Controlling a wind farm with wind turbines that are damping tower oscillations

Field of invention

The present invention relates to a method and to an arrange ment of controlling at least one wind turbine having a wind turbine tower and being connected to a utility grid. The pre sent invention further relates to a wind turbine having the arrangement.

Art Background

A wind turbine may comprise a wind turbine tower and a na celle mounted on top of the wind turbine tower which harbours a main rotor at which plural rotor blades are connected. The main rotor is mechanically coupled to a generator to produce electric energy upon rotation of the rotation shaft.

During operation of the wind turbine, tower oscillations may occur including back and forth oscillation of the wind tur bine tower. Conventionally, methods have been proposed to dampen tower oscillations by modulating the active power pro duction with the tower eigenfrequency of the tower oscilla tions. However, it has been observed that, when the wind tur bine is controlled and curtailed by the park level active power controller, the wind turbine will introduce an error in the active power production caused by the tower damping. Thereby, instability of the control may result or the utility grid stability may be hampered. Because the tower eigenfre- quency (for example 0.21 Hz to 0.25 Hz) lies within the band width range of the park level active power feedback control ler, the park level controller will try to counter the tower damping causing more fatigue load on the wind turbine and po tentially pushing the tower damping controller into its oper ational bounds. Thus, there may be a need for a method and an arrangement of controlling at least one wind turbine such that an error in the active power production may be reduced, while tower os cillations are effectively damped (thus, damping of tower os cillation is active). Furthermore, there may be a need for a control method, wherein grid stability can be improved and at the same time wind turbine tower oscillations are damped.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to an embodiment of the present invention it is provided a method of controlling at least one wind turbine having a wind turbine tower and being connected to a utility grid, the method comprising: dynamically filtering an utility grid active power in dependence of a damping signal provided for counteracting an oscillation of the wind turbine tower; and controlling the wind turbine in dependence of the fil tered utility grid active power.

The method may be performed partly or entirely by a wind tur bine controller and/or partly or entirely by a wind park con troller. Embodiments of the present invention operate on the wind turbine level and other embodiments of the present in vention operate on a wind park level. The utility grid active power may for example be obtained by measuring the active power provided by the wind turbine or the entire wind park to the utility grid. Thereby, measurement equipment for example installed at or close to a point of common coupling or in stalled between the wind park and the utility grid may be utilized.

The controlling the wind turbine may comprise supplying a re spective control signal to a converter of the wind turbine. The converter may comprise plural controllable switches, such as IGBTs, wherein pairs for each phase may be provided in se ries. The converter may comprise an AC-DC section, a DC link and a DC-AC section. The converter may be connected between the generator of the wind turbine and the output terminal of the wind turbine or a wind turbine transformer.

The damping signal is designed to counteract an oscillation of the wind turbine tower, in particular a back and forth os cillation of the wind turbine tower. The damping signal may define a supplemental active power signal which is added on a wind turbine level to for example a preliminary power refer ence. One or more eigenoscillations of the wind turbine tower may be counteracted by the damping signal. Thus, the damping signal may contain for a single wind turbine one or more fre quency components or frequency ranges and having correspond ing or associated amplitude information. If the method is performed on a park level, the damping signal may comprise a combination of individual wind turbine damping signals. The damping signal may comprise frequency components in one or more frequency ranges.

The filtered utility grid active power may have the amplitude of frequency components decreased which are comprised in the damping signal. When the utility grid active power is fil tered in dependence of the damping signal, the damping signal is in the further control method not considered as an error which a controller needs to counteract. Thereby, the output of the wind turbine may to a better degree comply with an ex ternally provided active power reference. For controlling the wind turbine, one or more controllers may be provided on park level and/or wind turbine level.

According to an embodiment of the present invention, control ling the wind turbine comprises: supplying a first active power difference between an externally defined active power reference and the filtered utility grid active power to an active power controller; outputting by the active power con- troller, a preliminary power reference to at least one wind turbine.

