Activating and deactivating a safe mode of operation of a wind turbine

Field of invention

The present invention relates to a method and to a corre- sponding arrangement of activating and/or deactivating a safe mode of operation of a wind turbine. Furthermore, the present invention relates to a wind turbine comprising the arrange- ment.

Art Background

A wind turbine, for example contained in a wind park, may be subjected to different environmental conditions, including normal environmental conditions where normal operation and production of electric energy is possible. However, a wind turbine may also be subjected to exceptional environmental conditions, involving high wind speed. The wind turbines need to be configured to respond to such exceptional environmental conditions. Therefore, a wind turbine may be designed to have a safe operation mode which increases the likelihood of avoiding damage during a severe weather event. This severe weather event might be a storm or a tropical cyclone de- scribed as a rotating storm with a low-pressure center and a closed low-level atmospheric circulation with strong winds.

An operation mode designed to avoid damage due to a severe weather event may involve operation of the wind turbine in a way which deviates from the standard mode of operation. This may include bringing the wind turbine out of operation or orienting the wind turbine's components in a way which reduc- es the likelihood of damage.

Conventional methods may have monitored the wind speed and may have activated or deactivated a safe mode of operation depending on whether the measured wind speed is below or above a wind speed threshold.

Conventional methods may have deactivated the safe mode of operation before the end of the severe weather event. Conven- tionally, a threshold margin may have been used so that the threshold to deactivate the safe mode of operation is on the safe side of the threshold used to activate the safe mode of operation. However, when the threshold margin is used so that deactivation of the safe mode of operation only occurs at relatively low wind speed, also the activation of the safe mode of operation is performed already if the wind speed is relatively low. Thereby, production may be reduced. Conven- tionally, also a delay of fixed time lengths may have been used for deactivation, such that a deactivation will not oc- cur before a predetermined duration of time has elapsed dur- ing which the deactivation criteria are fulfilled. However, thereby, also production may be reduced. Conventional methods may have used a time averaged value of for example the wind speed as a measured environmental condition. The conventional method may have activated the safe operation mode when the measurement or the averaged signal is above a predetermined threshold. The deactivation was conventionally performed, when the measurement or averaged signal is below the prede- termined threshold. However, in the conventional methods, the above indicated problems may arise.

It has however been observed that the conventional methods of activating or deactivating the safe mode of operation do not in all situations ensure a secure or safe operation of the wind turbine.

Thus, there may be a need for a method and a corresponding arrangement of activating and/or deactivating a safe mode of operation of a wind turbine, wherein the wind turbine can be operated in a safe manner and is appropriately responding to changing weather conditions. 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 activating and/or deactivating a safe mode of operation of a wind turbine, the method comprising: receiving at least one measurement signal related to a weath- er condition; filtering of a measuring signal dependent quan- tity to obtain a filtered signal, wherein the filtered signal and/or filtered signal depends on whether the measuring sig- nal dependent quantity and/or filtered signal is increasing or decreasing with time; activating and/or deactivating the safe mode of operation based on the filtered signal.

The method may be implemented in software and/or hardware.

The method may be performed by a module of a wind turbine controller for example. The safe mode of operation may com- prise to set the wind turbine in an operation mode which re- duces or even prohibits damage to components of the wind tur- bine which might occur due to a severe weather event, such as excessive wind speed and/or wind turbulence.

To activate the safe mode of operation may comprise to change the operation state of the wind turbine for example by chang- ing orientation or setting of one or more components, such as the yaw angle, the blade pitch angles, a converter reference signal or a combination thereof or others. Activating the safe mode of operation may disable the production of electric energy and disable supply of electric energy to the utility grid. Activating the safe mode of operation may also involve to disconnect the wind turbine from the utility grid.

For example, to activate the safe mode of operation it may be beneficial to stop operation of a upwind-facing horizontal- axis wind turbine and orient it such that the rotor is down- wind. In the downwind orientation it may be easier to ensure optimal orientation relative to the wind direction. In this orientation, however, the wind turbine may be unable to pro- duce energy for the grid. It is, therefore, undesirable to have a normally upwind wind turbine in this orientation un- less it is necessary. It also consumes power to orient the turbine to this downwind position and back again, which may cause a problem if the wind turbine is operating off-grid un- der battery or diesel power.

Another example for safe mode of operation is a downwind- facing horizontal-axis wind turbine with a cantilevered set of rotor blades that may fold or collapse inward in severe weather to reduce the cross-sectional surface area exposed to the wind. Another example is an upwind-facing horizontal-axis wind turbine with a cantilevered tail vane assembly.

