DK201870416A1 - Power optimisation of a wind turbine plant - Google Patents

Abstract

The invention relates to a method for controlling power production of a wind turbine park comprising a plurality of wind turbines in a situation where at least one of the wind turbines is operated to produce power at rated power level and at least one of the wind turbines is operated to produce power below the rated power level. A number of wind turbines, which able to produce power at the rated power, are selected and are provided with new up-rated power references, which are greater that the rated power level. Accordingly, some of the wind turbines are controlled to produce power above the rated power in order to compensate for the lower power production of some of the other wind turbines. In this way, it is possible to operate the wind park so that produced power from the parkapproaches the park capacity.

Description

POWER OPTIMISATION OF A WIND TURBINE PLANT

FIELD OF THE INVENTION

The invention relates to control of wind turbine plants, particularly to optimisation of the power production of wind turbine plants.

BACKGROUND OF THE INVENTION

In a wind turbine park, each wind turbine will be exposed to different wind speeds due to different locations in the park and due to wake effects from neighbour wind turbines. Accordingly, the power production of the wind park is often lower than the park's power capacity since not all wind turbines may be exposed to optimal wind conditions.

Accordingly, it is a problem that the power capacity of the wind turbine park is not utilized as often as desired.

EP 2 878 809 A1 discloses methods for operating a variable speed wind turbine as a function of a wind speed, the wind turbine having a rotor with a plurality of blades, a generator having a rated output power, and one or more pitch mechanisms for rotating the blades around their longitudinal axis, and a system for varying a torque of the generator. The methods comprise a sub-nominal zone of operation for wind speeds below a nominal wind speed and a supra-nominal zone of operation for wind speeds at or above the nominal wind speed, wherein at wind speeds at or near the nominal wind speed, the generator is allowed to generate more than its rated output power for a limited period of time. Also disclosed are wind turbines and wind farms adapted to perform these methods.

Whereas EP 2 878 809 A1 relates to power production optimization, the inventors of the present invention has appreciated that an improved solution is of benefit, and has in consequence devised the present invention.

SUMMARY OF THE INVENTION

It is an object of the invention to improve wind turbine power plant such as improving the power production capabilities of the wind turbine power plant and

DK 2018 70416 A1 particularly to improve the utilization of the wind turbine power plant's power production capacity.

In a first aspect of the invention, a method for controlling power production of a wind turbine park in a situation where at least one of the wind turbines is operated to produce power at a rated power level and at least one of the wind turbines is operated to produce power below the rated power level is provided, the wind turbine park comprises a plurality of wind turbines, the method comprises:

- determining a power difference between an amount of power produced by the wind turbine park and a power production capacity of the wind turbine park,

- determining at least one candidate wind turbine from the plurality of wind turbines which is operable to produce power at or above the rated power level,

- selecting at least one wind turbine from the candidate wind turbines,

- determining new power references for the selected wind turbines, where the new power references are increased relative to rated power references based on the power difference, and

- dispatching the new power references to the selected wind turbines to enable them to produce power above the rated power level.

Advantageously, the method includes the step of determining the increased power references based on a difference between the park capacity and the actual power production of the wind turbine park. Advantageously, the method also includes the step of determining which of wind turbines of the park should be operated to produce power at the nominal power level.

Due to these functions, the method enables control of the wind park's power production up to, but not above the rated park capacity, i.e. since only those wind turbines which has a capability to increase power are selected and since the new power references are determined on basis of the park capacity.

In comparison, EP 2 878 809 A1 does not distribute the increased power references only among wind turbine generators which are operated at full load. Furthermore, EP 2 878 809 A1 does also not determine power references based on the park capacity.

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According to an embodiment, the at least one candidate wind turbine is determined based on individual wind speeds associated the plurality of wind turbines.

Advantageously, by considering the wind speeds of each wind turbine, it is ensured that only those wind turbines which have the capability to be uprated are selectable.

For example, the at least one candidate wind turbine is determined to be operable to produce power at or above the rated power level if the individual wind speed associated with the at least one candidate wind turbine is above a rated wind speed and below a cut-out wind speed or possibly below an increased cut-out wind speed. Advantageously, by considering wind turbines up to the cut out wind speed, the power production of the wind park can be optimized also in the higher range of wind speeds.

