Wind turbines, wind farms and methods for operating a plurality of wind turbines

Abstract

The present disclosure relates to methods (100) for operating a plurality of wind turbines (10) of a string (105) of a wind farm (101), to wind farms (101) and to wind turbines (10). A method (100) comprises determining (110) that a common power conductor (108) does not receive power from a utility grid (102). The method further comprises, after a predetermined first period of time, a first wind turbine (111) trying to generate power and supply the power (120) to the common power conductor (108) to supply power to one or more other wind turbines (10) of the plurality of wind turbines. The method further comprises, after a predetermined second period of time, a second wind turbine (211) trying to generate power and supply the power (130) to the common power conductor (108) to supply power to one or more other wind turbines (10) of the plurality of wind turbines if the first wind turbine (111) fails. The predetermined second period of time is longer than the predetermined first period of time.

Description

GENERAL ELECTRIC RE OVABLES ESPANA S.L. NOVEMBER 27, 2024701048-WO-1 P5522PC00WIND TURBINES, WIND FARMS AND METHODS FOR OPERATING A PLURALITY OF WIND TURBINES

[0001] The present disclosure relates to methods for operating a plurality of wind turbines of a string of a wind farm, specifically when electrical connection to the utility grid is not available. The present disclosure further relates to wind farms and to wind turbines.BACKGROUND

[0002] Modern wind turbines are commonly used to supply electricity into the electrical grid, also known as utility grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft to a generator, either directly (“directly driven” or “gearless”) or through the use of a gearbox. This way, the generator produces electricity which can be supplied to the electrical grid.

[0003] The wind turbine hub may be rotatably coupled to a front of the nacelle. The wind turbine hub may be connected to a rotor shaft, and the rotor shaft may then be rotatably mounted in the nacelle using one or more rotor shaft bearings arranged in a frame inside the nacelle. The nacelle is a housing arranged on top of a wind turbine tower that may contain and protect the gearbox (if present) and the generator (if not placed outside the nacelle) and, depending on the wind turbine, further components such as a power converter, and auxiliary systems.

[0004] A wind farm may be disconnected from the utility grid due to different reasons, such as faults, e.g. in the power cables leading to the utility grid, or regular maintenance. Furthermore, the wind farm may also be disconnected from the utility grid during the erection and commissioning phase, i.e. prior to starting commercial operation of the wind farm.

[0005] Even if a wind farm is disconnected from the utility grid, operation of some auxiliary systems of the wind turbine may be required. Some relevant auxiliary systems for example include control systems, communication systems, air conditioning systems, temperature regulation systems or yaw operation systems. Operation of such auxiliary systems may bedesired for safety or maintenance reasons. For example, if a wind turbine is not capable of aligning to the wind, its structural integrity may be at risk.

[0006] In some known examples, such auxiliary systems may employ alternative energy resources when the wind farm is disconnected from the utility grid. Alternative energy sources may comprise battery-based systems, supercapacitors, uninterruptible power supplies (UPS) or diesel generators. A problem associated with these solutions is that auxiliary systems can only keep operating while electric power provided by such energy sources is available, i.e. until such auxiliary power sources do not run out of power as well. Other auxiliary energy sources, including energy sources based on renewable energy sources, such as solar panels, may also be used to provide auxiliary power. Another known approach to obtain auxiliary power to feed auxiliary systems of a wind turbine disconnected from the utility grid may include operating the wind turbine itself in a power generating mode so as to generate a small amount of electric power. The relatively small amount of generated electric power can be supplied to one or more of the corresponding auxiliary systems.

[0007] In some cases, a wind farm, or a portion of a wind farm, operates in an island mode. That is, in a situation in which the wind farm is disconnected from the utility grid, one or more wind turbines are operated to supply power to auxiliary systems. The auxiliary systems may be the auxiliary systems of the wind turbines generating the power or may be the auxiliary systems of the other wind turbines.

[0008] It is known that wind farms can be arranged in strings of wind turbines. Each string comprises a plurality of wind turbines which can be electrically connected in series. In known configurations, the different strings are connected to each other at a collection point, such that power generated by the wind turbines of the different strings is collected and, finally, injected into the utility grid.

[0009] In wind farms comprising strings, it is known to operate the wind turbines of a string to generate electric power to feed corresponding auxiliary systems when the string is electrically isolated from both the utility grid and from the remaining strings.

[0010] In some examples, the wind turbines in the string can be operated to generate power and supply the power to its own auxiliary systems. I.e., the individual wind turbines can operate in a so-called turbine island mode, TIM. The turbine island mode may herein be regarded as an autonomous mode in which a wind turbine generates power for its own auxiliary systems, but it does not supply or receive power from other entities (e.g. another wind turbine or a utility grid). In other examples, one or more wind turbines in a string can be operated to generate power and supply the power to their own auxiliary systems and to the auxiliarysystems of at least some of the other wind turbines in the string, particularly those lacking TIM hardware, and therefore uncapable of running in TIM mode. In such examples, the string is operated in a so-called string island mode, SIM. In these cases, the wind turbines receiving the electric power act as a load.

[0011] To enter an island mode in a string (or a portion thereof) of a wind farm, one or more wind turbines of the string may act as grid forming wind turbines. When disconnection from the utility grid is detected, these grid forming turbines may act as leaders and they may generate power and supply the power to their own auxiliary systems and to the auxiliary systems of other wind turbines. At least some of the remaining wind turbines of the string island may then follow the leading wind turbines, i.e. they may be “grid-following” or non-grid forming wind turbines, and also may be referred to as “followers”. Such remaining wind turbines may in particular consume power generated by the other wind turbines to feed their own auxiliary systems.

[0012] Throughout the present disclosure, a grid forming wind turbine may be regarded as a wind turbine in a grid forming operating mode in which a wind turbine creates and maintains a stable electrical signal independently. Accordingly, a wind turbine operating in a grid forming mode can establish or form a waveform with desired magnitudes, e.g. frequency, amplitude or phase, regardless of the presence of other electrical sources. The other wind turbines of the same string may be non-operational and act only as a load, i.e. they receive and consume power from the string. In some scenarios, it may be envisaged that another wind turbine may start to operate in a grid following mode which may be understood as an operating mode in which a wind turbine relies on an external, already existing, voltage waveform for its operation. Hence, the conditions of the external waveform are detected and followed by the wind turbine so that the power generated matches the frequency and phase of the already existing waveform.