Deactivating a safe mode of operation may involve to restore a normal operation of the wind turbine, wherein the wind tur- bine produces electrical energy and supplies it to a utility grid.

The measurement signal related to the weather condition may comprise information regarding wind speed, wind turbulence, atmospheric pressure, atmospheric pressure and/or pressure change across a particular distance, sun irradiance, rainfall and the like. The measurement signal related to the weather condition may in particular indicate the wind speed and/or wind turbulence and/or an atmospheric pressure or pressure change. The measurement signal may comprise an electri- cal/optical signal and/or a wireless signal.

The measuring signal dependent quantity may be a quantity which may depend on the measurement signal. In embodiments, the quantity may be equal to the measurement signal but in other embodiments, the measuring signal dependent quantity may be derivable from the measurement signal by processing and may be different from the measurement signal itself. The measuring signal dependent quantity may for example corre- spond to an average of the measurement signal over a particu- lar time as explained below in more detail.

The measurement signal dependent quantity is then filtered (for example using a (low-pass) filter having an adjustable or settable time constant or other filter parameters) to ob- tain the filtered signal. If the measurement signal dependent quantity and/or filtered signal is increasing, the filtered signal may be different from the filtered signal when the measuring signal dependent quantity and/or filtered signal is decreasing with time. Thereby, different requirements for a desired activation and requirements for a desired deactiva- tion of the safe mode of operation may be accounted for. In particular, a fast activation of the safe mode of operation may be achieved in order to reduce damage of components of the wind turbine. Furthermore, the deactivation of the wind turbine may be faster than conventionally performed, while components of the wind turbine are protected from damage by not deactivating the safe mode too early. Filtering may in- volve processing by a processor, for example utilizing appro- priate software.

Embodiments of the present invention may achieve e.g. the following beneficial effects:

1. Ensure quick activation at the onset of a severe weather event.

2. Avoid early deactivation before the severe weather event has ended.

3. Avoid a long delay (and therefore a loss of production) after the severe weather event has ended.

According to an embodiment of the present invention, the method further comprises activating the safe mode of opera- tion if the filtered signal is above or below an activation threshold; and/or deactivating the safe mode of operation if the filtered signal is below or above a deactivation thresh- old different from the activation threshold.

When the safe mode of operation is activated and/or deac- tivated based on the filtered signal, the increase for exam- ple of wind speed or the decrease of wind speed with time may in a more improved manner be accounted for by setting a cri- terion for activation or deactivation of the safe mode of op- eration. Thereby, components of the wind turbines may be pro- tected from damage and furthermore production efficiency may be improved.

If the filtered signal is above the activation threshold, further operation according to normal production would possi- bly damage components of the wind turbine. Therefore, acti- vating the safe mode of operation will protect components of the wind turbine from damage. When the safe mode of operation is deactivated (and possibly a normal operation mode involv- ing producing electric energy is entered), if the filtered signal is below a deactivation threshold, the prevailing weather condition, in particular wind speed, is not expected to damage components of the wind turbine. Therefore, normal operation may be resumed in order to continue to produce and supply electric energy to the utility grid.

When the deactivation threshold is different from the activa- tion threshold, the flexibility of the method may be in- creased and adaptation to the expected environmental charac- teristics and characteristic of the wind turbine may be ena- bled. E.g., the activation threshold may be set to be higher than the deactivation threshold. Thereby, secure operation of the wind turbine may be enabled while ensuring to improve en- ergy production.

According to an embodiment of the present invention, filter- ing includes, in particular low-pass filtering, employing a filter parameter, wherein the filter parameter comprises a filter time constant being different for an increasing or de- creasing measurement signal dependent quantity and/or fil- tered signal.

Low-pass filtering may be implemented in a number of differ- ent manners. The filter being used may comprise a multi-rate filter which may apply a low-pass filtering.

Thus, the method may involve a predetermined activation threshold and a predetermined deactivation threshold based on one or more signals associated with an environmental condi- tion (e.g. wind speed, atmospheric pressure, etc.) The meas- ured environment condition (s) may be converted into a fil- tered signal through the use of a multi-rate filter. This multi-rate filter may determine its output using different filter parameters depending on whether the input measurement and/or filtered signal is increasing or decreasing. The safe mode of operation may become activated when the filtered sig- nal is beyond the predetermined activation threshold and may become deactivated when the filtered signal is beyond the predetermined deactivation threshold in the opposite direc- tion.