The externally defined active power reference may refer, when the method is performed on the wind turbine level, to a de sired active power output of the individual or single wind turbine. When the method is performed on park level, the ex ternally defined active power reference may refer to a de sired wind park active power output of the entire wind park.

The preliminary power reference is derived by the active pow er controller based on the supplied first active power dif ference. The preliminary power reference is not finally used to control the wind turbine but to the preliminary power ref erence at least the damping signal of the respective wind turbine is added. Thereby, the effective damping of tower os cillations of the respective wind turbine is achieved while at the same time complying in an improved manner with the ex ternally defined active power reference.

In this embodiment, the active power controller is provided already with an input difference signal from which the fre quency components of the damping signal are reduced or even removed.

According to another embodiment of the present invention, controlling the wind turbine comprises: supplying a second active power difference between an externally defined active power reference and the utility grid active power to an ac tive power controller; outputting by the active power con troller, a control signal from which the filtered utility grid active power is subtracted in order to derive a prelimi nary power reference that is supplied to at least one wind turbine.

The second active power difference is, as may be convention ally known, derived as a difference between the externally defined active power reference and the utility grid active power (i.e. the unfiltered signal). According to this embodi ment, considering the damping signal is performed at the out put level of the active power controller. Thereby, also, counteracting the desired damping action is avoided.

According to an embodiment of the present invention, the ac tive power controller is configured to derive the preliminary control signal such as to decrease the active power differ ence. The active power controller may for example comprise a PID or PI controller. The active power controller may have been tuned in order to derive the respective output such that the input difference decreases more and more. Thereby, con ventionally known controllers may be utilized.

According to an embodiment of the present invention, dynami cally filtering the utility grid active power in dependence of the damping signal comprises using a band-stop filter which reduces, in the utility grid active power, amplitudes of frequency components of the damping signal, to between 0% and 10%, wherein the damping signal comprises in particular frequency components in a range between 0.1 Hz and 1 Hz, in particular between 0.2 Hz and 0.3 Hz.

The band-stop filter may be configured for filtering out or at least reducing frequency components of the respective in put signal in one or more frequency ranges. The one or more frequency ranges may be determined based on the damping sig nal to cover those frequencies which are comprised in the damping signal. The range may define a respective bandwidth of the filter. The filter may comprise several filter compo nents each for each frequency range. The respective frequency ranges are filtered out in the utility grid active power. Frequency components in the range between 0.1 and 1 Hz are typical for tower oscillations. Therefore, the damping signal counteracting the respective tower oscillations is effective ly filtered out in the filtered utility grid active power. Thereby, the method may still be improved regarding better compliance of the actual active power output of the wind tur bine with the externally defined active power reference.

According to an embodiment of the present invention, the method further comprises generating, by a damping controller, the damping signal based on a tower oscillation indicating signal, to counteract at least a back and forth oscillation of the wind turbine tower, wherein the tower oscillation in dicating signal is in particular measured by an accelerome ter.

The tower oscillation indicating signal may indicate frequen cy and amplitude of one or more tower oscillations. The damp ing signal may be derived to be phase offset to the tower os cillation indicating signal, in order to counteract the tower oscillation. For example, when the tower moves back and forth, in a time span during which the tower moves backwards, the load of the generator may be decreased (in particular in volving reducing active power output). When the wind turbine tower moves forward or is in a frontal position of the back and forth oscillation, it may be desired to increase the load (involving increasing active power output of the wind tur bine). In this counteracting way, the damping signal may in volve modulating the active power output of the wind turbine. The accelerometer may be arranged at a nacelle or at the wind turbine tower. Thereby, implementing the method is simplified including utilization of conventionally available equipment. Also the damping controller may comprise a PID or PI control ler.

According to an embodiment of the present invention, the method further comprises adding, at each wind turbine, the respective damping signal to the preliminary power reference to obtain a wind turbine active power reference signal; and controlling the wind turbine using the wind turbine active power reference signal. The preliminary power reference may not be designed to dampen tower oscillations. In contrast, the respective damping sig nal is designed to counteract the tower oscillation. Thus, adding the respective damping signal to the preliminary power reference results in the wind turbine active power reference signal to effectively causing damping of tower oscillation as well as complying with the desired or externally defined ac tive power reference.