[0013] Wind farms typically comprise a wind farm controller, which is in communication with each wind turbine. The wind farm controller is used to e.g. provide operating setpoints to the wind turbines or to receive operating conditions from the wind turbines.

[0014] During operation of a string (or a portion thereof) of a wind farm disconnected from the utility grid, it may happen that communication with the wind farm controller is not available. Also, the wind turbines may not be able to communicate with each other. In such examples, the wind turbines may not know which should be the first one to start to generate and supply power. Conflicts may arise between multiple wind turbines trying to form a local electrical grid at the same time.

[0015] The present disclosure aims to provide methods, systems and wind turbines for an efficient and free-of-conflicts operation of wind turbines, in particular of a wind farm string or a portion thereof, when electrical connection to the utility grid is not available.SUMMARY

[0016] In an aspect of the present disclosure, a method for operating a plurality of wind turbines of a string of a wind farm is provided. The wind turbines are connected to a common power conductor at respective connection points. The method comprises determining that the common power conductor does not receive power from a utility grid. The method further comprises, after a predetermined first period of time, a first wind turbine of the plurality of wind turbines trying to generate power and supply the power to the common power conductor to supply power to one or more other wind turbines of the plurality of wind turbines. The method further comprises, after a predetermined second period of time, a second wind turbine of the plurality of wind turbines trying to generate power and supply the power to the common power conductor to supply power to one or more other wind turbines of the plurality of wind turbines if the first wind turbine fails. The predetermined second period of time is longer than the predetermined first period of time.

[0017] According to this aspect of the disclosure, when electrical connection with a utility grid is missing, a first wind turbine of a string, after a first predetermined first period of time elapses, enters a grid forming mode and tries to generate power and supply power to the common power conductor. After a second predetermined period of time elapses, if the first wind turbine has failed in generating power and supplying the power, a second wind turbine enters a grid forming mode and tries to generate power and supply power to the common power conductor. Since the second wind turbine enters a grid forming mode if the first wind turbine fails, the second period of time is longer than the first period of time.

[0018] Therefore, if the first wind turbine is unable to form a local electrical grid, then reliability and redundancy is provided by another wind turbine, namely the second wind turbine, which tries to form the local grid. This may be particularly relevant if the plurality of wind turbines cannot communicate with a wind farm controller. In this manner, different wind turbines of the string may try to form a local electrical grid in a coordinated and controlled manner. Conflicts between two or more wind turbines acting as grid forming at simultaneous periods of time may be avoided.

[0019] In another aspect of the present disclosure, a wind farm is provided. The wind farm comprises a string including a plurality of wind turbines connected to a common powerconductor at respective connection points. A first wind turbine of the plurality of wind turbines is configured to monitor the common power conductor and to try to generate power and supply the power to the common power conductor and one or more other wind turbines of the plurality of wind turbines after a predetermined first period of time if no voltage is detected. A second wind turbine of the plurality of wind turbines is configured to monitor the common power conductor and to try to generate power and supply the power to the common power conductor and one or more other wind turbines of the plurality of wind turbines after a predetermined second period of time if no voltage is detected. The predetermined first and second periods of time are different.

[0020] The method of the previous aspect may specifically be performed in the wind farm of this aspect.

[0021] In still a further aspect of the present disclosure, a wind turbine is provided. The wind turbine is configured to be connected at a common power conductor at a point of connection to a common power conductor of a string. The wind turbine is configured to monitor the common power conductor and to try to generate power and supply the power to the common power conductor and one or more other wind turbines of the string after a predetermined first period of time if no voltage is detected.

[0022] This turbine may specifically be a turbine present in the wind farm of the previous aspect.

[0023] Throughout this disclosure, a string may be understood as a plurality of wind turbines that are electrically connected to form a series circuit. The wind turbines are connected through electrical conductors. In particular, power cables and power switches can be arranged between adjacent wind turbines to form the series circuit. In that sense, throughout this disclosure, a common power conductor is understood as the assembly of power cables used in a string to electrically connect the wind turbines of a string. The electric power generated by the wind turbines is injected in such common power conductor and transmitted along the common power conductor to a collection point of the wind farm, e.g. a wind farm substation. To this end, an end of the string, i.e. of the common power conductor, is connected to such collection point.BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Non-limiting examples of the present disclosure will be described in the following, with reference to the drawings, in which:Figure 1 illustrates a perspective view of one example of a wind turbine;Figure 2 illustrates a simplified, internal view of one example of the nacelle of the wind turbine of figure 1 ;Figure 3 schematically illustrates an example of a wind farm;Figure 4 shows a flowchart of an example of a method for operating at least a portion of string of a wind farm;Figure 5 schematically represents an example of four wind turbines trying to form a local grid after it has been determined that the utility grid is not available.DETAILED DESCRIPTION OF EXAMPLES

[0025] Reference now will be made in detail to embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation only, not as a limitation. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

[0026] Figure 1 is a perspective view of an example of a wind turbine 10. In the example, the wind turbine 10 is a horizontal-axis wind turbine. Alternatively, the wind turbine 10 may be a vertical-axis wind turbine. In the example, the wind turbine 10 includes a tower 15 that extends from a support system 14 on a ground 12, a nacelle 16 mounted on tower 15, and a rotor 18 that is coupled to nacelle 16. The rotor 18 includes a rotatable hub 20 and at least one rotor blade 22 coupled to and extending outward from the hub 20. In the example, the rotor 18 has three rotor blades 22. In an alternative embodiment, the rotor 18 includes more or less than three rotor blades 22. The tower 15 may be fabricated from tubular steel to define a cavity (not shown in Figure 1 ) between a support system 14 and the nacelle 16. In an alternative embodiment, the tower 15 is any suitable type of a tower having any suitable height. According to an alternative, the tower can be a hybrid tower comprising a portion made of concrete and a tubular steel portion. Also, the tower can be a partial or full lattice tower.