For cases where the activation requires an exceedance of an activation threshold, the multi-rate filter may only apply light low-pass filtering to increasing inputs of the measured condition and/or filtered signal. Light filtering may imply that the filter would have a small time constant, i, when filtering input signals. The filter could be such that there is no filtering of increasing input signals and/or filtered signal, i.e. t = 0. This case may be referred to as an enve- lope filter. This may ensure that as the measured condition exceeds the threshold for activation the safe mode of opera- tion is quickly activated with little or no delay.

The multi-rate filter may have a larger amount of low-pass filtering for decreasing inputs and/or filtered signal. The time constant here could be on the order of i = 1 to 4 hours.

This ensures that a rapid decrease in the measured condition does not result in corresponding rapid decrease in the fil- tered signal used in evaluation against the deactivation threshold. For gradual decreases in the measured input into the filter, the filtered output may only deviate slightly.

For cases where the activation of the safe mode of operation requires a signal to fall below an activation threshold, then the filtering of increasing and decreasing signals and/or filtered signal as described in the previous two paragraphs is reversed.

The benefit to this approach may be that a severe weather event usually ends with a gradual change in a measured envi- ronmental condition, such as wind speed.

The low-pass filtering of the measured signal when it is changing in the direction of the deactivation threshold may ensure that there is little delay from the end of the weather event to the return of operation. This is because fixed time delays, large threshold margins and/or long-duration averag- ing may not be needed to avoid deactivation due to short- duration behavior of the measured signal.

The filtered signal (e.g. y _i , e.g. output by a filter mod- ule) at a discrete time step i may depend on the measuring signal dependent quantity at this time step (e.g. u _±) and/or at a previous time step (e.g. u _±-i) and/or may depend on the filtered signal (e.g. y _{i-1 )} at the previous time step, in par- ticular in a linear manner. Filter coefficients (e.g. Bo and Bi) may be defined by a time constant (e.g. t), and the time duration between samples (timestep), Δt. The time constant may be dependent on whether the output y _i is larger or small- er than the previous output sample, y _i-1 .

When the filtered time constant is set to be different for an increasing or decreasing measurement signal dependent quanti- ty and/or filtered signal, the activation of the safe mode of operation may be performed without much delay after the weather condition changes to a potentially damaging condi- tion. Thus, a quick activation at the onset of a severe weather event may be ensured.

Further, when the filter time constant for the decreasing measurement signal dependent quantity and/or filtered signal is set to a larger value than the filter time constant set or utilized for the increasing measurement signal dependent quantity and/or filtered signal, it may be avoided to too early deactivate the safe mode of operation before the severe weather event has ended. Nevertheless, a long delay after the severe weather event has ended may also be avoided by appro- priately setting the filtered time constant for the decreas- ing measurement signal dependent quantity and/or filtered signal.

According to an embodiment of the present invention, for the case that the safe mode of operation is activated, if the filtered signal is above the activation threshold and the safe mode of operation is deactivated, if the filtered signal is below the deactivation threshold, the following applies: the time constant is set to a low time constant when the measuring signal dependent quantity and/or filtered signal is increasing; and the time constant is set to a high time con- stant when the measuring signal dependent quantity and/or filtered signal is decreasing, wherein the high time constant is greater than the low time constant.

The low time constant of the filter which is set when the measurement signal dependent quantity and/or filtered signal is increasing may ensure that deactivation or activation of the safe mode of operation is performed with only low or even without any delay, e.g. with only small delay after the meas- urement signal indicates a potentially damaging weather con- dition, such as too high wind speed. Further, when the time constant is set to the high time constant when the measure- ment signal dependent quantity and/or filtered signal is de- creasing, a premature deactivation of the safe mode of opera- tion may be avoided.

A low-pass filter may have the effect that when there are rapid changes in an input signal, the output signal will ad- just gradually. Rapid changes in the time domain, require high frequency content in the frequency domain. So if high frequency content of an input signal is filtered out, the output is prevented from changing rapidly.

The high time constant may for example be chosen depending on the particular application. The same holds for the low time constant.

The predetermined thresholds for activation and deactivation may have different values, such that there is a margin be- tween the activation of the safe operation mode and the deac- tivation. After the filtered signal is beyond the activation threshold, the mode will stay active until the filtered sig- nal has returned past the activation threshold to reach the deactivation threshold. The margin does not need to be large and for the example of wind speed measurements may only be 5- 10m/s between the two thresholds.

