A system for control of a wind turbine

Abstract

A system 70 for deterring birds 71 from a wind power plant 72 comprising a plurality of wind turbines 73 comprises one or more detection devices 74, 75 to detect the presence and location of one or mo

Description

A wind turbine typically comprises a rotor mounted to a tower. The rotor comprises a hub and a plurality of blades configured to extend from the hub. The rotor typically comprises three blades, although other numbers of blades are possible. Each blade is operably coupled to the hub by a blade bearing which allows for rotation of the blades relative to the hub, such that the pitch of the blades is adjustable. The rotor is coupled to a generator, such as via a gearbox. The generator is configured to convert the rotational energy of the rotor to electrical energy. A nacelle houses the generator and the optional gearbox. A main bearing supports the rotor and allows for rotation of the rotor relative to the nacelle and generator. In some examples, the wind turbine may include a brake to slow and stop the rotation of the rotor. The control of a wind turbine by a wind turbine controller may account for efficient extraction of energy from the wind field and the mitigation of excess wear and damage to the wind turbine. It can be advantageous for a wind turbine controller to have advance knowledge of the wind field that will be incident on the wind turbine. However, the collection of data in an efficient and effective manner presents a challenge. Summary According to a first aspect of the disclosure we provide a system for deterring birds from a wind power plant comprising a plurality of wind turbines, the system comprising: one or more detection devices configured to detect the presence and location of one or more birds proximal the wind power plant; one or more drones; a controller configured to receive information indicative of a detected location of one or more birds detected by the one or more detection devices, the controller configured to provide control signals to the one or more drones to fly to a location for deterring the detected one or more birds based on the detected location. In one or more embodiments, the one or more detection devices comprise one or more of: a RADAR, detection device; a thermal imaging device configured to detect birds based on thermal energy emitted by the birds; a camera configured to provide images for object recognition; and a LIDAR detection device. In one or more embodiments, the one or more detection devices are configured to be one of: ground-mounted, offshore structure mounted; wind turbine mounted, or drone mounted or a combination. In one or more embodiments, the number of drones is less than the number of wind turbines and the controller is configured to select at least one of the plurality of drones to receive the control signals to fly to the location for deterring the detected one or more birds based on predetermined criteria. In one or more embodiments, the one or more drones each include a camera and wherein the controller is configured to perform object recognition based on one or more images acquired by the camera to detect the presence and location of the one or more birds. In one or more embodiments, the one or more drones each include one or more of: a speaker for emitting a sound for deterring the detected birds and predator-imitating features. 06 03 25 In one or more embodiments, the controller is configured to cause the speaker of the one or more drones to emit the sound based on the proximity of the drone to the location for deterring the detected one or more birds. 5 In one or more embodiments, the one or more drones comprise a plurality of drones, each drone of the plurality of drones configured to have different default location distributed over the wind power plant, and wherein the controller is configured to receive information indicative of the different default locations, and io wherein the controller is configured to, in response to receipt of the detected location, select at least one of the plurality of drones based on a proximity of the detected location to the default location of the plurality of drones, and provide the control signals to the selected at least one drone. 15 In the present disclosure, the controller is configured to calculate whether the detected location received from the one or more detection devices is at a height lower than a predetermined threshold height, the predetermined threshold height based on a height of the wind turbines of the wind power plant, and 20 if the detected location is lower than the threshold height, provide the control signals to fly the at least one drone; and if the detected location is higher than the threshold height, do not provide the control signals to fly the at least one drone. 25 In one or more embodiments, the one or more detection devices include a camera and wherein the controller is configured to receive one or more images captured by the camera; and wherein the controller is further configured to: identify the type of bird based on the one or more images using an image detection algorithm; and 30 based on the identification of a plurality of birds by the one or more detection devices at any one time, prioritise the plurality of birds based on the identified bird type, and provide the control signals to fly the at least one drone based on said prioritization. In one or more embodiments, the one or more drones include a first part of a coupling device to couple to a bird deterring device comprising a second part of the coupling device and wherein said control signals are further configured to instruct at least one of the one or more drones to couple to the bird deterring device and then fly to the location for deterring the detected one or more birds. In one or more embodiments, at least a subset of the one or more drones include one or more sensors for measuring a characteristic of a wind field, and wherein the one or more drones are configured to operate in a bird deterrent mode in which they receive said control signals to fly to the location for deterring the detected one or more birds and a measurement mode wherein the one or more first drones are configured to fly upstream of one of the plurality of wind turbines to measure the characteristic of the wind field that will be incident on a rotor of that wind turbine, and wherein the controller is configured to operate the subset of the one or more drones in the measurement mode at times when the one or more detection devices do not detect the presence of one or more birds and operate the subset of the one or more drones in the bird deterrent mode at times when the one or more detection devices detect the presence of one or more birds. In one or more embodiments, at least a subset of the one or more drones include a camera and / or a sensor for performing inspections of a wind turbine, and wherein the one or more drones are configured to operate in a bird deterrent mode in which they receive said control signals to fly to the location for deterring the detected one or more birds and an inspection mode wherein the one or more first drones are configured to be flown to inspect the wind turbine by relaying images acquired by the camera and / or data acquired by the sensor, and wherein the controller is configured to operate the subset of the one or more drones in the inspection mode at times when the one or more detection devices do not detect the presence of one or more birds and operate the subset of the one or more drones in the bird deterrent mode at times when the one or more detection devices detect the presence of one or more birds. According to a second aspect of the disclosure, we provide a wind power plant comprising the plurality of wind turbines and the system of the first aspect. In one or more embodiments, the plurality of wind turbines are located over a spatial area and the one or more detection devices are located at edges of the spatial area. In one or more embodiments, the number of detection devices is less than the number of wind turbines. In one or more embodiments, the plurality of wind turbines are located over a spatial area and wherein the one or more drones comprise a plurality of drones and wherein the drones are positioned at different default locations over the spatial area. In one or more embodiments, the number of drones is less than the number of wind turbines and the controller is configured to select at least one of the plurality of drones to receive the control signals to fly to the location for deterring the detected one or more birds based on predetermined criteria. We further disclose a system for control of a wind turbine having a rotor and a plurality of blades, the system comprising: one or more first drones each having one or more sensors for measuring a characteristic of a wind field, wherein the one or more first drones are configured to fly upstream of the wind turbine to measure the characteristic of the wind field that will be incident on the rotor, wherein the one or more first drones are configured to follow a flight path during said measuring of the characteristic of the wind field; and a controller configured to provide said measured characteristic for control of the wind turbine. In one or more examples, the flight path of the one or more first drones is configured such that the one or more sensors of the one or more first drones measure the characteristic of the wind field that will be incident on at least one blade of the plurality of blades. For example, incident on a blade tip or inboard of the blade tip. In one or more examples, the flight path is based on a rotational position of the blades such that the one or more first drones fly in a position that tracks ahead of a leading edge of at least one blade, and wherein said flight path is upstream of the rotor. In one or more examples, the one or more first drones comprise a plurality of first drones corresponding to a number of blades of the wind turbine, wherein the flight path of each respective drone of the plurality of first drones is configured such that the one or more sensors of each drone measures the characteristic of the wind field that will be incident on a different respective blade of the plurality of blades. In one or more examples, the controller is configured to provide for one or more of: individual pitch control of the plurality of blades, collective pitch control of the plurality of blades, generator control, or yaw control, wherein said control is based on said measured characteristic from the one or more first drones. In one or more examples, the controller is configured to control the flight path such that the one or drones fly upstream of the wind turbine at a distance from the wind turbine that is a function of wind speed. In one or more examples, the system includes a plurality of further drones, the plurality of further drones each having one or more sensors for measuring a characteristic of the wind field wherein the plurality of further drones are configured to fly further upstream of the wind turbine than