[0027] The rotor blades 22 are spaced about the hub 20 to facilitate rotating the rotor 18 to enable kinetic energy to be transferred from the wind into usable mechanical energy, and subsequently, electrical energy. The rotor blades 22 are mated to the hub 20 by coupling ablade root region 24 to the hub 20 at a plurality of load transfer regions 26. The load transfer regions 26 may have a hub load transfer region and a blade load transfer region (both not shown in Figure 1 ). Loads induced to the rotor blades 22 are transferred to the hub 20 via the load transfer regions 26.

[0028] In examples, the rotor blades 22 may have a length ranging from about 15 meters (m) to about 90 m or more. Rotor blades 22 may have any suitable length that enables the wind turbine 10 to function as described herein. For example, non-limiting examples of blade lengths include 20 m or less, 37 m, 48.7 m, 50.2m, 52.2 m or a length that is greater than 91 m. As wind strikes the rotor blades 22 from a wind direction 28, the rotor 18 is rotated about a rotor axis 30. As the rotor blades 22 are rotated and subjected to centrifugal forces, the rotor blades 22 are also subjected to various forces and moments. As such, the rotor blades 22 may deflect and / or rotate from a neutral, or non-deflected, position to a deflected position.

[0029] Moreover, a pitch angle of the rotor blades 22, e.g. an angle that determines an orientation of the rotor blades 22 with respect to the wind direction, may be changed by a pitch system 32 to control the load and power generated by the wind turbine 10 by adjusting an angular position of at least one rotor blade 22 relative to wind vectors. Pitch axes 34 of rotor blades 22 are shown. During operation of the wind turbine 10, the pitch system 32 may particularly change a pitch angle of the rotor blades 22 such that the angle of attack of (portions of) the rotor blades are reduced, which facilitates reducing a rotational speed and / or facilitates a stall of the rotor 18.

[0030] In the example, a blade pitch of each rotor blade 22 is controlled individually by a wind turbine controller 36 or by a pitch control system 80 (see Figure 2). Alternatively, the blade pitch for all rotor blades 22 may be controlled simultaneously by said control systems.

[0031] Further, in the example, as the wind direction 28 changes, a yaw direction of the nacelle 16 may be rotated about a yaw axis 38 to position the rotor blades 22 with respect to wind direction 28.

[0032] In the example, the wind turbine controller 36 is shown as being centralized within the nacelle 16, however, the wind turbine controller 36 may be a distributed control system throughout the wind turbine 10, on the support system 14, within a wind farm, and / or at a remote-control center. The wind turbine controller 36 may include one or more processors 40 configured to perform one or more of the steps of the methods described herein. Further, many of the other components described herein include one or more processors. The wind turbine controller 36 may also include a memory, e.g. one or more memory devices. As used herein, a memory may comprise memory element(s) including, but not limited to, a computer readablemedium (e.g., random access memory (RAM)), a computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magnetooptical disk (MOD), a digital versatile disc (DVD) and / or other suitable memory elements.

[0033] Figure 2 is an enlarged sectional view of a portion of the wind turbine 10. In the example, the wind turbine 10 includes the nacelle 16 and the rotor 18 that is rotatably coupled to the nacelle 16. More specifically, the hub 20 of the rotor 18 is rotatably coupled to an generator 42 positioned within the nacelle 16 by the main shaft 44, a gearbox 46, a high-speed shaft 48, and a coupling 50. In the example, the main shaft 44 is disposed at least partially coaxial to a longitudinal axis (not shown) of the nacelle 16. A rotation of the main shaft 44 drives the gearbox 46 that subsequently drives the high-speed shaft 48 by translating the relatively slow rotational movement of the rotor 18 and of the main shaft 44 into a relatively fast rotational movement of the high-speed shaft 48. The latter is connected to the generator 42 for generating electrical energy with the help of a coupling 50. Furthermore, a transformer 90 and / or suitable electronics, switches, and / or inverters may be arranged in the nacelle 16 in order to transform electrical energy generated by the generator 42 having a voltage between e.g. 400V to 1000 V into electrical energy having medium voltage (e.g. 10 - 35 kV). Offshore wind turbines may have, for example, generator voltages between 650 V and 3500 V. Transformer voltages may for instance be between 30 kV and 70 kV, or even higher, e.g. over 130 kV. Said electrical energy is conducted via power cables from the nacelle 16 into the tower 15.

[0034] The gearbox 46, generator 42 and transformer 90 may be supported by a main support structure frame of the nacelle 16, optionally embodied as a main frame 52. The gearbox 46 may include a gearbox housing that is connected to the main frame 52 by one or more torque arms 103. In the example, the nacelle 16 also includes a main forward support bearing 60 and a main aft support bearing 62. Furthermore, the generator 42 can be mounted to the main frame 52 by decoupling support means 54, in particular in order to prevent vibrations of the generator 42 to be introduced into the main frame 52 and thereby causing a noise emission source.

[0035] Optionally, the main frame 52 is configured to carry the entire load caused by the weight of the rotor 18 and components of the nacelle 16 and by the wind and rotational loads, and furthermore, to introduce these loads into the tower 15 of the wind turbine 10. The rotor shaft 44, generator 42, gearbox 46, high speed shaft 48, coupling 50, and any associated fastening, support, and / or securing device including, but not limited to, support 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0036] In some examples, the wind turbine may be a direct drive wind turbine without gearbox 46. Generator 42 operates at the same rotational speed as the rotor 18 in direct drive wind turbines. They therefore generally have a much larger diameter than generators used in wind turbines having a gearbox 46 for providing a similar amount of power than a wind turbine with a gearbox.

[0037] The nacelle 16 may also include a yaw drive mechanism 56 that may be used to rotate the nacelle 16 and thereby also the rotor 18 about the yaw axis 38 to control the perspective of the rotor blades 22 with respect to the wind direction 28.

[0038] For positioning the nacelle 16 appropriately with respect to the wind direction 28, the nacelle 16 may also include at least one meteorological measurement system which may include a wind vane and an anemometer. The meteorological measurement system 58 can provide information to the wind turbine controller 36 that may include wind direction 28 and / or wind speed.