According to an embodiment, the method comprises determining or predicting if the wind speed for one or more of the plurality of wind turbines is or will be within a predefined wind speed distance from the cut out wind speed. Advantageously, by use of prediction methods or by checking if individual wind speeds are within a given range from the cut out wind speed, it is possible to predict if a given wind turbine is about to enter the cut out region or the full load region. The prediction may be based on e.g. Lidar technologies or extrapolation of recently obtained wind speeds of individual wind turbines.

Similarly, the method may comprise determining or predicting if the wind speed for one or more of the plurality wind turbines is or will be within a predefined wind speed distance from the rated wind speed.

According to an embodiment, the method further comprises determining an increased threshold value of the cut out wind speed for the selected wind turbines. Advantageously, by increasing the cut out wind speed, the park production can be increased towards the park capacity in situations of high wind speeds where some wind turbines are de-rated or shut down.

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According to an embodiment, the method further comprises determining if at least one of the wind turbines is operated in a de-rated mode or shut down mode to produce power below the rated power level. Advantageously, by simply monitoring which turbines are de-rated or shut down due to high wind speeds, the remaining wind turbines can be used as candidate wind turbines for possible uprated production.

According to an embodiment, the selection of the least one wind turbine from the candidate wind turbines is performed based on determined or estimated wind conditions and/or health data of the candidate wind turbines. Advantageously, the selection can be based on wind conditions such as turbulence and/or the health data in order to only select the best suited wind turbines.

According to an embodiment, the selection of the least one wind turbine is performed based on accumulated loads determined for the at least one candidate wind turbine. Advantageously, the selection can be based on accumulated loads in order to select those wind turbines with the lowest accumulated loads.

For example, the accumulated loads may comprise predicted accumulated loads determined from environmental parameters associated with locations of the wind turbines and/or calculated accumulated loads determined from measured load parameters of the wind turbines.

According to an embodiment, the selection of the least one wind turbine is performed based on instantaneous load measurements of the at least one candidate wind turbine. If a wind turbine for some reason is exposed to high loads, it may be preferred not to up-rate power production for that wind turbine.

According to an embodiment, the selection of the least one wind turbine is performed based on accumulated time periods of producing power above the rated power level associated with respective ones of the at least one candidate wind turbine. In this way it may be possible to obtain similar utilization times of up-rated production of different wind turbines.

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According to an embodiment, the selection of the least one wind turbine is performed based on a detection of blade icing of the at least one wind turbine. Advantageously, for wind turbines designed to produce power even with presence of blade icing, it is possible to utilize the extra load margin which is available when not icing is present for up-rated production.

A second aspect of the invention relates to a central controller for controlling power production of a wind turbine park comprising a plurality of wind turbines in a situation where one or more of the wind turbines are operated to produce power at a rated power level and at least one of the wind turbines are operated to produce power below the rated power level, where the central controller is arranged to perform the method according to the first aspect.

A third aspect relates to a wind turbine power plant which comprises a plurality of wind turbines and the central controller according to the second aspect.

A fourth aspect of the invention relates to a computer program product comprising software code adapted to control a wind power plant when executed on a data processing system, the computer program product being adapted to perform the method according to the first aspect.

In general, the various aspects and embodiments of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 illustrates a wind turbine power plant,

Fig. 2A illustrates the power production curve for the power production of a wind turbine for wind speeds near the rated wind speed,

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Fig. 2B illustrates the power production curve for the power production of a wind turbine for wind speeds near the cut out wind speed,

Fig. 3A illustrates a method which is able to increase the park power production in the vicinity of the rated wind speed,

Fig. 3B illustrates a method which is able to increase the park power production in the vicinity of the cut out wind speed, and

Fig. 4 illustrates methods for controlling power production of a wind turbine park for so that selected wind turbines can be controlled to produce power above the rated power in order to compensate for the lower power production of some of the other wind turbines. In this way, it is possible to operate the wind park so that produced power from the park approaches the park capacity.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a wind turbine power plant 100 which comprises a plurality of wind turbine generators 101 (WTG) arranged to form a wind turbine park 102.