the one or more first drones to measure the characteristic of the wind field that will be incident on the rotor, wherein the plurality of further drones are configured to fly in a predetermined formation; and wherein the controller is configured to provide said measured characteristic from the one or more sensors of the plurality of further drones for control of the wind turbine. In one or more examples, the plurality of further drones are configured to fly such that the predetermined formation is positioned in the wind field upstream of the wind turbine to measure the characteristic of the wind that will pass through a swept area of the rotor; and wherein the controller is configured to receive a drone data stream from each of the plurality of further drones representative of the measured characteristic at the position of the further drone in the formation over time; wherein the controller is configured to determine data indicative of the measured characteristic of the wind each blade will experience over its rotation based on the drone data streams. In one or more examples, the data indicative of the measured characteristic of the wind each blade will experience over its rotation is based on: a determination of a correspondence between the position of the further drone in the formation relative to a region of a swept area of the rotor; a measured wind speed and a distance between the predetermined formation and the swept area of the rotor; and a rotational speed of the rotor. In one or more examples, the controller is configured to provide for one or more of: individual pitch control of the plurality of blades, collective pitch control of the plurality of blades, generator control, or yaw control, wherein said control is based on said determined data indicative of the measured characteristic of the wind each blade will experience over its rotation based on the drone data streams. In one or more examples, the predetermined formation comprises a circular formation. In one or more examples, the plurality of further drones comprises at least four drones. In one or more examples, the characteristic of the wind field comprises one of: a) wind speed b) wind velocity comprising a longitudinal component indicative of wind speed in a longitudinal direction extending between the position of the drone and the rotor of the wind turbine, and at least one of a lateral component indicative of wind speed in a lateral direction and a vertical component indicative of wind speed in a vertical direction. In one or more examples, the one or more sensors comprises one or more of: one or more anemometer(s) configured to measure wind speed in a predetermined direction, an accelerometer, an accelerometer in combination with one or more absolute positions determined by a spatial positioning system, or the use of a spatial positioning system configured to measure how the wind field moves the drone in space, and one or more sensors of a flight system of the drone that are configured to provide the drone with feedback to counter movement of the drone caused by the wind field. In one or more examples, the one or more first drones comprise a plurality of first drones and wherein the controller, at any one time, is configured to control a first subset of the plurality of first drones to operate in a flying mode in which they follow their respective predetermined flight path and control a second subset of the plurality of first drones to operate in a charging mode in which the second subset fly to a designated charging station associated with the wind turbine and provide for recharging of a motive energy source of each of the first drones of the second subset. In one or more examples, the system includes the wind turbine including a wind turbine controller and wherein the controller is configured to provide the measured characteristic for control of the wind turbine to the wind turbine controller. In one or more examples, the system includes a plurality of wind turbines colocated in a wind power plant, each wind turbine comprising said wind turbine controller and an associated one or more first drones; and wherein the controller is configured to identify wind field events comprising a change in wind direction and / or wind speed above a predetermined threshold based on said measured characteristic obtained by the one or more drones associated with a first wind turbine of the plurality of wind turbines and wherein the controller is configured to, in response to detection of a wind field event, provide said measured characteristic for control of the first of the plurality of wind turbines and also to a second of the plurality of wind turbines. In one or more examples, (a) the controller is configured to receive blade position information indicative of a current position of the blades of the wind turbine or an estimate thereof, and the controller is configured to determine flight control signals based on the blade position information and provide the flight control signals to the one or more first drones to cause the one or more first drones to follow the flight path; or (b) the one or more first drones include a blade position sensor for determining the rotational position of the blades and wherein the flight path is based on the rotational position acquired from the blade position sensor. We further disclose a system for control of a wind turbine having a rotor and a plurality of blades, the system comprising: a plurality of drones, the plurality of drones each having one or more sensors for measuring a characteristic of a wind field wherein the plurality of drones are configured to fly upstream of the wind turbine to measure the characteristic of the wind field that will be incident on the rotor, wherein the plurality of drones are configured to fly in a predetermined formation; and wherein the controller is configured to provide said measured characteristic from the one or more sensors of the plurality of further drones for control of the wind turbine, wherein the plurality of further drones are configured to fly such that the predetermined formation is positioned in the wind field upstream of the wind turbine to measure the characteristic of the wind that will pass through a swept area of the rotor; and wherein the controller is configured to receive a drone data stream from each of the plurality of drones representative of the measured characteristic at the position of the further drone in the formation over time; wherein the controller is configured to determine data indicative of the measured characteristic of the wind each blade will experience over its rotation based on the drone data streams. In one or more examples, the data indicative of the measured characteristic of the wind each blade will experience over its rotation is based on: a determination of a correspondence between the position of the drone in the formation relative to a region of a swept area of the rotor; a measured wind speed and a distance between the predetermined formation and the swept area of the rotor; and a rotational speed of the rotor. In one or more examples, the controller is configured to determine a first mapping between the measured characteristic at the position of the further drone in the formation received at a first time and the corresponding region in the swept area of the rotor at a second, later, time accounting for a travel time of the wind, based on the measured wind speed and the distance between the formation and the swept area, and wherein the controller is configured to determine in which of the corresponding regions each blade will be present at the second time, based on the rotational speed of the rotor, wherein the data indicative of the measured characteristic of the wind each blade will experience over its rotation based on the drone data streams is based on said first mapping and the determination of which of the corresponding regions each blade will be present at the second time. In one or more examples, the controller is configured to provide for one or more of: individual pitch control of the plurality of blades, collective pitch control of the plurality of blades, generator control, or yaw control, wherein said control is based on said determined data indicative of the measured characteristic of the wind each blade will experience over its rotation based on the drone data streams. In one or more examples, the controller is configured to control the drones such that a distance between the predetermined formation and the rotor of the wind turbine is a function of wind speed. In one or more examples, the predetermined formation comprises a circular formation or a grid formation. In one or more examples, the plurality of further drones comprises at least four drones. In one or more examples, the characteristic of the wind field comprises one of: a) wind speed b) wind velocity comprising a longitudinal component indicative of wind speed in a longitudinal direction extending between the position of the drone and the rotor of the wind turbine, and at least one of a lateral component indicative of wind speed in a lateral direction and a vertical component indicative of wind speed in a vertical direction. In one or more examples, the one or more sensors comprises one or more of: one or more anemometer(s) configured to measure wind speed in a predetermined direction, an accelerometer, an accelerometer in combination with one or more absolute positions determined by a spatial positioning system, or the use of a spatial positioning system configured to measure how the wind field moves the drone in space, and one or more sensors of a flight system of the drone that are configured to provide the drone with feedback to counter movement of the drone caused by the wind field. In one or more examples, the controller, at any one time, is configured to control a first subset of the plurality of drones to operate in a flying mode in which they adopt the predetermined formation and control a second subset of the plurality of drones to operate in a charging mode in which the second subset fly to a designated charging station associated with the wind turbine and provide for recharging of a motive energy source of each of the first drones of the second subset. In one or more examples, the system includes the wind turbine including a wind turbine controller configured to control at least the pitch of the blades of the wind turbine and wherein the controller is configured to provide the measured characteristic for control of the wind turbine to the wind turbine controller. While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that other embodiments, beyond the particular embodiments described, are possible as well. All modifications, equivalents, and alternative embodiments falling within the scope of the appended claims are covered as well. The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future Claim sets. The figures and Detailed Description that follow also exemplify various example embodiments. Various example embodiments may be more completely understood in consideration of the following Detailed