[0039] In the example, the pitch system 32 (see Figure 1 ) is at least partially arranged as a pitch assembly 66 in the hub 20. The pitch assembly 66 includes one or more pitch drive systems 68 and at least one sensor 70. Each pitch drive system 68 is coupled to a respective rotor blade 22 (shown in figure 1 ) for modulating the pitch angle of a rotor blade 22 along the pitch axis 34. Only one of three pitch drive systems 68 is shown in figure 2.

[0040] In the example, the pitch assembly 66 includes at least one pitch bearing 72 coupled to hub 20 and to a respective rotor blade 22 (shown in figure 1 ) for rotating the respective rotor blade 22 about the pitch axis 34. The pitch drive system 68 includes a pitch drive motor 74, a pitch drive gearbox 76, and a pitch drive pinion 78. The pitch drive motor 74 is coupled to the pitch drive gearbox 76 such that the pitch drive motor 74 imparts mechanical force to the pitch drive gearbox 76. The pitch drive gearbox 76 is coupled to the pitch drive pinion 78 such that the pitch drive pinion 78 is rotated by the pitch drive gearbox 76. The pitch bearing 72 is coupled to pitch drive pinion 78 such that the rotation of the pitch drive pinion 78 causes a rotation of the pitch bearing 72.

[0041] Pitch drive system 68 is coupled to the wind turbine controller 36 for adjusting the pitch angle of a rotor blade 22 upon receipt of one or more signals from the wind turbine controller 36. In the example, the pitch drive motor 74 is any suitable motor driven by electric power and / or a hydraulic system that enables pitch assembly 66 to function as described herein. Alternatively, the pitch assembly 66 may include any suitable structure, configuration, arrangement, and / or components such as, but not limited to, hydraulic cylinders, springs, and / or servomechanisms. In certain embodiments, the pitch drive motor 74 is driven by energyextracted from a rotational inertia of hub 20 and / or a stored energy source (not shown) that supplies energy to components of the wind turbine 10.

[0042] The pitch assembly 66 may also include one or more pitch control systems 80 for controlling the pitch drive system 68 according to control signals from the wind turbine controller 36, in case of specific prioritized situations and / or during rotor 18 overspeed. In the example, the pitch assembly 66 includes at least one pitch control system 80 communicatively coupled to a respective pitch drive system 68 for controlling pitch drive system 68 independently from the wind turbine controller 36. In the example, the pitch control system 80 is coupled to the pitch drive system 68 and to a sensor 70. During normal operation of the wind turbine 10, the wind turbine controller 36 may control the pitch drive system 68 to adjust a pitch angle of rotor blades 22.

[0043] According to an embodiment, a power generator 84, for example comprising a battery and electric capacitors, is arranged at or within the hub 20 and is coupled to the sensor 70, the pitch control system 80, and to the pitch drive system 68 to provide a source of power to these components. In the example, the power generator 84 provides a continuing source of power to the pitch assembly 66 during operation of the wind turbine 10. In an alternative embodiment, power source 84 provides power to the pitch assembly 66 only during an electric power loss event of the wind turbine 10. The electric power loss event may include power grid loss or dip, malfunctioning of an electrical system of the wind turbine 10, and / or failure of the wind turbine controller 36. During the electric power loss event, the power generator 84 operates to provide electric power to the pitch assembly 66 such that pitch assembly 66 can operate during the electric power loss event.

[0044] In the example, the pitch drive system 68, the sensor 70, the pitch control system 80, cables, and the power source 84 are each positioned in a cavity 86 defined by an inner surface 88 of hub 20. In an alternative embodiment, said components are positioned with respect to an outer surface of hub 20 and may be coupled, directly or indirectly, to the outer surface.

[0045] As used herein, the term “processor” is not limited to integrated circuits referred to in the art as a computer, but broadly refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein.

[0046] Figure 3 schematically illustrates a wind farm 101 according to an example of the present disclosure in which method 100 (later illustrated herein) may be implemented. The wind farm 101 comprises a plurality of wind turbines 10 connected to a utility grid 102 at a pointof interconnection (POI) 109. The wind farm 101 may comprise a substation 106 including e.g. wind farm transformers configured for converting power from a wind farm voltage to a utility grid voltage.

[0047] The wind turbines 10 of the wind farm 101 may be arranged in strings 105. In the example depicted in Figure 4, a wind farm 101 comprising three strings 105 is shown. It is understood that, in other examples, a different number of strings 105 and a different number of wind turbines 10 per string 105 may be selected. Each string 105 may be connected to, or disconnected from, the substation 106 via a switch 104. In some examples, the switches 104 or breakers may be arranged at the substation 106.

[0048] The electric power generated by each wind turbine 10 in a string 105 may be collected in a string conductor 108, also referred to herein as common power conductor 108. Each wind turbine 10 may be connected to, or disconnected from, a corresponding string conductor 108 via one or more switches or breakers (not shown).

[0049] A string 105 of the wind farm 101 , or a portion thereof, may enter in a so-called string island mode if, for instance, electrical connection with the utility grid 102 is lost. In this case, the respective switch 104 may be opened and a local electrical grid may be formed at string level.

[0050] Hence, a first wind turbine 111 in the string 105 may operate in a grid forming mode to generate electric power. The electric power generated by the first wind turbine 111 may be transmitted to the string conductor 108. Such electric power may be used to feed other wind turbines 10 in the string 105, for example to feed the auxiliary systems of other wind turbines 10. Auxiliary systems may include, among others, one or more of communication systems, temperature and ventilation regulation systems, bearing lubrication systems, controller systems, yaw systems, or others.

[0051] The wind farm 101 may also comprise a wind farm controller 107, which may be communicatively coupled to the wind turbines 10, specifically to the wind turbine controllers 36 of the wind turbines 10. The wind farm controller 107 may also be communicatively coupled to the substation 106. If the wind turbines 10 are unable to communicate with the wind farm controller 107 and among each other, then conflicts, specifically coordination conflicts, may arise. For example, several wind turbines of a string 105 may try to form a local electric grid simultaneously. Or if the local electrical grid is not formed since a failure has occurred, then the wind turbines would not know which one of them should try to form the local grid next.