The power plant 100 is connectable with an electrical power grid (not shown) for supplying power from the wind turbine generators 101 to the electrical power grid. The connection with the power grid may be made through a point of common coupling to which all, or at least a plurality, of the wind turbine generators 101 is connected.

The power plant 100 is controlled by a central controller 110, also known as a power plant controller, a plant controller or park controller. The central controller 110 is arranged to control power generation from the wind turbine generators 101, e.g. according to a power reference Pref which defines the desired power to be supplied to the grid from the power plant 100.

The wind turbine 101 comprises a tower 111 and a rotor 112 with at least one rotor blade 113, such as three blades. The blades 113 are connected with the hub 114 which is arranged to rotate with the blades. The rotor is connected to a nacelle 115 which is mounted on top of the tower 111 and adapted to drive a generator situated inside the nacelle via a drive train. The rotor 112 is rotatable by action of the wind. Thus, the wind turbine 100 is capable of converting kinetic energy of the wind into mechanical energy by means of the rotor blades and,

DK 2018 70416 A1 subsequently, into electric power by means of the generator. The generator is connected with a power converter, which may comprise a generator side converter and a grid side converter. The generator side converter converts the generator AC power into DC power and the grid side converter converts the DC power into an AC power for injection into the utility grid via output inductors of the wind turbine 101.

The generator of the wind turbine generator is controllable to produce power, i.e. active power, corresponding to a power setpoint, i.e. an active power setpoint provided by the central controller 110. The output power may be adjusted according to the power setpoint by adjusting the pitch of the rotor blades 103 or by controlling the power converter to adjust the power production. Accordingly, the power setpoint is used for controlling the amount of wind power to be extracted by the wind turbine.

The power plant 100 further comprises a central controller 120 for controlling power production of the wind turbine park 102.

Fig. 2A illustrates the power production curve 201 for the power production of a wind turbine 101 for wind speeds near the rated wind speed v_rated. The power production is given as a function of wind speed v. For wind speeds below the rated wind speed v_rated, the power production increases as a function increasing wind speed. This lower range of wind speeds is referred to as the partial load region. For wind speeds above the rated wind speed v_rated, the power production is normally limited to a maximum power production Prated. This upper range of wind speeds is referred to as the full load region.

In a wind turbine park 102, at a given moment in time, some wind turbines 101 will experience wind speed below v_rated, others will experience wind speed above v_rated. Accordingly, even if the average wind speed at the wind turbine park 102 is at a level corresponding to or above the rated wind speed v_rated, some wind turbines 202 will produce power below the rated power Prated, whereas other wind turbines are able to produce the rated power Prated.

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Specifically as illustrated in Fig. 2A wind turbines 202a, 202b produces power below Prated, e.g. since the individual wind speeds associated the wind turbines 202a, 202b is below v_rated or due to wind turbulence. Wind turbines 202c, 202d are able to produce the rated power Prated, since the individual wind speeds associated the wind turbines 202c, 202d is above v_rated.

References to wind speeds herein, e.g. individual wind turbine wind speeds or average park wind speeds, may be instantaneous wind speeds or average wind speeds obtained as an average over a period of time.

Therefore, even with the average wind speed for the park 102 being above the rated wind speed v_rated, the wind turbine part 102 is only able to produce an amount of power with is less than the park capacity Pcap_park. For example, a 100 MW park capacity Pcap_park could be achieved by 50 wind turbines having a rated power Prated at 2 MW each.

Curve 203 illustrates the power produced by the wind turbine park 202 normalised with the number of wind turbines in the wind turbine park 202. Due to different power productions of different wind turbines 101, the park power Ppark is lower than the park capacity Pcap_park in the vicinity of the rated wind speed vrated. Accordingly, the maximum park capacity Pcap_park is not achieved around the rated wind speed.

Fig. 2B illustrates the power production curve 211 for the power production of a wind turbine 101 near the cut out wind speed v_cutout. For wind speeds up to the cut out wind speed v_cutout, the power production is normally limited to the maximum power production Prated. This range of wind speeds between v_rated and v_cutout is full load region. For wind speeds above the cut out wind speed v_cutout, the power production is reduced to a level below Prated. This may be achieved by an instantaneous shut down (curve 211a) or by a gradual reduction of the produced power, e.g. as a function of wind speed (curve 211b).