According to an embodiment of the present invention, the ac- tivation threshold is greater than the deactivation thresh- old, wherein the activation threshold is in particular be- tween 1.2 and 2.0 time the deactivation threshold.

When the activation threshold is greater than the deactiva- tion threshold, activation of the safe mode of operation may not be entered too early and deactivating the safe mode of operation may also not be entered too early, i.e. while po- tentially the severe weather event is not ended.

According to an embodiment of the present invention, for the case that the safe mode of operation is activated, if the filtered signal is below the activation threshold and the safe mode of operation is deactivated, if the filtered signal is above the deactivation threshold, the following applies: the time constant is set to a high time constant when the measuring signal dependent quantity and/or filtered signal is increasing; and the time constant is set to a low time con- stant when the measuring signal dependent quantity and/or filtered signal is decreasing, wherein the high time constant is greater than the low time constant.

This embodiment might be considered as a reverse setting com- pared to the previously described embodiment, wherein activa- tion is performed for the filtered signal to be below the ac- tivation threshold and deactivation is performed for the fil- tered signal to be above the deactivation threshold. Other- wise, effects of the described embodiments may be similar to those described with the previously described embodiment.

According to an embodiment of the present invention, the low time constant is between 0 and 60 minutes, in particular be- tween 0 and 10 minutes, and/or wherein the high time constant is between 30 minutes and 300 minutes, in particular between 60 minutes and 180 minutes.

Thereby, typical time values are provided which have been found by simulation and application to historical wind data to be appropriate for safely controlling the wind turbine. Other time values may be possible.

According to an embodiment of the present invention, filter- ing of the measuring signal dependent quantity comprises: av- eraging the measurement signal over a predetermined averaging time to obtain an averaged signal; filtering the averaged signal to obtain the filtered signal.

The averaged signal may therefore also have less high fre- quency components than the measurement signal itself. There- by, a kind of smoothing may be applied. Thereby, reliability of the method may be improved. The averaging may comprise for example also a weighted averaging in that the weighting of the past values of the measurement signals are accounted for or considered with less strength than values of the measure- ment signals closer to the present point in time. In other embodiments, a simple averaging may be performed without per- forming a waiting.

The measured signal(s) may be pre-filtered by e.g. an averag- ing window. However, the averaging time does not need to be on the order of hours and may be as short as 5 to 10 minutes. Especially in the case of wind speed measurements, this may ensure that fluctuations (e.g. due to turbulence) do not lead to inadvertent activations and/or deactivations. A 10-minute averaging of the input signal may also not have a significant effect on the delay of operation at the end of the severe weather event.

According to an embodiment of the present invention, the pre- determined averaging time is between 120 sec and 700 sec, in particular between 550 sec and 650 sec.

These values have been found by simulation to be appropriate according to embodiments of the present invention.

According to an embodiment of the present invention, filter- ing of the measuring signal dependent quantity comprises: limiting the measurement signal dependent quantity to be equal or greater than a floor value (e.g. V_0) not greater than the activation threshold; or limiting the measurement signal dependent quantity to be equal or lower than a ceiling value (V_0) not lower than the deactivation threshold; the method further comprising: filtering the limited measuring signal dependent quantity.

Limiting the measurement value to be equal or greater than the floor value may further improve the method and in partic- ular the reliability and safety and/or robustness. The ceil- ing value would be applied, if activation occurs below the activation threshold and deactivation occurs above the deac- tivation threshold. Furthermore, by using this implementation or embodiment, result of the filtering may be more restricted and predictable.

According to an embodiment of the present invention, the measurement signal related to the weather condition comprises a measurement signal of at least one of the following: a wind speed, an atmospheric pressure, a load on rotor blade and/or tower; a thrust on turbine rotor.

The weather condition may be measured by one or more measure- ment sensors installed at components of the wind turbine or for example at a met mast. Thereby, conventionally, available measurement signals may be utilized in embodiments of the present invention, thereby simplifying the method.

According to an embodiment of the present invention, the method is applied to plural types of measurement signals each being associated with respective type dependent filter param- eter and activation and deactivation threshold. Thereby, the method may be improved regarding reliability and adaptation to the particular application.

According to an embodiment of the present invention, activat- ing and/or deactivating the safe mode of operation comprises supplying a control signal to at least one actuator for es- tablishing the safe mode of operation, wherein a safe mode of operation comprises at least one of: shutting down the wind turbine; folding leewards (downwind) at least one rotor blade; adjusting a blade pitch angle of at least one rotor blade to a feathered position; adjusting or allowing to ad- just the yaw to a downwind position; idling in yaw downwind position; disconnecting the wind turbine from the electrical grid; securing and/or braking the rotor.