[0052] In an aspect of the present disclosure, a method 100 for operating a plurality of wind turbines of a string 105 of a wind farm 101 is provided. The plurality of wind turbines isconnected to a common power conductor 108 at respective connection points. For example, switches connecting the wind turbines to the string conductor may be closed. This method 100 is schematically illustrated in the flowchart of figure 4.

[0053] The method comprises, at block 110, determining that the common power conductor 108 does not receive power from a utility grid 102. The method further comprises, at block 120 and after a predetermined first period of time, a first wind turbine 111 of the plurality of wind turbines trying to generate power and supply the power to the common power conductor 108 to supply power to one or more other wind turbines of the plurality of wind turbines. The method further comprises, at block 130 and after a predetermined second period of time, a second wind turbine 211 of the plurality of wind turbines trying to generate power and supply the power to the common power conductor 108 to supply power to one more other wind turbines of the plurality of wind turbines if the first wind turbine fails 111. The predetermined second period of time is longer than the predetermined first period of time.

[0054] Therefore, when there is no electrical connection to the utility grid, the first wind turbine 111 tries to generate power and supply the power to the common power conductor 108 and therefore to at least some of the wind turbines of the plurality of wind turbines. If the second wind turbine 211 determines that there is still no electric power available, then the second wind turbine 211 tries to generate power and supply it to the common power conductor 108. The moments at which the first wind turbine 111 and the second wind turbine 211 enter a grid forming mode are predetermined.

[0055] Accordingly, if the first wind turbine 111 fails to generate and supply power to other wind turbines, then the second wind turbine 211 tries to do so. Conflict is avoided, as the first wind turbine 111 tries to form the local electrical grid at a certain time, and the second wind turbine 211 tries to form the local grid at a subsequent time in case there is still no electrical grid.

[0056] The second wind turbine 211 may determine that the wind turbine 111 has failed, i.e. that there is still no local electrical grid, by measuring voltage and frequency of the common power conductor 108. Likewise, monitoring voltage and frequency of the common power conductor 108 may allow wind turbines to know when the local electrical grid has been formed, and thus power is available in the common power conductor 108 and can be started to be consumed.

[0057] The plurality of wind turbines 105 of method 100 may specifically not be able to communicate with a wind farm controller 107, at least temporarily. Also, the plurality of wind turbines may not be able to communicate with each other, at least temporarily. For example,optic fiber connecting the wind farm controller 107 and the different wind turbines may be damaged or faulty. Therefore, the wind turbines may not be able to receive indications or commands from the wind farm controller 107. If communication with the wind farm controller was available, the wind turbines 10 could be instructed to try to form the local grid by the wind farm controller. The wind farm controller 107 may thus act as coordinator, indicating which wind turbine(s) should act as grid forming wind turbines and when. Since at least some of the wind turbines 10 are configured, e.g. programmed, to start to try to form a local grid at certain times, then if communication with the wind farm controller 107 is not available, an effective and controlled grid forming process may still be performed.

[0058] This is likewise applicable to communication between the wind turbines 10. If such communication was available, then the wind turbines could coordinate to organize and form a local grid in a suitable manner. The above method specifically allows a reliable and coordinated grid forming process when such communication is not available.

[0059] The above steps of method 100 may be repeated once or more than once. For example, the method may further comprise, after a predetermined third period of time, a third wind turbine 311 of the plurality of wind turbines trying to generate power and supply power to one or more other wind turbines of the plurality of wind turbines if the second wind turbine 211 fails. The predetermined third period of time is longer than the predetermined second period of time. This and other additional wind turbines may therefore act as grid forming wind turbines and try to form the local electrical grid if necessary.

[0060] It should be noted that in some examples, the first wind turbine 111 does not need to be the actual first turbine trying to form the local grid. Likewise, the second wind turbine 211 does not need to be the actual second turbine trying to form the local grid. For example, additional wind turbines could have tried to form the local grid between the first and second wind turbines. Or additional wind turbines could have tried to form the local grid before the first wind turbine.

[0061] The predetermined period of time may be given with respect to a “time zero”. For example, the predetermined first and second periods of time may start with the determination that the common power conductor does not receive power from the utility grid 102. Since all the wind turbines generally monitor whether they are connected to an electrical grid (either the utility grid or a local grid), this may be a simple manner to program which wind turbine acts as grid forming if necessary. Figure 5 schematically represents an example of four wind turbines trying to form a local grid after it has been determined that the utility grid 102 is not available.

[0062] In this figure, time to represents the time at which it has been determined that the common power conductor 108 does not receive power from the utility grid 102. A first wind turbine WT1 may start to act as grid forming at a time t1 > to. Le., grid loss is detected and then the first wind turbine WT1 starts to try to form the local grid at t1 = tO + Pt1 = tO + AtO. Pt1 represents a predetermined first period of time. If necessary, a second wind turbine WT2 may act as grid forming at a time t2 = tO + Pt2 (Pt2 is another longer predetermined period of time), if necessary a third wind turbine WT3, e.g. a third wind turbine 311 of figure 4, may act as grid forming at a time t3 = tO + Pt3 (Pt3 is still another longer predetermined period of time), and so on.

[0063] The amount of time between successive wind turbines (successive in terms of acting as forming wind turbines) may be the same. For example, a difference between the predetermined third Pt3 and second Pt2 periods of time may substantially be the same as the difference between the second Pt2 and first Pt1 periods of time. In other words and with respect to figure 5, At2 = At1. In the example of figure 5, except for AtO, the other amounts of time between successive wind turbines are the same: At1 = At2 = At3 and so on.

[0064] In some examples, the predetermined first period of time Pt1 may be three minutes or less, optionally between one minute and three minutes, for example between one minute and two minutes. In some examples, it may be less than one minute. The predetermined second period of time Pt2 may be at least one minute longer than the predetermined first period of time Pt1 , specifically at least two minutes longer than the predetermined first period of time Pt1 . This amount of time may be sufficient for allowing the first wind turbine WT 1 , 111 to try to form the local grid, and at the same time may allow the second wind turbine WT2, 211 to try to generate the local grid relatively quickly. Specifically, the amount of time between subsequent wind turbines trying to form the local grid may be between one minute and three minutes. For example, the amount of time may be around two minutes. In some examples, At3 = At2 = At1 ~ 2 minutes.