According to embodiments of the present invention, further supplemental control signals may be added for other control purposes, such as torque ripple reduction and so on.

According to an embodiment of the present invention, the method further comprises filtering the filtered utility grid active power using a transition filter triggered by a detect ed transient in the utility grid active power, the output of the transition filter being derived as a weighted sum of the filtered utility grid active power and the (unfiltered) util ity grid active power.

The transition filter may be provided for reducing a ripple which may occur after a large transient in the measured grid active power. Thus, the filtered utility grid active power and the (unfiltered) utility grid active power is added in a weighted manner to obtain a composite signal which may be ef fective in reducing a ripple due to a large change in the measured grid active power. The transient may be defined ac cording to the particular application. Detection of a transi ent may for example be defined when the utility grid active power changes in a particular time span more than a thresh old. The threshold and the length of the time span may be adapted according to the particular application. Thereby, the method may still be improved.

According to an embodiment of the present invention, weighting coefficients sum to one and change with a time elapsed since the occurrence of the transient. Thus, for each transient, the transition filter is triggered again and the weighted sum is calculated starting at the occurrence of the transient. The longer after the occurrence of the transient, the higher is the weighting of the filtered utility grid ac tive power. Thereby, a smooth and reliable and safe control of the wind turbine may be achieved.

According to an embodiment of the present invention, the weighting coefficient of the utility grid active power is highest at (or immediately or shortly after) the occurrence of the transient and then decreases with time to zero, in particular according to an exponential function. Thereby, an effective implementation is enabled. Further, a smooth tran sition to the situation when there is no transient is achieved, since the output of the transition filter more and more approaches the filtered utility grid active power which is utilized when there is no transient detected in the utili ty grid active power.

According to an embodiment of the present invention, the transient is detected, if the utility grid active power changes by more than an active power threshold, in particular between 5% and 20% of a rated power, during a time span, in particular between 0.1 s and 10 s. According to other embodi ments, the threshold may be set to different values.

According to another embodiment of the present invention, the method is performed on a park level, wherein the method is adapted to control plural wind turbines of a wind park, wherein the externally defined active power reference is an externally defined wind park active power reference; wherein the damping signal is a (sum) signal combined from individual damping signals of all the wind turbines, each provided for counteracting an oscillation of an respective wind turbine tower, wherein the preliminary active power reference is de fining a preliminary wind park active power reference. Herein, the damping signal may be the sum of the individual damping signals of all the wind turbines. This sum of the in dividual damping signals may comprise plural frequency compo nents which may lie within one or more frequency ranges. The respective filtering may involve reducing the amplitude of the frequencies in these one or more frequency ranges.

According to another embodiment, however, the method may be performed individually for each wind turbine of the wind park. In this case, the externally defined active power ref erence may have the meaning of an externally defined wind turbine active power reference. The damping signal may be the damping signal of the considered wind turbine. The prelimi nary control signal may define a preliminary wind turbine ac tive power reference. The utility grid active power may re late to the actual active power output of the considered wind turbine.

According to this embodiment, the method may further comprise dividing the preliminary wind park active power reference in to plural preliminary power references for all wind turbines.

Thereby, each wind turbine may receive a respective prelimi nary power reference which does not interfere with a damping controller which may be comprised in and is operating within the wind turbine for damping tower oscillations.

It should be understood, that features, individually or in any combination, disclosed, described, explained or applied for a method of controlling at least one wind turbine having a wind turbine tower is also, individually or in any combina tion, applicable to an arrangement for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the present invention and vice versa.

According to an embodiment of the present invention it is provided an arrangement for controlling at least one wind turbine having a wind turbine tower and being connected to a utility grid, the method comprising: a band-stop filter adapted to dynamically filter an utility grid active power in dependence of a damping signal provided for counteracting an oscillation of the wind turbine tower; and a controller adapted to control the wind turbine in dependence of the fil tered utility grid active power.