Description in connection with the accompanying Drawings. Brief Description of the Drawings One or more embodiments will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows an example embodiment of a system according to the invention including a controller, a drone and a wind turbine; Figure 2 shows an first example of a flight path being followed by the drone; Figure 3 shows a second example including three drones following a flight path and further drones in a formation; Figure 4 shows a third example including three drones following a flight path and further drones in a different formation; Figure 5 shows an example spatial correspondence between the formation and the swept area of the rotor; Figure 6 shows an example controller as part of a wind turbine controller; and Figure 7 shows a further example aspect for deterring birds. Detailed Description Figure 1 shows an example wind turbine 1, as will be known to those skilled in the art. The wind turbine comprises a tower 2 and a rotor 3. The rotor 3 includes a plurality of blades 4, such as three, mounted to a hub 5. The turbine 1 includes a nacelle 6 at the top of the tower 2. The nacelle 6 may house a generator 7 and, optionally, a gearbox 8. The wind turbine 1 sits in a wind field 10 that impinges on the rotor 3 thereby rotating it for the purpose of generating power. The nacelle contains a yaw system that enables rotation of the nacelle and therefore the direction the rotor faces, typically to keep the rotor facing upwind. While an onshore wind turbine is shown, the present invention is applicable to other wind turbine designs, including offshore or floating wind turbines. The wind turbine includes a wind turbine controller (not shown) for controlling operation of the wind turbine, such as generator control (e.g. generator torque or generator current) and / or blade pitch angle among others. Example embodiments of the present invention relate to the control of the wind turbine 1. In particular, embodiments relate to a system configured for the acquisition of characteristics of the wind field 10 for said control of the wind turbine 1 using drones. The drones are configured to fly in the wind field 10 upstream, such as upstream of the direction the rotor 3 of the wind turbine 1 is facing to measure the characteristics of the wind field 10 that will pass through the swept area of the rotor of the wind turbine. The capture of those characteristics ahead of when the wind having those characteristic will impinge on the wind turbine blades 4 is advantageous. Control of a wind turbine using such advance knowledge of the characteristics of the wind field is known in the art as predictive control or feed-forward control. Control of wind turbines 1 using advance knowledge of the wind conditions just upstream of the rotor 3 of the wind turbine 1 is an area that has been investigated previously. The basic premise is that, with knowledge of some characteristics of the wind a few seconds before it hits the rotor 3, the turbine can be controlled to reduce fatigue and / or extreme loads on the wind turbine structure, and / or increase the energy capture, to thereby reduce a levelized Cost Of Energy. As will be familiar to those skilled in the art, this type of control feeds advance information forward to the wind turbine controller, for control of one or more of the blade pitch angles, generator torque, generator current, operating modes, the yaw system, and other controlled sub-systems. The distances upstream that provide information that is useful for predictive control are generally from 25m (e.g. 1 second of advance notice at 25 m / s windspeed) to 250m (e.g. 10 seconds at 25 m / s; or e.g. 20 seconds at 12.5 m / s). Previous attempts at predictive control using advance information of wind conditions have used nacelle-mounted LIDAR, which generate laser beams to observe the wind upstream of the rotor. However, it has been found that LIDAR systems have a relatively high capital cost by comparison to the gains that they can deliver in terms of reduction of turbine loads and increase in turbine energy capture, plus their cost increases significantly if they are used to provide wind conditions over a large part of the swept area of the rotor upstream of the wind turbine, rather than based only on a few laser lines of sight, such as between one and five points across the swept area of the rotor 3 upstream of the wind turbine. Further disadvantages of nacelle-mounted LIDAR systems include their lack of accuracy in the region 25m-100m upstream of the outboard sections of the blades, as the beams have such a high angle to the incident wind that they provide low confidence in the longitudinal wind-speed, that is the wind direction towards the direction the rotor is facing. LIDAR based systems have been proposed for mounting on the leading edge of blades to overcome this problem, but industry experience has indicated high operational costs for such equipment. Figure 1 shows a system 11 for control of the wind turbine 1 comprising one or more aerial drones 12. One drone is shown in schematic figure 1 for simplicity but other examples include a plurality of drones as will be described. Drone technology has advanced rapidly over the past twenty years and continues to do so today. The capital cost and operational cost of a plurality of drones is relatively low and, in the examples described herein, are used as a platform for wind field characteristic measurement. The drone(s) may be of helicopter type or quad-copter type. In one or more examples, the drones may be of a type capable of hovering in a stationary location. Each drone 12 may include one or more sensors 13 for measuring a characteristic of the wind field 10. Accordingly, the one or more drones 12 are configured to fly upstream of the wind turbine 1 in the wind field 10 to measure the characteristic of the wind field that will be incident on the rotor 3. It will be appreciated that the wind field 10 is changeable and therefore the upstream location of the drones may be relatively fixed at some times and changeable at others, depending on the level of turbulence present in the wind field 10. Accordingly, upstream may be considered to be in front of the rotor 3 spaced therefrom in a direction the rotor 3 is facing. It will be appreciated that the rotor 3 is generally controlled by the wind turbine controller to face into the prevailing wind field 10. In other examples, the determination of the upstream direction may be made by measurement of the wind direction or by determination of an average wind direction over a recent time period relative to the location of the wind turbine 1. In either case, the drones 12 are configured to be upstream such that they are in a position where they may capture characteristics of the wind field that will impinge on a swept area of the blades 4 and therefore, depending on the blade position at a particular time, on the blades 4 themselves. While drones 12 have been used previously for performing blade inspections, it has been determined that they may have advantageous application in the capture of wind field characteristics for control of the wind turbine 1, and, in particular, predictive control of the wind turbine. In some examples, the characteristic of the wind field, such as wind speed and / or direction, captured by the drone(s) 12 may obviate the need for nacelle mounted anemometry or, if present, the use of such nacelle mounted anemometry at particular times, such as at start-up of the wind turbine. The characteristic of the wind field is typically wind speed, although in other examples a measure of pressure or temperature (e.g. for density determination) may be acquired. The wind speed may be measured directly, such as by an anemometer, or measured indirectly or inferred from other sensor measurements. For example, in one or more examples an anemometer may be provided on each drone that is configured to measure wind speed, such as wind speed in a predetermined direction. Thus, the one or more sensors 13 may be configured to measure wind velocity comprising a longitudinal component indicative of wind speed in a longitudinal direction 14 extending between the position of the drone 12 of the rotor 3. Further the measurement may include a lateral component indicative of wind speed in a lateral direction (i.e. in and out of the page as shown in figure 1) and a vertical component indicative of wind speed in a vertical direction 15. In other examples, an accelerometer may measure how the drone 12 is moved by the wind and a measure of wind speed in various component directions may be inferred or calculated therefrom. In one or more examples, a spatial positioning system such as a satellite based positioning system (e.g. GPS, Global Positioning System) may provide an absolute position and determination of changes in that absolute position may provide information from which characteristics of the wind field can be determined. In other examples, a combination of an accelerometer and an absolute position from a spatial positioning system may be used. In one or more examples, the drone's flight control system may include means to stabilise the drone against wind-caused movement, such that the drone may accurately follow a flight path or hold a position. In such an example, the output of the flight control system of the drone or sensors thereof may be used to infer the wind speed information. For example, an indication of the drone's rotor thrust used to stabilise the position of the drone may be indicative of vertical wind speed. In some examples, the one or more sensors 13 may comprise LIDAR, for measuring wind speed proximal the drone's position. It will be appreciated that the flight of the drone 12 may affect the measured wind speed. Thus, any measurement of wind speed may be processed, by the one or more sensors 13 of the drone 12 or the controller 16 or any other entity, such that it is independent of the movement of the drone 12. Thus, the measurements will then be representative of the wind field. Advantageously, in the present example, the one or more drones 13 are configured to follow a flight path based on a rotational position of the blades, as will be described in more detail with reference to Figure 2. Accordingly, the wind characteristics gathered by the sensor(s) of the drones may be more representative of the wind that will be encountered by the blades 4 of the wind turbine 1, which is advantageous. However, the scope of the disclosure covers other flight paths based on other factors, as will also be described. The system 11 further comprises a controller 16 configured to provide said measured characteristic from the one or more sensors 13 of the one or more drones 12 for control of the wind turbine 1. The controller 16 may be mounted on (or its functionality may be distributed over) the one or more drones 12 or it may be mounted anywhere on the wind turbine, including on / in its nacelle, rotor or tower. The controller 16 may be configured to transmit live information representing the measured characteristic to the wind