The actuator may for example be configured for adjusting a yaw angle defining an azimuthal angle of the nacelle relative to the wind turbine tower. In other embodiments, the actuator may comprise an actuator for a blade pitch angle readjust- ment. The actuator may additionally or alternatively also ac- tuate one or more breakers for example for disconnecting or connecting the wind turbine to a utility grid. Idling in the yaw downwind position may involve to stop production of elec- tric energy. Then the wind turbine rotor does not face any- more the wind, as is in the normal operation, but the rotor will be oriented in the opposite direction. In particular, the hub where the rotor blades are connected may be oriented leewards (downwind) in the safe mode of operation.

It should be understood that features, individually or in any combination, disclosed, explained, described or provided for a method of activating and/or deactivating a safe mode of op- eration of a wind turbine, are also, individually or in any combination, applicable to an arrangement for activating and/or deactivating a safe mode of operation of a wind tur- bine according to embodiments of the present invention and vice versa.

According to an embodiment of the present invention it is provided an arrangement for activating and/or deactivating a safe mode of operation of a wind turbine, the arrangement comprising: an input module configured to receive at least one measurement signal related to a weather condition; a pro- cessor configured: to filter a measuring signal dependent quantity to obtain a filtered signal, wherein the filtered signal depends on whether the measuring signal dependent quantity and/or filtered signal is increasing or decreasing with time; to derive at least on control signal for activat- ing and/or deactivating the safe mode of operation based on the filtered signal.

The arrangement may for example be a portion of a wind tur- bine controller. According to an embodiment of the present invention it is provided a wind turbine, comprising: a tower; a nacelle mounted on top of the tower; a rotor harboured in the na- celle; plural rotor blades mounted on the rotor; an arrange- ment according to the preceding embodiment; a measurement sensor configured to generate the measurement signal in de- pendence of a weather condition; an actuator for receiving the control signal.

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.

Figs. 1 and 2 schematically illustrate a wind turbine accord- ing to an embodiment of the present invention in two differ- ent operation modes;

Figs. 3, 4, 5 schematically illustrate a wind turbine accord- ing to an embodiment of the present invention in different operation transition states;

Figs. 6, 7, 8, and 9 illustrate graphs for explaining activa- tion and deactivation of a safe mode of operation of a wind turbine according to embodiments of the present invention for different weather events in the past by simulation;

Figs. 10 and 11 illustrate activation and deactivation of a safe mode of operation according to the prior art and accord- ing to an embodiment of the present invention respectively; and

Figs. 12 schematically illustrates an implementation of a filter used in embodiments of the present invention.

Detailed Description

The illustration in the drawings is in schematic form. It is noted that in different figures, elements similar or identi- cal in structure and/or function are provided with the same reference signs or with reference signs, which differ only within the first digit. A description of an element not de- scribed in one embodiment may be taken from a description of this element with respect to another embodiment.

The wind turbine 100 schematically illustrated in Fig . 1 ac- cording to an embodiment of the present invention comprises a wind turbine tower 102, a nacelle 103 mounted on top of the tower 102 and a rotor 104 having plural rotor blades 105 mounted thereon. The wind turbine 100 further comprises an arrangement 110 according to an embodiment of the present in- vention for activating/deactivating a safe mode of operation of the wind turbine 100. The arrangement 110 may for example be installed within the nacelle 103.

The arrangement 110 comprises an input terminal 111 for re- ceiving a measurement signal 112 which is related to a weath- er condition. The measurement signal 112 may for example be generated by a wind speed sensor 113 which may be mounted at the nacelle 103. The arrangement 110 comprises a not illus- trated processor which is configured to filter a measuring signal dependent quantity (i.e. a quantity which depends on the measurement signal 112) to obtain a filtered signal.

The filtered signal is for example indicated in Figs. 6, 7,

8, 9, 11 and is thereby denoted or labelled with reference sign ending with the digits 15 and followed by a letter. Such a filtered signal is for example illustrated in Fig. 6 and labelled with reference signs 615a, ..., 615d. The arrangement 110 is further configured for deriving at least one control signal 116 for activating and/or deactivating the safe mode of operation based on the filtered signal (for example sig- nals 615a, ..., 615d illustrated in Fig. 6). The control signal 116 may be supplied to a not in detail illustrated yawing system which may be configured to change a yawing angle of the nacelle as indicated by the arrow 117.