Correspondingly with the situation in Fig. 2b, at a given moment in time, some wind turbines 101 will experience wind speed below v_cutout, others will experience wind speeds above v_cutout. Accordingly, even if the average wind

DK 2018 70416 A1 speed at the wind turbine park 102 is at a level corresponding to or some level below the cut out wind speed v_cutout, some wind turbines 202 will produce power below the rated power Prated - since they have been actively derated whereas other wind turbines are still allowed produce the rated power Prated.

Specifically as illustrated in Fig. 2B wind turbines 202g, 202h produces power below Prated since the individual wind speeds associated with the wind turbines 202g, 202h is above v_cutout. Wind turbines 202e, 202f are still allowed to produce the rated power Prated since the individual wind speeds associated the wind turbines 202e, 202f is below v_cutout. Here the cut out wind speed v_cutout is the wind speed limit above which the turbine may be shut down or may be operated at a lower power level to reduce the loading on turbine.

Therefore, even with the average wind speed for the park 102 being below the cut out wind speed v_cutout, but still near v_cutout, the wind turbine part 102 is only able to produce an amount of power with is less than the park capacity Pcap_park.

Curve 213 illustrates the power produced by the wind turbine park 202 normalised with the number of wind turbines in the wind turbine park 202. Due to different power productions of different wind turbines 101, the park power Ppark is lower than the park capacity Pcap_park in the vicinity of the cut out wind speed v_cutout. Accordingly, the maximum park capacity Pcap_park is not achieved around the cut out wind speed.

Fig. 3A illustrates an embodiment according to the invention which is able to increase the park power production Ppark in the vicinity of the rated wind speed vrated. This is achieved by allowing wind turbines 101 which experiences wind speeds above v_rated to produce power at a level above the rated power Prated. The power level above the rated power Prated may be determined according to the power production curve 301, being a function of the wind speed, so the wind turbines may be able to produce power up to the up-rated power level Puprated. Power references for selected wind turbines 101 may be determined from the uprated power production curve 301 by the central controller 120.

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Specifically as illustrated in Fig. 3A wind turbines 202a, 202b produces power below Prated whereas wind turbines 202c, 202d are able to produce power at a level above the rated power Prated according the power curve 301. Thus, the power production of the wind turbines 202c, 202d operated in the full load and uprated region compensates the low power production of the wind turbines 202a, 202b being operated in the partial load region.

The up-rated power production curve 301 may be defined to extract the maximum wind energy as a function of the wind speed v up to the uprated power level Puprated. For wind speeds above a given speed, the power production curve 301 may be decreased again from Puprated to Prated. The increase of the power production from Prated to Puprated may be achieved by applying optimal pitch angles to the rotor blades 103 until the uprated power Puprated is achieved. After this the wind turbine is controlled according to the power production curve 301 and the generator speed is controlled via pitch angle adjustments.

Accordingly, since some of the wind turbines 101 are capable of producing power above the rated power Prated, the park power production Ppark approaches maximum park capacity Pcap_park for wind speeds around the rated wind speeds as illustrated by power curve 303 which is closer to the principal power curve 201 than the power curve 203 in Fig. 2A.

Fig. 3B illustrates an embodiment according to the invention which is able to increase the park power production Ppark in the vicinity of the cut out wind speed v_cutout. This is achieved by allowing wind turbines 101 which experiences wind speeds below v_cutout to produce power at a level which is higher than the rated power Prated. The power level above the rated power Prated may be determined according to the power production curve 311, being a function of the wind speed, so the wind turbines may be able to produce power up to the up-rated power level Puprated. Thus, the power production curves 301, 311 have similar functions for different ranges of the wind speed.

Specifically as illustrated in Fig. 3B, wind turbines 202g, 202h produces power below Prated whereas wind turbines 202e, 202f are able to produce power at a level above the rated power Prated according the power curve 311. Thus, the power production of the wind turbines 202e, 202f operated near but below the cut

DK 2018 70416 A1 out wind speed compensates the low power production of the wind turbines 202a, 202b being operated in the cut out region.

The up-rated power production curve 311 defines the uprated power production as a function of the wind speed with a maximum power given by Puprated. The uprated power for wind speeds near v_rated and the uprated power for wind speeds near v_cutout may be equal or different.

