be appreciated from
Also, the greater the error in alignment, the more protected it is advisable for the wind turbine to be against new changes in direction and gusts of wind, so that in one embodiment, the added value is growing with the alignment error from a minimum threshold error.
Said additional minimum limit value ΔβMIN is obtained by means of the calculation block of the control system which allows calculation, among other things, of the lower limit value of the blade pitch angle βMIN on the basis of the misalignment as shown in
If the wind direction is maintained after a predefined time interval ΔT since a sudden change in wind direction (which can be determined by means of the comparison of the rate of change of misalignment with respect to time), the lower limit value of the blade pitch angle βMIN is calculated again only on the basis of the comparison of a signal indicative of wind speed with a predetermined curve or table which defines the blade pitch angle lower limit value βMIN that marks a stalling threshold for each value of the signal indicative of wind speed. Which is to say, after a ΔT since a sudden change in wind direction, ΔβMIN is cancelled or τ regains its original value. This is so because, if the wind orientation direction is maintained for a while, it is not advisable to maintain a lower limit value of the blade pitch angle βMIN that is too high, as it might not be appropriate for the speed of the incident wind. This is a transient protection measure for the machine, until it is determined that the wind turbine is in a stable situation.
Number | Date | Country | Kind |
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ES201331903 | Dec 2013 | ES | national |
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Number | Date | Country | |
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20150176568 A1 | Jun 2015 | US |