[0065] Determining that the common power conductor 108 does not receive power from the utility grid 102 may be performed in any suitable manner known in the art. In some examples the determination may include the wind turbines determining that voltage has been below a voltage threshold during a certain amount of time. Electric frequency may also be monitored. If there was connection to the utility grid 102 and it was lost, switches 104 connecting the strings 105 to the substation 106 may be opened to isolate the strings 105. The wind turbine closest to the substation 106 (in terms of length of common power conductor between the wind turbines and the substation 106) may be the first one that determines that there is no electrical connection to the utility grid 102.

[0066] The steps of this method 100 may be performed in more than one string 105. In the example of figure 3, the steps may be performed in each string 105. In this regard, a first wind turbine 111 and a second wind turbine 211 are indicated for each string 105. In this and other examples, the second wind turbine 211 which may act as a grid forming wind turbine at a certain time, may also act as non-grid forming wind turbine at other time(s). For example, the second wind turbine 211 acts as non-grid forming (the non-grid forming wind turbines monitor to check if there is an electrical grid so they can get power at a certain time are represented with dashed lines in figure 3) when the first wind turbine 111 is acting as grid forming (the grid forming wind turbines at that certain time are represented with solid lines in figure 3). And the second wind turbine 211 may also act as non-grid forming if other wind turbines of the group try to form the local grid at other times.

[0067] In some examples, all the wind turbines of the plurality of wind turbines may be configured to act as grid forming. For example, in the middle string shown in the example of figure 3, all the wind turbines are configured to act as grid forming if necessary. In other examples, one or more wind turbines of the group may not be configured to be grid forming. For example, if a string 105 includes a large number of wind turbines, then only some of them may try to form the grid in a staggered manner. Also, in some examples, the wind turbines which are not configured as grid forming may lack turbine island mode (TIM). Therefore, if there is no connection to the utility grid 102, they may obtain power from a grid forming wind turbine when the local grid is formed.

[0068] In some examples and as explained further below, one or more wind turbines having turbine island mode (TIM) may be left out of the grid forming process such that these turbines do not interfere with it. For example, turbines 59 represented with dotted lines in figure 5 represent wind turbines having TIM capabilities which are configured to generate power, but are not to belong to the local grid of the near wind turbines. I.e., these turbines 59 do not try to form a local grid and they are not to receive power from the turbines forming the local grid. In this regard, a switch connecting these turbines to the common power conductor 108 may be open.

[0069] The method may further comprise, if the second wind turbine 211 succeeds in generating power, the second wind turbine 211 supplying the power to the common power conductor 108 and supplying power to the one or more other wind turbines of the plurality of wind turbines. The method may further comprise, determining that the utility grid 102 can supply power to the common power conductor 108, and starting other wind turbines (i.e. wind turbines different from the second wind turbine 211 ) of the plurality of wind turbines to supplypower to the utility grid 102. I.e., if the utility grid 102 becomes available, the wind turbines may stop operating in string island mode and may then be connected to the utility grid 102.

[0070] Determining that the utility grid 102 can supply power to the common power conductor 108 may include recovering communication with a wind farm controller 107. The wind farm controller 107 may indicate the availability of the utility grid 102 to the plurality of wind turbines.

[0071] Therefore, in some examples, the second wind turbine 211 succeeds in forming the local grid and, as the local grid now exists, there is no need that additional wind turbines try to form it. At a subsequent time, it is detected that electrical connection to the utility grid 102 can be regained. For example, the wind farm controller 107 becomes available and the wind farm controller 107 initiates communication with the wind turbines 10. The wind farm controller indicates the wind turbines the availability of the utility grid 102. String island mode (SIM) is ended, and then the wind turbines are reconnected to the utility grid 102. Switches 104 connecting the strings 105 to the substation 106 may in particular be closed after SIM is finished. The wind turbines 10 of the string may then start to operate in a normal operation mode, generating power and providing it to the utility grid.

[0072] In some other examples, the switches 104 connecting the strings 105 to the substation 106 may first be closed. The wind turbines may identify then that they are electrically connected to the utility grid 102. For example, the power sent by the grid-forming wind turbine may include an electric signature such as a voltage ripple, whereas the power from the utility grid may lack it. The wind turbines may therefore know that the utility grid is available, and they change the operational mode to the normal operation mode where they generate power and provide it to the utility grid.

[0073] In some examples, the first wind turbine 111 may already succeed in forming the local grid. I.e., the wind turbines may be configured to monitor connection to a grid and to try to form one if necessary. But it may happen that the first wind turbine trying to form the local grid already succeeds in doing so. In some other examples, the third wind turbine 311 or another wind turbine may be the wind turbine succeeding in forming the local grid. This local grid may be maintained until connection to the utility grid 102 is (re-)gained or until a failure occurs and a new local grid may need to be formed.

[0074] Which wind turbine is to be the first one to try to form the local grid and the order in which the wind turbines are to try to form the local grid if necessary may be fixed depending on one or more criteria. In some examples, the first wind turbine 111 (and optionally the second wind turbine 211 and other grid forming wind turbines) may be selected from the plurality ofwind turbines based on electrical features of an electrical circuit including the plurality of wind turbines and the common power conductor 108. When a string 105 is disconnected from a remainder of the wind farm 101 , specifically by opening switch 104 (see figure 3), a new electrical circuit is formed. The features of this new electrical circuit, e.g. impedance, resonance frequency, etc. may be different than before, and this may be taken into account.

[0075] In some examples, the electrical features may include electrical resonance performance. A risk of electrical resonance may vary depending on which wind turbine tries to generate power and supply it to the common power conductor 108. Computational simulations may be performed before wind turbine installation to estimate the risk associated to having different wind turbines 10 acting as grid forming. For example, analysis of harmonics may be performed. The order in which the wind turbines should start to try to form the local grid would then be fixed in ascending order or resonance risk. For example, the first wind turbine to try to form the local grid would be the one having a lowest risk of causing electrical resonance, etc.