The arrangement may be comprised partly or entirely in a wind park controller or/and a wind turbine controller. The ar rangement may comprise hardware components and software com ponents.

According to an embodiment of the present invention it is further provided a wind turbine comprising a wind turbine tower and an arrangement according to the preceding embodi ment.

The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi ment but to which the invention is not limited.

Brief Description of the Drawings

Embodiments of the present invention are now described with reference to the accompanying drawings. The invention is not restricted to the illustrated or described embodiments.

Fig. 1 schematically illustrates a wind park comprising an arrangement for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the pre sent invention;

Fig. 2 schematically illustrates a wind turbine according to an embodiment of the present invention; Figs. 3, 4 and 5 illustrate graphs showing performance of a control method according to the present invention and the prior art.

Detailed Description

The illustration in the drawings is in schematic form. It is noted that in figures 1 and 2, elements similar or identical in structure and/or function are provided with the same ref erence signs or with reference signs, which differ only with in the first digit. A description of an element not described in one embodiment may be taken from a description of this el ement with respect to another embodiment.

The wind park 100 schematically illustrated in Fig. 1 com prises plural wind turbines 103 and a park level controller 105 as an implementation of an arrangement for controlling at least one wind turbine having a wind turbine tower according to an embodiment of the present invention. The arrangement 105 comprises a band-stop filter 107 which is adapted to dy namically filter an utility grid active power 109 in depend ence of a damping signal 111 provided for counteracting an oscillation of a respective wind turbine tower of all the wind turbines 103. The damping signal 111 is the sum of indi vidual wind turbine damping signals 110 of the individual wind turbines.

The arrangement 105 further comprises control equipment in particular including an active power controller 113 and an active power dispatcher 115 representing an implementation of a controller 117 which is adapted to control the wind tur bines 103 in dependence of the filtered utility grid active power 119 which is output by the band-stop filter 107 in de pendence of the measured grid active power 109 and the damp ing signal 111. The wind park 100 further comprises a wind turbine transformer 121 and comprises grid measurement equip ment 123 for measuring the utility grid active power 109. During normal operation, the wind turbines 103 provide elec- trical energy via the wind farm transformer 121 to a utility grid 125.

In the embodiment as illustrated in Fig. 1, the arrangement 105 further comprises a transition filter 127 which may be triggered by a detected transient of the utility grid active power. Thus, the transition filter also receives the utility grid active power 109 and detects whether a transient has oc curred. The transient may involve a large change in the util ity grid active power 109 in a particular time window. The transition filter 109 further receives as input the filtered utility grid active power 119 as output by the band-stop fil ter 107. The transition filter 127 derives a weighted sum of the filtered utility grid active power 119 and the (unfil tered) utility grid active power 109 and outputs the weighted sum 129 (e.g. as controller grid active power) supplying it to a difference element 131. Thereby, the difference element 131 computes a first active power difference 133 between an externally defined active power reference 135 and the weighted sum 129 output by the transition filter 127.

The first active power difference 133 is supplied to the ac tive power controller 113 which outputs based thereon, a pre liminary power reference 136, here a preliminary wind park power reference. This preliminary wind park power reference 136 is supplied to an active power dispatcher 115 which di vides the wind park active power reference 136 into plural preliminary power references 137 for all wind turbines.

As is depicted in Fig. 1, the park level controller 105 re quests the tower damping frequency and amplitudes from the turbine (for example as damping signal 111). The resulting frequency and bandwidth is calculated in the park level con troller 105 online and used in a dynamic band-stop filter 107 to remove the resulting tower damping frequency from the measured active power feedback to the park level active power feedback controller 113. If there is a large transient in the measured grid active power, the band-stop filter 107 could introduce a ripple into the filtered grid active power 119. Therefore, a transition filter 127 is introduced to combine the filtered grid active power and the measured grid active power (signals 109 and 119) with different weights wA and wB such that the weighted sum 129 (also referred to as controller grid active power) is given by controller grid active power = wA * Filtered grid active pow er (signal 119) + wB * grid active power (signal 109) with wA + wB = 1

When a large change in the measured grid active power 9 is detected, wB = 1 and wA = 0 and wB will go towards 0 through a mathematical function such as: wB = exp(^ *t)

Thus, the weighted sum 29 (controller grid active power as given by controller grid active power = (1 - exp(^ *t)) * Filtered grid active power + exp(^ *t) * grid active power

Herein, t will be increasing over time (since the occurrence of the transient) and l is selected such that minimal ripple at transient responses is generated and the setting time of the filter is within a reasonable time.