turbine controller. The location at which the characteristic was captured may also be transmitted by the controller to the wind turbine controller. The location at which the measured characteristic was acquired may be provided by the drone or may be acquired from a drone position tracking system. It will be appreciated that in some examples, such as the present example, the drone is controlled to follow a flight path and therefore its location is known and thus the part of the wind field the measured characteristic represents is also known. In other examples, the drone or other drone position tracking system may report the location such that the wind field characteristics measured are associated with a location at which the measured characteristic was made. Likewise, in some examples, the time at which the measured characteristic was made may be reported by the drone or sensors thereof or determined by other means. In other examples, the controller 16 may be a processor remote from the drones 12 and configured to receive information, such as a data stream, wirelessly transmitted from each drone 12 representing the measured characteristics experienced by each drone at its current location in flight over time. Accordingly each drone may comprise a transmitter and / or transceiver for transmitting the measured characteristic(s) to the controller 16. In other examples, the controller 16, which receives the measured characteristic from the one or more drones and, optionally, controls the flight of the drones, is part of the wind turbine controller which provides for control of the wind turbine, such as by providing functions such as blade pitch angle control of the blades 4 and / or yaw control. In other examples, the controller 16 may be a server and the drones may communicate with the server using a mobile data network or other communication means. The server may communicate with the wind turbine controller(s) by one or more wired / wireless networks and / or transmitters. Thus, in general, it will be appreciated that the function of the controller in receiving the information representing the acquired wind field characteristic from one or more drones 12, the receiving of measurement locations and the possible processing of that information and the provision of the measured characteristic, in whatever form, to the wind turbine controller may be achieved in many different ways. Figure 2 shows the wind turbine 1 and the drone 12 represent by a four-pointed star. Figure 2 also shows the flight path 20 the drone 12 is configured to follow. The flight path 20 follows the rotational path traced by the blades but positioned upstream of the rotor 5. Thus, the blades 4 sweep an area 21 and the blades have a rotational position in that swept area 21 as they rotate. The position of one or more of the blades may be determined by the controller 16 in various ways. In one or more examples, the controller 16 may be configured to receive blade position information indicative of a current position of the blades 4 of the wind turbine or an estimate thereof from the wind turbine controller. Accordingly, the controller 16 may be configured to determine flight control signals based on the blade position information and provide the flight control signals to the one or more first drones to cause the one or more first drones to follow the flight path 20. The controller 16 may be configured to program the drone 12 to follow the flight path 20, such as intermittently or periodically. In other examples, the controller 16 may actively control (i.e. fly) the drone to follow the flight path 20. In other examples, the blade position may be determined by the drone 12. Thus, the controller 16 may be part implemented in the drone(s) 12 which may include a blade position sensor, such as a camera operating with feature recognition, for determining the rotational position of the blades. Thus, the flight path 20 is based on the rotational position acquired from the blade position sensor. In this example, the drones acquire the location of the blade and determine a flight path based on the rotational position of the blade. In the present example, the flight path 20 is configured such that drone 12 flies in a position 22 that tracks ahead by an angle 0 of a leading edge 15 of at least one blade 4, and wherein said position 22 is upstream of the rotor 3 by a distance 23. The flight path 20 is generally circular given that it tracks the movement of the blade 4. However, it need not be perfectly circular. In some examples, the flight path 20 may be limited by the performance of the drone 22 to change altitude and horizontal position. In some examples, the flight path 20 may track ahead of the leading edge of the blade at the blade tip (while also at the upstream distance 23). In other examples, the flight path 20 may track ahead of the leading edge of the blade at an inboard position along the blade 4, thereby leading to a smaller radius flight path 20, also at an upstream distance 23. In other examples, the flight path may have other shapes and be configured to track ahead of the leading edge 15 but at a variable longitudinal position along the blade 4. The degree to which the flight path 20 tracks ahead of the blade 4 is such that the drone(s) measure the characteristic of the wind field 10 that will be incident on the blade 4. Thus, considering one measurement as an example, the drone 12 or sensors thereof may be configured to measure a part of the wind field at the position 22 of the drone 12 as shown. That part of the wind field takes time, based on the wind speed, to travel to the swept area 21 over the distance 23. That part of the wind field may thus arrive at a corresponding region 24 at a later time. By that later time, the blade 4 has rotated by an angle such that it is in the region 24 and will therefore experience those measured characteristics. Thus, the amount the flight path 20 tracks ahead of the blade position may be a function of one or more of the distance 23 between the rotor 5 and the plane of the flight path 20 (or more generally the position of the drone in its flight path 20), the wind speed and the rotational speed of the rotor. Thus, if the rotor speed increases, the flight path 20 may be adjusted to increase the angle 0 that the flight path 20 tracks ahead of the leading edge, and vice versa. Further, if the wind speed decreases, the flight path 20 may be adjusted to increase the angle that the flight path 20 tracks ahead of the leading edge 15 and vice versa. In other examples, rather than control the angle 0, if the wind speed decreases, the controller may adjust the flight path to increase the distance 23 and vice versa. If the rotational speed of the rotor increases beyond a performance limit i.e. a maximum flying speed of the drone, the controller may adjust the flight path to move the flight path such that it tracks a position ahead of the leading edge but at a longitudinal position along the blade that is more inboard, and vice versa. Such control may be provided for other reasons as well. The controller may be configured to control position of the flight path ahead of an inboard-outboard position along the leading edge 15 as a function of the rotor speed. Such functions may account for flying speed limits of the drones and / or weather conditions. Thus, the flight path may be controlled in various ways including as a function of wind speed (however acquired) and / or rotor position and / or rotational speed of the rotor 5. The flight path may be modified in terms of the longitudinal distance 23 upstream of the wind turbine and / or the angle that the flight path tracks ahead of the leading edge of any one blade 4. In some examples, the controller 16 is configured to control the upstream distance 23 based on wind speed or a recent average wind speed. The wind speed may be measured by the sensors 13 of the drones themselves or by other sensors, such as a nacelle mounted sensor. In some examples, the controller is configured to control the upstream distance of the flight path 20 such that the time between measurement of the wind field characteristic and when the wind having that characteristic impinges the blade(s) is within a predetermined range or is substantially constant. Accordingly, the predictive control need not constantly adapt to different time delays and instead the upstream drone position is modified. It will be appreciated that the controller 16 being configured to control the upstream distance 23 based on wind speed or a recent average wind speed includes the use of proxy measurements or parameters that are indicative of wind speed. For example, the wind turbine's operating point can be used to back-calculate / estimate the wind-speed and then use that estimate in place of a measured wind-speed. Also, wind-speed estimators are already present in many wind turbine controllers and, accordingly, the output of the wind speed estimator may be used. In some examples or conditions, the rotational speed of the rotor can be used as a proxy for wind-speed. In other examples, if the wind turbine is operating at full load then the use of a combination of power output, air density and pitch angle can provide an estimate of wind-speed. Accordingly, the examples herein in which the controller 16 controls the upstream distance 23 based on wind speed includes such estimates and proxies. While a single drone 12 is shown in figures 1 and 2, it is preferable that a plurality of drones 12 are provided, such as at least one drone for each blade 4. In such an example, each drone is associated with a different blade 4. Thus, the flight path 20 of each respective drone is configured such that the one or more sensors 13 of each drone measures the characteristic of the wind field that will be incident on its associated blade 4. More than one drone per blade may be provided to enable wind field characteristics to be acquired representing parts of the wind field 10 that will impinge on a plurality of different parts of each blade 4 along their length. In general, the controller 16 is configured to provide the measured characteristics for control of the wind turbine. The control of the wind turbine may include one or more of individual pitch control of the plurality of blades, collective pitch control of the plurality of the blades, yaw control, or generator control, such as of generator torque or generator current. In examples where the controller 16 is integrated or coupled with the wind turbine controller, the controller 16 may be configured to provide the wind turbine control signals, such as to the blade pitch actuators or the generator and the like. It will be known to those skilled in the art of wind turbine control that it may be desirable to control the pitch angles of the blades such that a pitch angle of one of the blades differs from the pitch angle of the other blade or blades, thereby providing individual pitch control (IPC). Further, the instantaneous pitch angle of any one blade may follow a function of the instantaneous azimuth angle of that blade relative to a notional fixed