Thereupon, for activating the safe mode of operation illus- trated in Fig. 2, the control signal 116 may cause a yawing system to rotate the nacelle 103 around the vertical longitu- dinal axis of the wind turbine tower 102 from the normal ori- entation as depicted in Fig. 1 to the downwind position as illustrated in Fig. 2. As can be appreciated from Fig. 1, the wind 118 impacts on the rotor blades 105 for normal energy production, while the wind 218 in Fig. 2 hits the wind tur- bine from the back, since the wind turbine is in the yaw downwind position. In Fig. 1, the hub at which the rotor blades 105 are mounted faces the wind 118, while in Fig. 2, the hub is in an opposite direction.

Beside changing the yaw position of the nacelle for entering the safe mode of operation, further actuations may be per- formed according to embodiments of the present invention, for example adjusting blade pitch angle for example.

Figs. 3, 4 and 5 illustrate further embodiments of entering a safe mode of operation. Therein, the wind turbine 300 com- prises rotor blades 305 which may fold or collapse inward in a severe weather event to reduce the cross-sectional surface area exposed to the wind. Thereby, in Fig. 3, the wind tur- bine 300 is already adjusted in a yaw downwind position. In Figs. 4 and 5, further steps of the folding process of the rotor blades 305 are illustrated. In Fig. 5, entering the safe mode of operation is completed. Also, wind turbine 300 illustrated in Fig. 3, 4 and 5 comprises an arrangement 310 for activating and/or deactivating a safe mode of operation which may be configured and function in a similar manner as has been described with reference to Figs. 1 and 2.

A heavy weather event may for example comprise a tropical cy- clone having an eye in the center. When the wind turbine is within the eye of the center, the wind turbine may experience relatively low wind speed. In conventional methods, the meth- od may mistakenly consider the calm conditions within the eye as evidence that the tropical cyclone has completely passed and is over. Thereby, conventionally, the wind turbine may experience potentially hazardous, time-consuming and/or ener- gy-consuming situations. Within the eye, the wind speed may considerably drop from the peaks at the eye wall to the rela- tive calm of the hurricane eye. A safe operation mode that activates based only upon measured wind speed as has been conventionally applied, may mistakenly deactivate during this period of low wind. Other heavy weather conditions which may be addressed in embodiments of the present invention may com- prise multi-cell thunderstorms, where periods of relative calm may occur within a severe weather system.

Figs . 6 , 7 , 8 , 9 illustrate graphs as considered in embodi- ments of a method of activating and/or deactivating a safe mode of operation of a wind turbine according to embodiments of the present invention.

Therein, corresponding abscissas 620, 720, 820, 920 indicate the time while ordinates 621, 721, 821, 921 indicate the wind speed, as for example measured by wind speed sensor 113 il- lustrated in Fig. 1. An activation threshold 622 as well as a deactivation threshold 623 are defined as depicted in Fig. 6. Corresponding thresholds are also depicted in Figs. 7, 8 and 9, their reference sign differing only in the first digit. Those thresholds may be set to be different or may be set to be the same and may be selected or set depending on the par- ticular application. Those thresholds may for example be set depending on the geographical location, expected weather con- ditions, expected wind speeds and so forth.

The thick curves 624, 724, 824, 924 in Figs. 6, 7, 8, 9 label the wind speed averaged over an average time duration of 600 seconds. This wind speed average may represent a measuring signal dependent quantity according to embodiments of the present invention.

Fig. 6 indicates measurement and simulation values from a tropical cyclone. In a time interval 625 illustrated in Fig.

6, the average wind speed 624 increases, while in a time in- terval 626, the average wind speed 624 decreases. According to embodiments of the present invention, different filterings are applied in the time period 625 (average wind speed is in- creasing) in the time period 626 (average wind speed is de- creasing). According to the depicted embodiment, a time con- stant Ʈ of a filter is set to zero in the time window 625 as well as in other time windows or time intervals 627 in which the average wind speed is also increasing. In contrast, in the time intervals 626, 628, where the average wind speed 624 is decreasing, the curves 615a, 615b, 615c, 615d are indicat- ed corresponding to filtering results with time constants of Ʈ = 1 h, 2 h, 3 h, 4 h, respectively. The filtering with in- creased time constants in the time intervals 626, 628 results in the avoidance of too early deactivation of the safe mode of operation. Deactivation of the safe mode of operation oc- curs when selected one of the curves 615a, ..., 615d represent- ing a filtered signal, drops below the deactivation threshold 623. In the illustrated embodiment, deactivation may for ex- ample occur at the point in time 629 when curve 615a crosses the horizontal line representing the deactivation threshold 623. Other embodiments may select another of the curves 615a, ..., 615d as filtered signal that is compared with the deacti- vation threshold 623.