Fig. 2 schematically illustrates a wind turbine 203 according to another embodiment, wherein the arrangement 205 is com prised in the wind turbine 203. The arrangement 205 receives a measured utility grid active power 209. Further, the con troller 205 receives a damping signal 211 from a damping con troller 239 also comprised in the wind turbine 203. The ar rangement 205 has similarly as the arrangement 105 illustrat ed in Fig. 1. A band-stop filter derives from the utility grid active power 209 and the damping signal 211 a filtered active utility grid active power. The filtered utility grid active power is then subtracted from the utility grid active power and supplied to an active power controller (similar to active power controller 113 illustrated in Fig. 1) which de rives therefrom a preliminary active power reference 237.

The damping signal 211 is added to the preliminary power ref erence 237. The output of the addition element 241 is the wind turbine active power reference signal 243. This signal is supplied to a converter 245 which is connected to the gen erator 247 which is connected to the main rotation shaft 249 at which plural rotor blades 251 are connected. Thus, the ac tive power reference signal 243 is adapted to effectively dampen tower oscillations of the tower 202 of the wind tur bine 203 as well as to comply with the externally defined ac tive power reference 209 defining the desired active power of the wind turbine active power output.

The arrangement 205 receives an externally defined wind tur bine active power reference (an implementation of a external ly defined active power reference) 235. The damping control ler 239 receives an oscillation indicating signal 246 from an accelerometer installed in nacelle 210.

Figs. 3 and 4 indicate measurement results according to the prior art and according to the present invention or according to an embodiment of the present invention, respectively, wherein abscissas 3 indicate the sample number or the time while the ordinates 4 indicate power output in Megawatt.

Fig. 3 shows for the conventional control method the wind turbine active power reference 35 as derived by a convention al wind park controller and the turbine active power output 38 as output by the respective wind turbine. As can be seen from Fig. 3, the wind turbine active power reference 35 com prises an oscillating behaviour having the same frequency as the active power output 38 but being phase-shifted. Thus, both signals counteract each other.

In the Fig. 4 according to an embodiment of the present in vention, also the wind turbine active power reference, in particular the preliminary active power reference 37 is il lustrated as well as the actual power output 38 of the wind turbines. It can be seen that the preliminary active power reference 37 does not comprise an oscillating behaviour as the conventional curve as illustrated in Fig. 3. However, the actual active power output 38 of the wind turbine comprises an oscillating behaviour causing an effective damping of tow er oscillations.

Fig. 5 illustrates a graph having the abscissa 3 indicating the sample number or the time and having an ordinate 4 indi cating the power. The curve 60 indicates the wind turbine ac tive power reference while the curve 62 indicates the turbine active power. These data have been obtained after a reset of the transition filter 27. It can be seen that the turbine ac tive power reference 60 quickly decays to small values. Thereby, ripples are effectively avoided or at least reduced.

Embodiments of the present invention may include the follow ing features or achieve the following advantages:

- The park level controller using the tower damping fre quency and amplitudes from the turbine avoids counter acting the tower damping which will allow the tower damping controller to operate within its full dynamic range to counter tower oscillations which will cause less fatigue on the turbine and extend the lifetime of the turbine.

- The park level active power controller will not experi ence additional error caused by the tower damping on the turbine which is uncontrollable from the park level, making it possible to design a park level active power controller with a narrower operational range which will allow for less active power overshoot which will improve compliance to grid codes and protection of transmission equipment such as high voltage transformers.

It should be noted that the term "comprising" does not ex clude other elements or steps and "a" or "an" does not ex clude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be con strued as limiting the scope of the claims.