reference angle that is independent of the function followed by the other blades. Obtaining measurements of wind characteristics upstream of the wind turbine's rotor is advantageous because, currently, blade loads for individual blade pitch control are measured at the blade root, but there is a significant dynamic lag between the wind hitting the outboard sections of the blade, where most of the rotor torque is generated, and the load being transmitted along the blade to the blade root. Measuring the wind speed just upstream of the outboard sections of the rotor by the drone 22 therefore provides advance information on both the wind flow and how it may impact the dynamics of the blade 4 itself. In one or more examples, the blades 4 may have active aerodynamic control devices mounted on them. For example, ailerons. The controller may be configured to provide the measured characteristics for control of these active aerodynamic control devices or may be configured to control them based upon the wind field characteristic measured by the drones 12. How the wind turbine controller or the controller 16 uses the measured characteristic in the control of the wind turbine is not the main focus here. However, known means of predictive control or feed-forward control may be used. It will nevertheless be appreciated that the provision of advance characteristics of the wind field using drones may thus provide for more accurate and / or effective control of the wind turbine. Example figure 3 shows a further example. In this example three "blade tracking" drones 12a, 12b and 12c are provided each following a flight path based on the rotational position of a respective blade 4a, 4b and 4c. Accordingly, the measurements from each drone are provided for IPC of its corresponding blade 4a, 4b and 4c. Thus, the controller 16 may wirelessly receive the measurements from the three drones and provide those measurement for control of the wind turbine 1. For ease of reference the "blade tracking" drones 12a, 12b and 12c may be referred to as "first drones". Optionally, the system 11 may include a plurality of further drones 30-39. The further drones 30-39 are substantially similar to the first drones 12a-12c. Thus, the plurality of further drones may each have one or more sensors 13 for measuring a characteristic of the wind field 10, such as wind speed as described above. In some examples, only the further drones are provided and the blade tracking first drones are absent from the system 1. Such an example comprises an aspect of the disclosure. However, in the present example, the plurality of further drones 30-38 are configured to fly further upstream, such as at a distance 40, relative to the wind turbine 1, than the distance 23 of the one or more first drones. Again, the further drones 30-38 are configured to measure one or more characteristics of the wind field that will be incident on the rotor 4 and, in some examples, on each blade 4. Further, rather than following a flight path 20 based on the rotational position of the blades, the further drones 30-38 are configured to hover in a predetermined formation 39. In this first example, the formation 39 comprises a grid, such as a 3x3 grid, although this is not the only formation. In this or other examples, the controller 16 is configured to control the upstream distance of the formation of further drones 30-38 based on wind speed. In some examples, the controller 16 is configured to control the upstream distance 23 based on wind speed or a recent average wind speed. The wind speed may be measured by the sensors 13 of the drones 30-38 themselves or by other sensors, such as a nacelle mounted sensor. In some examples, the controller is configured to control the upstream distance of the formation 39 such that the time between measurement of the wind field characteristic and when the wind having that characteristic impinges the blades 4a, 4b and 4c is within a predetermined range or is substantially constant. Accordingly, the predictive control need not constantly adapt to different time delays and instead the upstream drone position is modified. In general, the further drones 30-38 are configured to hold a relatively stationary position upstream of the wind turbine other than for upstream distance control mentioned above. As before, the wind direction and the direction the rotor faces can change over time. Thus, the further drones 30-38 may hold a stationary position or formation upstream of the wind turbine while accounting for wind direction and / or yaw direction changes above a threshold level. Similarly, the controller 16 in whichever way it may be embodied, is configured to provide said measured characteristic from the one or more sensors of the plurality of further drones 30-38 for control of the wind turbine. The location at which the measured characteristics are obtained from each individual further drone 30-38 may also be provided or otherwise determined. The formation 39 is sized and positioned such that the further drones 30-38 are configured to measure the characteristic of the wind field upstream of the turbine 1 that will pass through the swept area 21 of the rotor 3. The controller 16 is configured to receive a drone data stream from each of the plurality of further drones 30-38 representative of the measured characteristic at the position of the further drone in the formation over time. Thus, the sensors 13 onboard each drone may sample the wind field at a sample rate and the drone data stream may represent that live stream of samples or measurements. In general, the controller 16 is configured to determine data indicative of the measured characteristic of the wind field each blade 4a, 4b and 4c will experience over its rotation based on the drone data streams. Thus, taking one blade at a 12 o'clock position, it will experience the wind field that was measured by the drone 31 a time period earlier. That time period, as before, may be a function of the distance 40 between the rotor 3 and the formation 39 (or more generally the position of the drone 31 in the formation), and the wind speed (measured by the drone 31 or otherwise). When that same blade is at 1-2 o-clock position, it will experience the wind field that was measured by the drone 32 a time period earlier. Then that measured by drone 35, then drone 38, then drone 37, then drone 36, then drone 33, then drone 30 and then back to drone 31 as it makes its full rotation. Accordingly, the controller 16, when providing information for individual pitch control or other purposes may be configured to take time slices of the drone data streams that correspond to the wind field that will be incident on that blade at a particular point in its rotational travel. Example figure 4 shows a further example. In this example, eight further drones 41-48 are provided in a circular predetermined formation 49. The upstream location is positioned at a distance 40 from the swept area of the rotor. In some examples, irrespective of the formation type 39, 49, outboard further drones 50 may be provided at locations upstream of the wind turbine but offset from the direction the rotor 5 is facing by more than 5 degrees. In some examples, the angular offset may be greater than 5, 10, 15 or 20 degrees, and may be up to 90 degrees. These still further drones 50 may be configured to detect changes in wind direction above a threshold amount. Accordingly, measured characteristics from these drones may be provided to the wind turbine controller for yaw control or other purposes. Example figure 5 shows the formation 49 side-by-side with the swept area 21. In determining the data to provide for control of the wind turbine 1, such as for individual pitch control, the controller 16 may be programmed with or determine a correspondence between the position of the further drone 41-48 in the formation 49 relative to a region 51-58 of a swept area of the rotor. Accordingly, the position in the formation 39, 49 may have a spatial correspondence with regions of the swept area based on parts of the wind field 10 that, on average, pass through both parts. Thus, the drone 41 at the top of the formation may be spatially aligned with, but upstream of, a region 51 of the swept area. Likewise, the drones at other positions in the formation may align, along the wind direction, with a corresponding region 52-58 of the swept area. As before, a part of the wind field that is measured by one of the further drones takes a time period to travel to the corresponding region 51-58 of the swept area based on measured wind speed (such as measured by the same drone, a different drone or otherwise) and the distance 40 between the predetermined formation 49 and the swept area of the rotor 21. Further, the rotational speed of the rotor determines which of the regions 51-58 a blade will be in at a time with the part of the wind field reaches the swept area. Thus, the controller 16 may be configured to determine measurements relevant for control of an individual blade as a function of the determined / programmed correspondence between the formation 39, 49 and spatially corresponding regions of the swept area, the distance 40, the rotor speed, and the blade position. In some examples the system includes a fleet of first drones and / or further drones. The fleet may include drones assigned for measurement collection and therefore flying, while others are charged or held in reserve. The fleet may be held in or on the tower, the nacelle or other store associated with the wind turbine 1. The controller 16, at any one time, may be configured to control a first subset of the plurality of first drones to operate in a flying mode in which they follow their respective predetermined flight path 20 or adopt the formation 39, 48. The controller 16 may also control a second subset of the plurality of first drones to charge themselves or return to an area for charging, i.e. operate in a so called charging mode. The drones in the charging mode may be flown or instructed to fly to a designated charging station associated with the wind turbine and provide for recharging of a motive energy source, such as a battery or hydrogen store for a fuel cell. In one or more examples, the controller 16 may operate the drones disclosed herein in an inspection mode. In other examples, the system may include drones specifically for inspection rather than measuring the wind field. In the inspection mode, one or more of the drones may be controlled to inspect the internal structure of the wind turbine 1. It is known to use drones for inspection of the blades'external structures. However, drones provided in the tower or nacelle, possibly also for use in measuring characteristic of the wind as disclosed herein, may be controlled to perform inspection of internal structures and components. In the inspection mode, the controller may cause the drone to follow a flight path that is internal to the wind turbine tower and / or nacelle. A camera and / or other sensors provided on the drone may be provided to relay images or video of those internal structures and components to an