[0076] In these or other examples, the electric features may include voltage drop performance. If a wind turbine is to supply power to several other wind turbines, then electrical losses increase with increasing distance between the supplying wind turbine and the supplied wind turbine. This may be taken into account to decide which is the first wind turbine to act as grid forming and also the order of the other wind turbines in acting as grid forming. For example, if power should be supplied to seven wind turbines, then a turbine arranged in the middle may be given higher priority in acting as grid forming than a turbine arranged at an end of the wind turbines.

[0077] Other criteria may similarly be used. For example, the optimal energy distribution along the string 105 may be considered when fixing the order of the wind turbines in the string 105. As already mentioned, simulations may be used to determine an optimized order in which the wind turbines should act as grid forming.

[0078] As already mentioned, method 100 may be applied to a whole string 105 or to a portion thereof. For example, method 100 may be applied to two or more different portions of a same string independently.

[0079] In some examples, at least a part of the wind farm may have not been commissioned yet. In this situation, one or more of the wind turbines of the wind farm have not been commissioned and are not operational yet. I.e., the wind turbines may try to form a local grid before starting of normal operation for the first time after installation. Still in some examples, the above method 100 may be performed to black start at least some of the wind turbines of the plurality of wind turbines.

[0080] In a further aspect of the disclosure, a wind farm 101 is provided. An example of a wind farm 101 is schematically illustrated in figure 3. The wind farm comprises a string 105 including a plurality of wind turbines connected to a common power conductor 108 at respective connection points. A first wind turbine 111 of the plurality of wind turbines is configured to monitor the common power conductor 108 and to try to generate power and supply the power to the common power conductor 108 to supply power to one or more other wind turbines of the plurality of wind turbines after a predetermined first period of time if no voltage is detected. A second wind turbine 211 of the plurality of wind turbines is configured to monitor the common power conductor 108 and try to generate power and supply the power to supply power to one or more other wind turbines of the plurality of wind turbines after a predetermined second period of time if no voltage is detected. The predetermined first and second periods of time are different.

[0081] Therefore, at least some of the wind turbines of the plurality of wind turbines are configured to act as grid forming wind turbines. Each wind turbine configured to act as grid forming wind may do so if necessary and at predetermined specific times known by the corresponding wind turbines beforehand. For example, this information may be programmed in the wind turbines in a wind turbine factory, before the wind turbines are installed.

[0082] Features and explanations of the previous aspect may be applied to this aspect, and vice versa. In this regard, the above method 100 may be performed in the wind farm of this aspect.

[0083] For example, electrical criteria may be used to select which wind turbines are to act as grid forming wind turbines, and specifically which wind turbine is to be the first one to try to form a local grid. In this regard, the first wind turbine 111 and the second wind turbine 211 may for example have a lower risk of causing electric resonance in the common power conductor 108 than one or more other wind turbines of the plurality of wind turbines.

[0084] Additionally or alternatively, the first 111 and second 211 wind turbines may be positioned substantially in a middle of the plurality of wind turbines. In his manner, electrical losses may be relatively low for the surrounding wind turbines when the first wind turbine 111 and the second wind turbine 211 act as grid forming than when other wind turbines act as grid forming. For example, if a wind turbine arranged at an end portion of a string (or string portion) acts as grid forming, then the wind turbines at the other end portion may receive remarkably less power than the wind turbines which are closer to the grid forming wind turbine. If the first and second wind turbines are at a middle portion, then more wind turbines may receive a higher amount of power due to less electrical losses.

[0085] The plurality of wind turbines may comprise one or more wind turbines different from the first wind turbine 111 and the second wind turbine 211 which do not have wind turbine island mode capabilities and which are configured as non-grid forming wind turbines. Accordingly, these wind turbines lacking TIM may still be able to operate their auxiliary systems when electric connection to the utility grid 102 is not available. Retrofitting these turbines, specifically by providing the necessary hardware to be able to perform TIM, may be expensive. But by acting only as non-grid forming wind turbines, they could still be energized and e.g. be aligned to the wind for increased structural safety.

[0086] In some examples, the string of wind turbines 105 may further comprise one or more wind turbines which have turbine island mode capabilities in which the wind turbine generates power for its own auxiliary systems only. I.e., one or more wind turbines having TIM capabilities may be left out of the staggered sequence in which several wind turbines may act as grid forming at different times and in which some wind turbines check if they receive power. In these examples, the wind turbine(s) having TIM “left out” may simply generate power for auto consumption and do not interfere with the formation of the local grid. In the example of figure 3, turbines 59 of the top string 105 and the bottom string 105 may have TIM but do not play a role in grid formation and do not form part of the local grid if formed. These turbines 59 may be not connected to the common power conductor 108 at corresponding points of connection. For example, corresponding switches connecting the turbines to the common power conductor may be open.

[0087] This may be advantageous, as a successful grid forming wind turbine may avoid supplying power to additional wind turbines. For example, if there is not a lot of wind, it may be advantageous for a grid forming wind turbine to have less wind turbines to which supply power. Also, if for any reason the local grid ends up not being formed after the different wind turbines configured to be grid forming wind turbines have tried to form it, then at least these turbines with TIM 59 may power their auxiliary systems.

[0088] As previously mentioned, the wind farm 101 may further comprise an offshore substation 106. A switch between the string 105 and the offshore substation 106 may be open.

[0089] In a further aspect of the disclosure, a wind turbine 10 comprising electrical terminals and configured to be connected to a common power conductor 108 at a point of connection of a string 105, is provided. The wind turbine 10 is configured to monitor the common power conductor 108 and to try to generate power and supply the power to the common power conductor 108 to supply power to one or more other wind turbines of the string 105 after a predetermined first period of time if no voltage is detected.

[0090] Therefore, the wind turbine may detect that there is no electrical grid at its terminals, e.g. that electrical connection to the utility grid 102 has been lost or that a local grid has not been formed yet, and act as a grid forming wind turbine at a corresponding predetermined time.