For wind speeds above a given speed, the power production curve 311 may be increased from Prated to Puprated. The uprated power level may be achieved by setting the power reference for the wind turbine according to the power production curve 311 and controlling the generator speed via pitch angle adjustments to extract wind power according to the power production curve 311. Above the cut out wind speed v_cutout, the uprated power production curve 311 is decreased, either gradually as a function of wind speed to allow some power production above v_cutout, alternatively uprated power production curve 311 commands a complete shut down.

Accordingly, since some of the wind turbines 101 are capable of producing power above the rated power Prated, the park power production Ppark approaches maximum park capacity Pcap_park for wind speeds around the cut out wind speeds as illustrated by power curve 313 which is closer to the principal power curve 211 than the power curve 213 in Fig. 2A.

As an alternative or in addition to uprating the power production for one or more wind turbines 101 operating near the cut out wind speed, the threshold value of the cut-out wind speed v_cutout may be increased to v_cutout2 and applied to one or more wind turbines 101. The increased cut-out wind speed may be applied to the original power production curve 211 to allow wind turbines to continue production at Prated above v_cutout. Alternatively, the increased cut-out wind speed may be applied to the uprated power production curve 311 to allow wind turbines to continue production at Puprated above v_cutout.

Fig. 4 illustrates a method according to an embodiment for controlling power production of a wind turbine park 102 in a situation where at least one of the wind

DK 2018 70416 A1 turbines is operated to produce power at a rated power level Prated and at least one of the wind turbines is operated to produce power below the rated power level Prated.

Thus, the method addresses situations where there is possibility to increase to park production and where at least one of the wind turbines 101 experiences wind speeds above the rated wind speed v_rated and below the cut-out wind speed v_cutout or v_cutout2. In this situation, at least one other wind turbine 101 is not able to produced the rated or nominal power Prated, e.g. since the wind turbine experiences wind speeds below the rated wind speed v_rated and above the cutout wind speed v_cutout.

This situation may be determined on basis of determined or estimated wind speeds for individual wind turbines 101. Alternatively, or in combination with the analysis of individual wind speeds, the situation may be determined on basis of a determined power difference AP between an amount of power produced by the wind turbine park Ppark and a power production capacity of the wind turbine park Pcap_park.

If the power produced by the wind turbine park Ppark is lower than the power production capacity of the wind turbine park Pcap_park due to one or more wind turbines 101 being operated on basis of wind speeds below the rated wind speed v_rated or above the cut-out wind speed v_cutout, one or more candidate wind turbines WTG_cand are determined. The candidate wind turbines are operable to produce power at or above the rated power level since they experience wind speeds above v_rated and below v_cutout or v_cutout2. Accordingly, the one or more candidate wind turbines are determined based on individual wind speeds associated with individual wind turbines.

Optionally, a selection WTG_sel of at least one wind turbine from the candidate wind turbines may be performed for the purpose of selecting particular ones of the candidate wind turbines. For example, some candidate wind turbines may be less preferred due to presence of turbulent wind conditions, low remaining life time, high structural loads, a high number of alarm logs, due to a maximum number of wind turbines which are allowed to be uprated and other conditions.

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On basis of the determined power difference AP and the selected wind turbines WTG_sel, new power references Pref are determined for the selected wind turbines. The power references may be determined from the uprated power production curves 301, 311 and are subsequently dispatched to the selected wind turbines to enable them to produce power above the rated power level Prated.

In case a wind turbine is not able to produce Puprated, but only a power between Puprated and Prated since the wind speed is not high enough to enable production at Puprated, the wind turbine may still be uprated by setting the power reference Pref to Puprated. The wind turbine will then produce power according to the available wind energy, i.e. a power between Puprated and Prated, as long as the wind speed is sufficient for producing at least Prated.

For example, with reference to Fig. 3A, wind turbine 202d may be selected in order to compensate fully or partially the low production from wind turbines 202a, 202b. Wind turbine 202c may be a candidate wind turbine, but not selected to be uprated due to wind speeds being close to the threshold wind speed, v_rated.