operator. The flight path may be preprogrammed by the controller in order to safely navigate the tight internal spaces of the wind turbine. In other examples, the inspection mode may hand-over control of the drone to a remote-control of an operator, thus allowing the operator to fly the drone manually or semi-automatically to inspect the internal structures and components. The control of one or more drones to perform an internal inspection of a wind turbine may comprise an aspect of the disclosure. Thus, there is disclosed a wind turbine inspection system comprising one or more drones and a controller configured to provide for control of the drone in an internal space of the wind turbine, the one or more drones having a camera or other sensor and the controller configured to acquire images and / or video obtained by said camera or data acquired by the other sensor for the inspection of internal structures and components of the wind turbine. We also disclose a wind turbine in combination with the wind turbine inspection system, wherein the wind turbine includes one or more apertures in one or more of a nacelle, a door structure of a tower, a hub, or the tower configured to allow access to the internal structure of the wind turbine by the one or more drones of the wind turbine inspection system. This is advantageous as the inspection can be performed remotely by the drones in advance of the arrival of an engineer. Also, the internal structure of a wind turbine is typically compartmentalised and therefore providing a plurality of drone-access apertures may advantageously allow for a thorough internal inspection by the wind turbine inspection system. In some examples, the wind turbine 1 is part of a plurality of wind turbines in a wind power plant. In such an example, the drones of wind turbines at the edge of the wind power plant may detect extreme events earlier than the drones of other wind turbines. Thus, in such examples, the controller may be configured to send the measurements of the characteristics, or other warning, not only to a wind turbine with which it is associated but also to other wind turbines for control of the wind turbines. Figure 6 shows an example of the controller 16, which in this example is integrated with a wind turbine controller 60. Thus, the controller 16 receives the measured characteristic(s) from each of the first drones and / or further drones and / or still further drone(s) 12. The controller 16 then provides the information for control of the wind turbine. For example, the measured characteristic(s) may be provided to an IPC block 61. The IPC block may determine pitch control signals based on the measurements and provide them to an interface 62, which may provide a conduit to blade pitch actuators and or other actuators / components of the wind turbine. The measured characteristics may be provided to a yaw system block 63 for control of the yaw and / or yaw error. The yaw system block 63 may provide yaw control signals to the interface 62 for control of yaw direction actuators. The measured characteristics may be provided to a mode controller block 64 which controls an operating mode of the wind turbine. The mode controller block 64 may also provide control signals via the interface 62. The operating modes may include one or more of: a start-up mode, a shutdown mode, a safe mode in which rotor speed may be reduced and which may be invoked during extreme weather conditions, and a damage mitigation mode for use in high winds or gusty conditions where rapid changes in wind speed or wind direction are present or any other operating mode familiar to those skilled in the art. The measured characteristics may be provided to a speed and power control block 65 which may generate generator control signals, such as torque demand or generator current demand control signals for the generator via the interface 62. In other examples, the wind turbine controller 60 may include a model-based wind turbine controller. As will be understood by those skilled in the art, the functional blocks 61, 63, 64, 65 may be wholly or partially replaced by a model based control block. In one or more examples, control of the wind turbine based on the measured characteristics may be control using input from a lifetime usage estimator which provides an indication of the remaining life of the blades. It will be appreciated that other lifetime usage estimators which provide indications of the remaining life of other components of the wind turbine may be used. The lifetime usage estimator for a given blade receives the measured characteristics that pertain to that blade. The lifetime usage estimators for other components of the wind turbine may use the measured characteristics as inputs to a dynamic model of the turbine that is used to calculate the loadings on those other components. The use of the measured characteristics enables more accurate lifetime estimates to be made for the blades and for other components. Figure 7 illustrates a further aspect of the disclosure. Wind turbines present a hazard to birds. Further, birds striking a wind turbine may potentially cause damage to wind turbines. Accordingly, discouraging birds from flying near to wind turbines is advantageous. The use of drones to deter birds may be advantageous. The deployment of drones may be reactive to the detection of birds and therefore less disruptive to the environment. Figure 7 shows an example system 70 for deterring birds 71 from a wind power plant 72 comprising a plurality of wind turbines 73. The system 70 shown in example figure 7 comprises two detection devices 74, 75 configured to detect the presence and location of one or more birds proximal the wind power plant. It will be appreciated that in other examples a single detection device may be sufficient. For large wind power plants, more than two detection devices may be required. In some examples, a detection device is provided for each wind turbine of the wind power plant. However, preferably, the number of detection devices 74, 75 is less than the number of wind turbines 73. The one or more detection devices 74, 75 may be one or more of RADAR, detection devices; thermal imaging devices configured to detect birds based on thermal energy emitted by the birds; and LIDAR detection devices. In some examples, the detection devices 74, 75 may include a camera to detect birds 71 by visual recognition. The system 70 may receive images from the camera and use an image recognition algorithm, such as a trained machine learning model to detect birds 71 in the images and derive a location for the detected bird(s). In some examples, the system includes one or more processors or a communicative connection to a server for performing such processing of the images received from the camera. In some examples, a combination of different detection devices 74, 75 may be provided. In general, the detection devices 74, 75 are capable of detecting birds proximal the wind power plant 72 and configured such that a location for said detected birds can be determined based on information they generate. The one or more detection devices 74, 75 are shown ground-mounted but in other examples, they may be mounted on one or more of the wind turbines 73 or mounted on the drones 76, 77, 78, or on an offshore structure, or in a combination of mounting positions. Further, the detection devices 74, 75 are shown at the edges of the wind power plant in the present example, although in other examples, the detection device(s) could be centrally located or located at the corners. The system 70 further comprises one or more drones. In the present example, three aerial drones 76, 77, 78 are shown. The drones 76, 77, 78 may be of helicopter type or quad-copter type. In some examples, they may be of fixed wing type. The drones are configured to be remote controlled by wirelessly received control signals. The control signals may be configured to actively fly the drone to a location or may provide a location to which the drone is capable of autonomously flying. In some examples, the number of drones 76, 77, 78 is less than the number of wind turbines 73. This provides efficient use of resources for deterring birds from a wind power plant. The system 70 further comprises a controller 80 configured to receive information indicative of a detected location of one or more birds detected by the detection devices 74, 75. The information may be received via a wired or wireless network or using the SCADA network of the wind power plant 72. The controller 80 may be configured to process the information to derive the detected location such as based on a predetermined position of the detection device 74, 75. The controller 80 is configured to provide control signals to the drones 76-78 to fly to a location for deterring the detected one or more birds based on the detected location. The location to which the drone is sent may be configured to track the movement of the bird 71, as detected by systems of the drone or from the information from the detection devices. This is advantageous as the controller 80 is able to dispatch drones to deter birds wherever they may be detected over a spatial area 81 of the wind power plant thereby providing protection for multiple wind turbines 73 on demand. The location for deterring the detected one or more birds may be the same as the detected location. In other examples, the location to which the drone is dispatched may be based on the detected location and the location of one or more wind turbines 73 proximal the detected location. The controller 80 may have predetermined information of the location of the wind turbines 73 in the wind power plant. For example, the drone may be flown to a position in between the detected location and the one or more wind turbine 73 determined to be proximal the detected location to increase the chance the bird is deterred away from the wind turbines 73. In other examples, the controller 80 may be configured to determine a direction of travel of the bird, such as based on at least two detected locations for the detected bird 71. Then, the location to which the drone 76, 77, 78 is flown may be ahead of the bird's direction of travel in order to intercept the bird 71. The controller 80 may be configured to select at least one of the plurality of drones 76, 77, 78 to receive the control signals to fly to the location for deterring the detected one or more birds 71 based on predetermined criteria. Thus, in examples where the number of drones is less than the number of wind turbines, the controller may efficiently dispatch drones as required to serve multiple wind turbines. The predetermined criteria may include the proximity of the drone to the detected location. Thus, the closest drone, e.g. drone 76, may be sent the control signals to deter the bird 71. In such an example, each drone may be configured to have different default or "home" location distributed over the spatial area 81 of the wind power plant 72. The default location may change should the drones be configured to "patrol" the area 81. The default locations may be specified