[0091] Features and explanations of the previous aspects may be applied to this aspect, and vice versa. In this regard, the wind turbine of this aspect may be used in above method 100 and may be part of the plurality of wind turbines in the wind farm of the previous aspect.

[0092] In some examples, the predetermined period of time may be between one minute and two minutes and a half, for example about two minutes. If the wind turbine is not the first wind turbine in trying to form the local grid, then the period of time may be longer than if the wind turbine is the first to try to form the local grid.

[0093] If an amount of time measured between the times at which two wind turbines subsequently (one after the other, no further wind turbines acting as grid forming in between) start to try to form the local grid, this amount of time may be between one minute and two minutes and a half, for example two minutes.

[0094] In some examples, the wind turbine may further be configured to retry to generate power after a second predetermined time delay. Therefore, the wind turbine may be configured to be grid forming for the first time after a certain period of time if necessary, and may further be configured to be grid forming for a second time after another period of time if necessary. With respect to the example of figure 5 and the second wind turbine WT2, this turbine is configured to be grid forming at a time t2, after a period of time Pt2, and is also configured to be grid forming at a time t6, after a period of time Pt6. The period of time Pt6 may be referred to as time delay to differentiate the first time and the second time at which the wind turbine may act as grid forming.

[0095] This written description uses examples to disclose a teaching, including the preferred embodiments, and also to enable any person skilled in the art to put the teaching into practice, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Aspects from the various embodiments described, as well as other known equivalents for each such aspects, can be mixed and matched by one of ordinary skill in the art to construct additional embodiments and techniquesin accordance with principles of this application. If reference signs related to drawings are placed in parentheses in a claim, they are solely for attempting to increase the intelligibility of the claim, and shall not be construed as limiting the scope of the claim.

Claims

22CLAIMS1 . A method (100) for operating a plurality of wind turbines (10) of a string (105) of a wind farm (101 ), wherein the wind turbines (10) are connected to a common power conductor (108) at respective connection points, the method (100) comprising: determining (110) that the common power conductor (108) does not receive power from a utility grid (102), after a predetermined first period of time, a first wind turbine (111 ) of the plurality of wind turbines trying to generate power and supply the power (120) to the common power conductor (108) to supply power to one or more other wind turbines (10) of the plurality of wind turbines; and after a predetermined second period of time, a second wind turbine (211 ) of the plurality of wind turbines trying to generate power and supply the power (130) to the common power conductor (108) to supply power to one or more other wind turbines (10) of the plurality of wind turbines if the first wind turbine (111 ) fails, wherein the predetermined second period of time is longer than the predetermined first period of time.

2. The method of claim 1 , wherein the plurality of wind turbines at least temporarily cannot communicate with a wind farm controller (107).

3. The method of claim 1 or claim 2, wherein the plurality of wind turbines at least temporarily cannot communicate with each other.

4. The method of any of claims 1 - 3, wherein the predetermined first and second periods of time start with the determination that the common power conductor (108) does not receive power from the utility grid (102).

5. The method of any of claims 1 - 4, wherein the predetermined first period of time is three minutes or less, optionally between one minute and three minutes.

6. The method of any of claims 1 - 5, wherein the predetermined second period of time is at least one minute longer than the predetermined first period of time, specifically at least two minutes longer than the predetermined first period of time.

7. The method of any of claims 1 - 6, wherein the first wind turbine (111 ) is selected from the plurality of wind turbines based on electrical features of an electrical circuit including the plurality of wind turbines and the common power conductor (108), the electrical features including one or more of electrical resonance performance, voltage drop performance or optimal energy distribution along the string.

8. The method of any of claims 1 - 7, further comprising, after a predetermined third period of time, a third wind turbine (311) of the plurality of wind turbines trying to generate power and supply power to one or more other wind turbines (10) of the plurality of wind turbines if the second wind turbine (211 ) fails, wherein the predetermined third period of time is longer than the predetermined second period of time.

9. The method of claim 8, wherein a difference between the predetermined third and second periods of time is substantially the same as the difference between the predetermined second and first periods of time.

10. The method of any of claims 1 - 7, further comprising: if the second wind turbine (211 ) succeeds in generating power, the second wind turbine (211 ) supplying the power to the common power conductor (108) and supplying power to the one or more other wind turbines (10) of the plurality of wind turbines; determining that the utility grid (102) can supply power to the common power conductor (108); and starting other wind turbines of the plurality of wind turbines to supply power to the utility grid.11 . The method of claim 10, wherein determining that the utility grid (102) can supply power to the common power conductor (108) includes recovering communication with the wind farm controller (107), the wind farm controller (107) indicating the availability of the utility grid (102) to the plurality of wind turbines.

12. A wind farm (101) comprising a string (105) including a plurality of wind turbines (10) connected to a common power conductor (108) at respective connection points, wherein: a first wind turbine (111 ) of the plurality of wind turbines is configured to monitor the common power conductor (108) and to try to generate power and supply the power to thecommon power conductor (108) to supply power to one or more other wind turbines (10) of the plurality of wind turbines after a predetermined first period of time if no voltage is detected; and a second wind turbine (211 ) of the plurality of wind turbines is configured to monitor the common power conductor (108) and to try to generate power and supply the power to the common power conductor (108) and one or more other wind turbines (10) of the plurality of wind turbines after a predetermined second period of time if no voltage is detected; and wherein the predetermined first and second periods of time are different.

13. The wind farm of claim 12, wherein the first (111 ) and second (211 ) wind turbines are positioned substantially in-between the plurality of wind turbines.

14. The wind farm of any of claims 13 - 15, wherein the string of wind turbines (105) further comprises one or more wind turbines (59) which have turbine island mode capabilities in which the wind turbine generates power for its own auxiliary systems only.

15. A wind turbine (111 , 211 , 311 ) configured to be connected at a point of connection to a common power conductor (108) of a string (105), the wind turbine being configured to monitor the common power conductor (108) and to try to generate power and supply the power to the common power conductor (108) to supply power to one or more other wind turbines (10) of the string (105) after a predetermined first period of time if no voltage is detected, and optionally wherein the wind turbine is further configured to retry to generate power after a predetermined time delay longer than the predetermined first period of time.

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