Activation of the safe mode of operation is performed, when the averaged wind speed (corresponding to a filtered value obtained by filtering with a time constant of i = 0) crosses and exceeds the activation threshold 622. Activation of the safe mode of operation occurs at the time point 630 in Fig. 6 at which point in time the averaged wind speed 624 crosses the horizontal line representing the activation threshold 622.

In Fig. 6, the activation threshold 622 is set at a wind speed of 35 m/s, while the deactivation threshold 623 is set at a value of a wind speed value of 25 m/s.

Fig. 7 comprises data from an extra-tropical storm. In Fig.

7, activation of the safe mode of operation occurs at the point in time 730 and deactivation occurs at the point in time 729.

The embodiments illustrated in Figs. 8 and 9 further comprise to define and employ a floor value V_0 defining the minimum input value to a filter which is utilized for deriving the filtered signal. The floor value V_0 illustrated in Figs. 8 and 9 is set at a value of 22 m/s, i.e. is below the deacti- vation threshold 823 and further also below the activation threshold 822. In Fig. 8, activation of the safe mode of op- eration is entered at a point in time 830 and deactivation is entered in a point in time 829.

As can be appreciated from Figs. 8 and 9, the filtered sig- nals 815a, ..., 815d and 915a, ..., 915d representing filtered signals as derived in embodiments of the present invention, are different from the filtered signals 615a, ..., 615d, 715a, ..., 715d, as derived in the embodiments illustrated in Figs. 6 and 7. Thereby, the method for activating/deactivating the safe mode of operation may further be tuned in accordance to the particular application.

Figs. 10 and 11 illustrate in comparison a conventional meth- od and a method according to an embodiment of the present in- vention for addressing activation and/or deactivation of a safe mode of operation. Therein, the abscissas 1020, 1120 in- dicate time while the ordinates 1021, 1121 indicate the wind speed.

The curve 1031 in Fig. 10 represents an average of a wind speed and the quantity 1032 represents a conventionally used speed threshold. Conventionally, activation occurs at the point in time 1033 where the wind speed 1031 crosses the threshold 1032. At a point in time 1034, the wind speed crosses for a second time the threshold 1032. Conventionally, a time interval or delay time At is waited before the safe mode of operation is deactivated at a point in time 1035. Thereby, a relatively large delay may lead to a considerable reduction of energy output.

The curve 1115 represents a filtered signal which is derived based on filtering the averaged wind speed 1124 for time du- rations in which the averaged wind speed 1124 decreases. Ac- cording to embodiments of the present invention, the filtered signal 1115 derived from the average wind speed 1124 is con- sidered for deactivation. Activation may occur at the point in time 1130, where the average wind speed 1124 crosses and exceeds the activation threshold 1122. Deactivation of the safe mode of operation occurs at the point in time 1129 at which the filtered signal 1115 crosses and lies below the ac- tivation threshold 1122 which is equal in the present embodi- ment to the deactivation threshold 1123. Thereby, as can be appreciated from Figs. 10 and 11, the point in time 1129 where the normal operation may be resumed, is by a time dura- tion At' earlier than the point in time 1035 where the nor- mal operation is resumed according to a conventional method.

Below further details of particular embodiments are disclosed to which the invention is not restricted:

For the case where activation of the safe operation mode is related to an exceedance of the activation threshold, a limit may be used for the input to the multi-rate filter such that the input value may not be lower than a predetermined floor not greater than the deactivation threshold. This floor can be used to ensure that the rate at which the filtered signal changes in response to decreases in the input signal is not affected by deep decreases in the signal. (An example of this floor is shown in Figure 8 and 9 as V_0). For the case where activation of the safe operation mode is related to a condi- tion falling below the activation threshold then the floor would act instead as a ceiling not lower than the deactiva- tion threshold. One or more signals may be used for activa- tion and deactivation of the safe mode of operation.