near known roosting or feeding areas near the wind power plant. The controller 80 is configured to receive information indicative of the different default locations, which may be preprogrammed or the live current locations of the drones 76-78. Accordingly, the controller 80 may then select at least one of the plurality of drones based on a proximity of the detected location (or the location to which the drone will be sent to deter the bird) to the default location and provide the control signals to the selected drone. The predetermined criteria may include the level of power, such as state of charge or level of fuel, each drone has. Accordingly, the closest drone 76 may not be dispatched if it is charging and its level of power is still low and, instead, the next closest drone may be sent. The predetermined criteria may include determination of an operational mode the drone 76, 77, 78 is in at the time the bird 71 is detected. For example, at least some of the drones 76, 77, 78 may include one or more sensors for measuring a characteristic of a wind field or for carrying out wind turbine inspections, and may operate in a bird deterrent mode or a measurement mode. In the measurement mode, they may perform the task of measuring one or more characteristics of the wind field, as described above, wherein they are configured to fly upstream of one of the plurality of wind turbines to measure the characteristic of the wind field that will be incident on a rotor of that wind turbine, such as shown for drone 78 following a flight path 20 upstream of wind turbine 82. Likewise, at least some of the drones 76, 77, 78 may include one or more sensors for performing inspections of the wind turbines, and may operate in a bird deterrent mode or an inspection mode. In the inspection mode, they may perform the task of carrying out internal or external inspections of the wind turbine, as described above. In some examples, one or more of the drones may be configured to operate in all of the inspection mode, the measurement mode and the bird deterrent mode. The controller 80 may be configured to operate the drone 78 in the measurement mode or inspection mode at times when the one or more detection devices do not detect the presence of one or more birds and operate the drone 78 in the bird deterrent mode at times when a bird 71 is detected nearby. It will be appreciated that the criteria for determining when to switch to the bird deterrent mode may be based on the importance of the measured characteristic of the wind field to the wind turbine controller of turbine 82, the importance of wind turbine inspections to turbine operation, and / or the availability of other "dedicated" bird deterring drones 76, 77 or ones that are already in the bird deterrent mode. The drones 76-78 may include a speaker, such as a siren, for emitting a sound for deterring the detected birds 71. Alternatively, they may include lights for deterring birds. In other examples, the generic noise made by the drone or simply the presence of the drone may be sufficient to deter birds 71. In some examples, the drones 76-78 may include predator-imitating features, such as wings or a silhouette of a hawk or other predator. In some examples, the continual emission of sound from the speaker would be undesirable. Accordingly, the controller 80 may be configured to cause the speaker of the drones 76-78 to emit the sound based on the proximity of the drone to the location for deterring the detected birds 71. Thus, the drone may only emit the sound when it has arrived at or is near to the location of the bird. For drones configured to operate in the bird deterrent mode and at least one of the measurement mode or inspection mode, the continual carrying of a bird deterring device, such as a siren, may be wasteful of energy. Accordingly, the drone 78, for example, may include a first part of a coupling device to couple to a bird deterring device 83 (shown schematically mounted to a designated coupling zone on the nacelle of wind turbine 82). The bird deterring device 83 comprises the complementary second part of the coupling device such that it can be picked up by the drone 78 when it is instructed to switch from the measurement mode to the bird deterring mode. Accordingly, the control signals from the controller 80 may include control signals instructing the drone 78 to switch modes and couple to the bird deterring device 83 and then fly to the location for deterring the detected one or more birds. The coupling zone may be located elsewhere such as in the tower, in the nacelle or in an ancillary structure. The controller 80 may also be configured to evaluate the risks to the birds and to the wind turbines 73 of the wind power plant 72 when dispatching drones to deter birds. For example, the controller 80 may be configured to calculate whether the detected location received from the detection devices 74, 75 is at a height lower than a predetermined threshold height before providing the control signals to fly the drone(s) to deter the bird. Thus, the predetermined threshold height may be based on a height of the wind turbines, such as to the blade tip plus a margin. The controller 80 may therefore be configured to dispatch drones more efficiently, such as only if the flight path of the bird 71 is low enough to present a risk based on the predetermined threshold height. In some examples, the rotor of the wind turbine are high above the ground level and therefore birds may safely move through the wind power plant below rotor height. Thus, the controller 80 may be configured to calculate whether the detected location received from the detection devices 74, 75 is at a height higher than a second predetermined threshold height before providing the control signals to fly the drone(s) to deter the bird. The second predetermined threshold height may be the lowest point of the rotor relative to ground level minus some margin. The controller 80 may be configured to perform further risk-based assessments to provide for the efficient use of the drones 76, 77, 78. For example, one or more of the detection devices 74, 75 may include a camera and the controller 80 is configured to receive one or more images captured by the camera. By using an image detection algorithm which may involve an appropriately trained model, the controller 80 may be configured to identify whether birds are present and, if so, the type or size of bird 71 based on the one or more images. This may be advantageous when selecting whether to dispatch a drone to deter the bird or not. Further, in instances where a plurality of birds are detected near or in the spatial area 81 of the wind power plant, the controller 80 may prioritise the plurality of birds based on the identified bird type or size. Thus, larger birds or breeds that have a higher probability of striking the blades, may be prioritized over small birds for example. Accordingly, the controller 80 may be configured to provide the control signals to fly the drones 76, 77, 78 based on said prioritization, such as to the higher risk birds first. Communication between the drones and the controller may be provided by radio frequency control signals. Communication between the detection devices and the controller may be provided by wired or wireless communication networks, such as a SCADA communication network. Accordingly, it will be appreciated that the drones, the controller 80 and the detection devices may have communication means such as transmitters and receivers for providing for said communications. The drones 76-78 may be provided with recharging points in or on the wind turbines 73, such as in or on a station at the tower base, in / on a separate building at the tower base, inside or on the outside of the tower base or in / on the nacelle. The recharging points may be at the same locations as those provided for the aspects of the disclosure described in relation to figures 1 to 6. In some examples, the controller 80 may be configured to monitor, using information from the detection devices 74, 75 and / or drones 76-78 whether the deterring has been effective. If so, such as the bird 71 has changed direction away from the wind power plant, the controller 80 may be configured to provide control signals to fly the drone back to its default location or change its mode, such as to the measurement mode. If not, the controller 80 may provide further control signals to one or more of: move the dispatched drone closer to the detected bird(s) 71, increase the volume of the siren, or dispatch additional drones for deterring the detected bird(s) 71. In any of the examples, the drones may include one or more processors and / or memory to process the information acquired by any sensors, detection device or cameras they may carry. Further the drones may include a processor to control any bird deterring devices, cameras or coupling devices they may be carrying based on the control signals from the respective controller. Likewise, the drones may be provided with appropriate communication devices to send any information and / or receive the control signals. Further, the controller 16, 80 may include one or more processors and / or memory for performing the functions described herein as well as appropriate communication devices to receive any information and / or transmit the control signals. The detection devices 74, 75 may also include one or more processors and / or memory for performing their functions. In some example embodiments the set of instructions / method steps described above are implemented as functional and software instructions embodied as a set of executable instructions which are effected on a computer or machine which is programmed with and controlled by said executable instructions. Such instructions are loaded for execution on a processor (such as one or more CPUs). The term processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. A processor can refer to a single component or to plural components. In other examples, the set of instructions / methods illustrated herein and data and instructions associated therewith are stored in respective storage devices, which are implemented as one or more non-transient machine or computer-readable or computer-usable storage media or mediums. Such computer-readable or computer usable storage medium or media is (are) considered to be part of an article (or article of manufacture). An article or article of manufacture can refer to any manufactured single component or multiple components. The non-transient machine or computer usable media or mediums as defined herein excludes signals, but such media or mediums may be capable of receiving and processing information from signals and / or other transient mediums. 5 In this specification, example embodiments have been presented in terms of a selected set of details. However, a person of ordinary skill in the art would understand that many other example embodiments may be practiced which include a different selected set of these details. It is intended that the following claims cover all possible example embodiments. 10 06 03 25