According to an embodiment, the candidate wind turbines are arranged in a prioritized list so that wind turbines having wind speeds closest to v_rated or v cutout have the lowest priority. In generated candidate wind turbines may be selected dependent on the wind speed associated with them or dependent on the distance between the associated wind speed and the threshold v_rated or v cutout.

The method of various embodiments can be implemented in the central controller 120 or other controller or processor/computing device of the wind turbine power plant 100.

In order to predict if any of the wind turbines 101 is approaching the cut out wind speed v_cutout, the method may include monitoring of the wind speed for individual wind turbines in order to determine if the wind speed is within a predefined wind speed distance from the cut-out wind speed v_cutout or is within a wind speed range near the cut our wind speed. Alternatively, a prediction of

DK 2018 70416 A1 future wind speeds for individual wind turbines may be obtained, e.g. on basis of met mast or Lidar technologies, in order to forecast if wind speeds will be within a predefined wind speed distance from the cut-out wind speed or within a predefined wind speed range.

By determining if one or more wind turbines 101 are approaching the cut-out wind speed limit, actions could be taken to prepare an increase of power production of other wind turbines. Such actions could include determination of candidate wind turbines which can be selected to increase the power production when needed.

Similarly, the determination or estimation of future wind speeds may be used to determine wind turbines which are likely to re-enter rated wind speeds below v_cutout, i.e. full load operation, by determining wind speeds which approach the cut out wind speed from above.

Similarly, in order to predict if any of the wind turbines 101 is approaching the rated wind speed v_rated from wind speeds below or above v_rated the method may include determining or predicting if the wind speed for individual wind turbines is or will be within a predefined wind speed distance from the rated wind speed or within a suitably defined range of wind speeds.

The method may include a simple determination of which, if any, of the wind turbines is operated above the cut out wind speed v_cutout, i.e. wind turbines which are operated in a derated power mode or shut down mode. This check may be performed by monitoring the status of a wind turbine controller signal. This method may be used in addition to or as an alternative to determining wind turbines operated above the cut out wind speed on basis of measured individual WTG wind speed. Similarly, wind turbines which are operated in the partial load region can be determined on basis of monitoring the status of a suitable wind turbine controller signal.

As previously mentioned, selected wind turbines WTG_sel may determined from the candidate wind turbines on basis of various conditions.

For example, selected wind turbines WTG_sel may be determined on basis of

DK 2018 70416 A1 determined or estimated wind conditions and/or health data of the candidate wind turbines. Wind conditions may include turbulent wind conditions which can be used to give wind turbines with the highest turbulent conditions lower selection priority compared to wind turbines exposed to lower turbulence. Health conditions can include alarms statistics or alarm logs. Similarly, wind turbines having a high alarm frequency may be given lower priority since they may be not be sufficiently reliable to be used for uprated power production.

Alternatively or additionally, selected wind turbines WTG_sel may be determined on basis of accumulated loads determined for candidate wind turbine or all wind turbines 101. Accordingly, a wind turbine having high values of accumulated loads, e.g. accumulated loads of the blades 113 may not be available to produce uprated power levels.

The accumulated loads may be determined from measured or estimated load parameters of the wind turbines, such blade loads. Alternatively or additionally, the accumulated loads may be determined from environmental parameters associated with locations of the wind turbines. For example, site specific environmental data for the wind park 102, such as turbulence data, obtained prior to the establishing the wind park or after establishment, can be used to predict the general load levels and therefore accumulated loads of individual wind turbines.

As another alternative or a supplement, selected wind turbines WTG_sel may be determined based on instantaneous load measurements of the candidate wind turbines or wind turbines 101. Accordingly, a wind turbine 101 which experiences a high load, e.g. due to some fault, may not be available for uprated power production.

In order to achieve a distribution of the loading of the wind turbines due to uprated operation, wind turbines 101 from the group of candidate wind turbines may be selected on basis of the history of previous situations where the wind turbines have been operated to produce uprated power, e.g. based on an integrated period of time where individual wind turbines have been operated to produce power above the associated rated power levels. The integrated period of

DK 2018 70416 A1 time may be the time from the erection of the wind turbines until present, or over other time span such as the time since the last major maintenance service.