The use of multiple measured conditions may be done in dif- ferent ways. For example, wind speed, atmospheric pressure, load sensors on the rotor blades or tower, or thrust measure- ments on the turbine rotor may be used to activate and deac- tivate a safe operation mode during a tropical cyclone. One or more of these measured signals could be treated as de- scribed in this method and compared against unique activation and deactivation thresholds. The activation and deactivation conditions could then be combined using Boolean logic such that activation occurs when any signal's activation threshold is reached, and deactivation occurs only when all signals' deactivation threshold is reached. Other Boolean combinations could be used where appropriate. Similarly, multiple measured signals could be used to improve the accuracy of a single signal treated by the method described here. In the example of the tropical cyclone again, the atmospheric pressure meas- urement and rotor blade load measurements could be used to improve the accuracy of the measured wind speed signal through the use of state estimation from measurements and mathematical models, such as observers and Kalman Filters.

In the embodiments illustrated in Fig. 6 to 9 the 10-minute average of the wind speed is used as input to a multi-rate filter. The filter has a time constant of i = 0 for increas- ing wind speeds and results are shown for different time con- stants for decreasing wind speeds: t = 1, 2, 3, 4 hours. The activation threshold is set to 35 m/s and the deactivation threshold is set to 25 m/s. In Figures 6, 7, a floor VO on the input signal to the multi-rate filter is set to 0 m/s (i.e. no floor) and in Figures 8, 9 a floor of VO = 22 m/s is used.

The effect of the method is clear to see in the figures be- low. In Figure 6, by setting a time constant of only 1 hour the safe mode of operation is not deactivated by the wind speed decrease during the eye of the tropical cyclone. When the tropical cyclone is finally over, the wind speed is de- creasing at a slower rate and there is no additional delay due to the filtering for all but the case with a 4-hour time constant. This remains the case for the extra-tropical storm (Fig. 7). As the wind speeds decrease below there is little to no delay.

Figures 8, 9 are included just to show the effect of changing the floor VO.

Using conventional solutions to avoid deactivation of a safe operation mode due to the eye of a tropical cyclone, could result in a significant delay in returning to operation after the cyclone's passing. The eye of a tropical cyclone is typi- cally 30-65km in diameter but may range from just a few kilo- meters to very large diameters such as 370km for Typhoon Car- men (1960). The eye typically progresses at around 25 to 30 km/h, meaning that the duration of relative calm experienced by a wind turbine may last from one to a few hours. This would require a very large averaging window or fixed delay to avoid deactivation. Wind speeds in the eye can be very low, meaning that a margin would need to be very large to avoid deactivation. These deactivation conditions would lead to significant delays in returning to operation at the end of the severe weather event. The described solution has there- fore been developed. Fig . 12 schematically illustrates an implementation of a fil- ter 1250 (e.g. executing by a processor and comprised e.g in arrangement 110 illustrated in Fig. 1) used in embodiments of the present invention to derive a filtered signal (e.g. y _i , e.g. output by a filter module 1250) at a discrete time step i.

At an input module 1251 a measuring signal dependent quantity u _i at this time step i is received. A delay module 1252 gen- erates therefrom the measuring signal dependent quantity u _i-1 at the previous time step i-1.

The measuring signal dependent quantity u _i at the time step i and the measuring signal dependent quantity u _i-1 at the pre- vious time step are provided to gain modules 1253, 1254 hav- ing same gain coefficients B1 and B1.

At an addition element 1255 the outputs of modules 1253 and 1254 are added to the output of gain module 1256 having gain coefficient B0different than gain coefficient B1. The gain module 1256 receives the filtered signal y _i-1 at step i-1 from a delay element 1260 which in turn receives the filtered sig- nal yi at time step i from box 1261.

The output of the adder 1255 (provided to box 1261) is the filtered signal y _i at the discrete time step i.

This output, i.e. filtered signal y _i , is provided to gain change module 1257 that changes the gain coefficients as in- dicated in boxes 1258, 1259 depending on whether the filtered signal y _i at step i is greater of smaller than filtered sig- nal y _i-1 at step i-1.

Filter coefficients (e.g. Bo and Bi) may be defined by a time constant (e.g. t), and the time duration between samples (timestep), At. The time constant may be dependent on whether the output yi is larger or smaller than the previous output sample, y _i-1 . The following expression determines the output, i.e. filtered signal y _i for time step i:

The filter coefficients Bo and B ₁ are defined by the time constant, and the time duration between samples (timestep), Δt.

The time constant is dependent on whether the output y _i is larger or smaller than the previous output sample, y _i-1 :

Other equations are possible to derive the filtered signal.

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.