Claims

1. A system for deterring birds from a wind power plant comprising a plurality of wind turbines, the system comprising:5 one or more detection devices configured to detect the presence andlocation of one or more birds proximal the wind power plant;one or more drones;a controller configured to receive information indicative of a detected location of one or more birds detected by the one or more detection devices, io the controller configured to provide control signals to the one or more drones to fly to a location for deterring the detected one or more birds based on the detected location,wherein the controller is configured to calculate whether the detected location received from the one or more detection devices is at a height lower 15 than a predetermined threshold height, the predetermined threshold height based on a height of the wind turbines of the wind power plant, andif the detected location is lower than the threshold height, provide the control signals to fly the at least one drone; andif the detected location is higher than the threshold height, do not 20 provide the control signals to fly the at least one drone.

2. The system of claim 1, wherein the one or more detection devices comprise one or more of:a RADAR, detection device;25 a thermal imaging device configured to detect birds based on thermalenergy emitted by the birds;a camera configured to provide images for object recognition; anda LIDAR detection device.30 3. The system of claim 1 or 2, wherein the one or more detection devicesare configured to be one of: ground-mounted, offshore structure mounted; wind turbine mounted, or drone mounted or a combination.

4. The system of any preceding claim, wherein the number of drones is less 35 than the number of wind turbines and the controller is configured to select at06 03 25least one of the plurality of drones to receive the control signals to fly to the location for deterring the detected one or more birds based on predetermined criteria.5 5. The system of any preceding claim, wherein the one or more droneseach include a camera and wherein the controller is configured to perform object recognition based on one or more images acquired by the camera to detect the presence and location of the one or more birds.io 6. The system of any preceding claim, wherein the one or more drones each include one or more of: a speaker for emitting a sound for deterring the detected birds and predator-imitating features.

7. The system of claim 6, wherein the controller is configured to cause the 15 speaker of the one or more drones to emit the sound based on the proximity of the drone to the location for deterring the detected one or more birds.

8. The system of any preceding claim, wherein the one or more drones comprise a plurality of drones, each drone of the plurality of drones configured 20 to have different default location distributed over the wind power plant, and wherein the controller is configured to receive information indicative of the different default locations, andwherein the controller is configured to, in response to receipt of the detected location, select at least one of the plurality of drones based on a 25 proximity of the detected location to the default location of the plurality of drones, andprovide the control signals to the selected at least one drone.

9. The system of any preceding claim, wherein the one or more detection 30 devices include a camera and wherein the controller is configured to receive one or more images captured by the camera; and wherein the controller is further configured to:identify the type of bird based on the one or more images using an image detection algorithm; and06 03 25based on the identification of a plurality of birds by the one or more detection devices at any one time, prioritise the plurality of birds based on the identified bird type, andprovide the control signals to fly the at least one drone based on said 5 prioritization.

10. The system of any preceding claim, wherein the one or more drones include a first part of a coupling device to couple to a bird deterring device comprising a second part of the coupling device and wherein said control io signals are further configured to instruct at least one of the one or more drones to couple to the bird deterring device and then fly to the location for deterring the detected one or more birds.

11. The system of any preceding claim, wherein at least a subset of the one 15 or more drones include one or more sensors for measuring a characteristic of a wind field, andwherein the one or more drones are configured to operate in a bird deterrent mode in which they receive said control signals to fly to the location for deterring the detected one or more birds and a measurement mode wherein 20 the one or more first drones are configured to fly upstream of one of the plurality of wind turbines to measure the characteristic of the wind field that will be incident on a rotor of that wind turbine, andwherein the controller is configured to operate the subset of the one or more drones in the measurement mode at times when the one or more 25 detection devices do not detect the presence of one or more birds and operate the subset of the one or more drones in the bird deterrent mode at times when the one or more detection devices detect the presence of one or more birds.

12. The system of any preceding claim, wherein at least a subset of the one 30 or more drones include at least one camera and / or at least one sensor for performing inspections of a wind turbine, andwherein the one or more drones are configured to operate in a bird deterrent mode in which they receive said control signals to fly to the location for deterring the detected one or more birds and an inspection mode wherein 35 the one or more first drones are configured to be flown to inspect the wind06 03 25turbine by relaying images acquired by the at least one camera and / or data acquired by the at least one sensor, andwherein the controller is configured to operate the subset of the one or more drones in the inspection mode at times when the one or more detection 5 devices do not detect the presence of one or more birds and operate the subset of the one or more drones in the bird deterrent mode at times when the one or more detection devices detect the presence of one or more birds.

13. A wind power plant comprising the plurality of wind turbines and the io system of any one of claims 1 to 12.

14. The wind power plant of claim 13 wherein the plurality of wind turbines are located over a spatial area and the one or more detection devices are located at edges of the spatial area.1515. The wind power plant of claim 13 or 14, wherein the number of detection devices is less than the number of wind turbines.

16. The wind power plant of claim 13, wherein the plurality of wind turbines 20 are located over a spatial area and wherein the one or more drones comprise a plurality of drones and wherein the drones are positioned at different default locations over the spatial area.

17. The wind power plant of claim 13, wherein the number of drones is less 25 than the number of wind turbines and the controller is configured to select at least one of the plurality of drones to receive the control signals to fly to the location for deterring the detected one or more birds based on predetermined criteria.

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