Wind turbines located in relative cold areas may be exposed to icing where layers of ice is formed on the blades. For that reason, wind turbines operated at such locations are designed or operated to be able to withstand loads due the extra weight caused by icing. When such wind turbines are not exposed to icing, the unused load margin can be used to produce power at a level above the rated power Prated. Therefore, for wind turbines which are configured to operate with blade icing, selected wind turbines WTG_sel may be determined from a blade icing status of the individual wind turbines. The blade icing status may be available from a blade icing control signal from the wind turbine, or it may be detected otherwise e.g. from measurements of blade loads.

As explained the selected wind turbines WTG_sel may be determined on basis of various control signals 401 provided by the individual wind turbines, such as icing status, partial- and full load operation signals, load signals, etc. Data such as time data for uprated operation, time data relating to period where a WTG has been operated above a given load, alarm log data and accumulated load data may be provided by the individual wind turbines 101 or their associated controllers, or such data may be determined by the central controller 120 or other controller or processing device.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is to be interpreted in the light of the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims (16)

1. A method for controlling power production of a wind turbine park (102) comprising a plurality of wind turbines (101) in a situation where at least one of the wind turbines is operated to produce power at a rated power level (Prated) and at least one of the wind turbines is operated to produce power below the rated power level, the method comprises:

- determining a power difference (AP) between an amount of power produced by the wind turbine park (Ppark) and a power production capacity (Pcap_park) of the wind turbine park,

- determining at least one candidate wind turbine (202d) from the plurality of wind turbines which is operable to produce power at or above the rated power level,

- selecting at least one wind turbine from the candidate wind turbines,

- determining new power references (Pref) for the selected wind turbines, where the new power references are increased relative to rated power references based on the power difference, and

- dispatching the new power references to the selected wind turbines to enable them to produce power above the rated power level.

2. A method according to claim 1, wherein the at least one candidate wind turbine is determined based on individual wind speeds associated the plurality of wind turbines.

3. A method according to claim 2, wherein the at least one candidate wind turbine is determined to be operable to produce power at or above the rated power level if the individual wind speed associated with the at least one candidate wind turbine is above a rated wind speed (v_rated) and below a cut-out wind speed (v_cutout).

4. A method according to claim any of the preceding claims, further comprising determining or predicting if the wind speed for one or more of the plurality of wind turbines is or will be within a predefined wind speed distance from the cut-out wind speed (v_cutout).

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5. A method according to claim any of the preceding claims, further comprising determining or predicting if the wind speed for one or more of the plurality wind turbines is or will be within a predefined wind speed distance from the rated wind speed (v_rated).

6. A method according to any of the preceding claims, further comprising determining an increased threshold value (v_cutout2) of the cut-out wind speed for the selected wind turbines.

7. A method according to claim any of the preceding claims, further comprising determining if at least one of the wind turbines is operated in a de-rated mode or shut down mode to produce power below the rated power level.

8. A method according to claim any of the preceding claims, wherein the selection of the least one wind turbine from the candidate wind turbines is performed based on determined or estimated wind conditions and/or health data of the candidate wind turbines.

9. A method according to claim any of the preceding claims, wherein the selection of the least one wind turbine is performed based on accumulated loads determined for the at least one candidate wind turbine.

10. A method according to claim 9, wherein the accumulated loads comprises predicted accumulated loads determined from environmental parameters associated with locations of the wind turbines and/or calculated accumulated loads determined from measured load parameters of the wind turbines.

11. A method according to claim any of the preceding claims, wherein the selection of the least one wind turbine is performed based on instantaneous load measurements of the at least one candidate wind turbine.

12. A method according to claim any of the preceding claims, wherein the selection of the least one wind turbine is performed based on accumulated time periods of producing power above the rated power level associated with respective ones of the at least one candidate wind turbine.

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13. A method according to claim any of the preceding claims, wherein the selection of the least one wind turbine is performed based on a detection of blade icing of the at least one wind turbine.

14. A central controller (120) for controlling power production of a wind turbine park (102) comprising a plurality of wind turbines in a situation where one or more of the wind turbines are operated to produce power at a rated power level and at least one of the wind turbines are operated to produce power below the rated power level, where the central controller is arranged to perform the method according to claim 1.

15. A wind turbine power plant (100) which comprises a plurality of wind turbines (101) and the central controller (120) according to claim 14.

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