Methods for operating a wind turbine during grid events and wind turbines

Abstract

The present disclosure relates to a method (100) for operating a wind turbine (10) during a grid event. The wind turbine (10) comprises a generator (42) and a power converter (61), which includes a generator side converter (63), a grid side converter (65), a DC-link, and an energy dissipating unit (69). The method (100) comprises detecting the grid event. The method (100) also comprises reducing a power of the generator (42) and reducing a power injected into the grid (95) in response to the grid event. Furthermore, an end of the grid event is detected and a power injected into the grid (95) is increased with a first rate (204). The method also comprises increasing the power generated by the generator (42) in response to a difference between the power generated by the generator (42) and the power injected into the grid (95) reaching a threshold (203). The present disclosure also relates to the corresponding wind turbine (10).

Description

GENERAL ELECTRIC RE OVABLES ESPANA S.L. NOVEMBER 19, 2024701012-WO-1 P5520PC00METHODS FOR OPERATING A WIND TURBINE DURING GRID EVENTS AND WIND TURBINES

[0001] The present disclosure relates to methods for operating a wind turbine and, in particular, to methods for operating a wind turbine connected to an electrical grid. Specifically, the present disclosure relates to methods for controlling active power in a wind turbine in the presence of grid events. The present disclosure also relates to wind turbines suitable for such methods.BACKGROUND

[0002] Modern wind turbines are commonly used to supply electricity into the electrical 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 generator may be connected to the electrical grid through a power converter. The power converter may comprise a generator side converter, connected to the generator, and a grid side converter, connected to the electrical grid. A DC-link may be arranged between the generator side converter and the grid side converter. An energy dissipating unit may be typically provided in the DC-link.

[0004] The power converter regulates the power output from the generator to the electrical grid such that grid integration requirements can be satisfied. In particular, the power converter is used to adjust the variable-frequency and variable-voltage AC power from the generator into AC power with compatible characteristics, e.g. phase and frequency, so that it can be injected into the electrical grid. The power converter is also used to control the torque applied to the electrical generator such that the operating point of the wind turbine can be controlled to, e.g. optimize energy capture at moderate wind speeds. Furthermore, the power converter enables the wind turbine to remain connected to the electrical grid during certain grid events or disturbances, such as voltage dips or overvoltages. In other words, the power converter allowsthe wind turbine to ride through grid events without disconnecting, thus supporting the stability of the electrical grid.

[0005] During such grid faults or events, the wind turbine may be required to reduce the active power injected into the grid. Furthermore, existing grid codes typically require very fast reaction to such abnormalities in the electrical grid.

[0006] On the one hand, in response to such a grid event, the generator of the wind turbine is typically controlled such that electrical power generated is reduced. Moreover, the pitch angle of the wind turbine blades is also adjusted so as to reduce the captured energy. However, physical limitations, e.g. in the loads that can be withstood by the blades, and a maximum pitch rate, and also communication delays exist. Consequently, the generated power cannot be reduced instantaneously and the maximum rate at which generated electrical power can be reduced is constrained.

[0007] On the other hand, in order to satisfy the requirements imposed by the grid codes, the power injected into the grid from is very quickly constrained. In other words, during the grid event, the power injected into the grid is typically lower than the power generated by the generator due to the limited capabilities of the wind turbine to reduce the generated power.

[0008] The different dynamics result in a power imbalance. Hence, during a grid event, more power is typically generated than can be injected into the electrical grid. The excess power or power surplus, i.e. the difference between the power generated by the generator and the power injected into the grid, is commonly dissipated in an energy dissipating unit. In some known examples, the energy dissipating unit comprises a dynamic brake or chopper, which is commonly arranged in the DC-link of the power converter. To this end, the dynamic brake comprises a resistive unit, which is used to convert electrical energy into heat. The resistive unit is dimensioned such that it can dissipate a certain amount of energy without exceeding limits, e.g. thermal limits.

[0009] Upon clearing of the grid event, a recovery phase is carried out. During such a recovery phase, the wind turbine is controlled so as to increase the power injected into the electrical grid. In particular, in known methods, the generator of the wind turbine is controlled so as to increase the generated power as soon as possible once the grid event is cleared.

[0010] As already mentioned, grid codes generally include requirements for the handling of grid events. In some cases, such requirements may also include requirements, not only for the duration of the grid event itself, but also for the recovery phase after the grid event is cleared. As an example, some grid codes require wind turbines to recover injection of power into the grid with a relatively slow ramp after a low voltage event. Such a slow ramp rate canresult in a prolonged imbalance in the power generated by the generator and the power injected into the grid. Such prolonged imbalance can eventually lead to overloading of the energy dissipating unit.

[0011] Different solutions have been proposed to avoid such overloading while fulfilling the requirements imposed by existing grid codes. In particular, some known solutions comprise over-dimensioning of the energy dissipating unit by arranging very large resistors. Other solutions aim at improving the capabilities of the wind turbine to reduce generated power and / or torque very quickly, i.e. by increasing the load bearing capabilities of certain critical components. Nevertheless, such solutions comprise significant changes in the design of at least some of the wind turbine components. Consequently, these solutions are not applicable in already installed wind turbines unless a retrofit is carried out. Furthermore, the use of, e.g. larger resistors, may also require an over-dimensioning of other system like the cooling system of the wind turbine. Overall, increased cost inevitably arises from those solutions comprising over dimensioning and / or re-design of components to increase their load handling capabilities.

[0012] The present disclosure aims to provide improved methods for operating a wind turbine to cope with grid events. Specifically, the present disclosure aims to at least partially overcome some of the limitations of the already existing solutions. The present disclosure also relates to wind turbines incorporating, or being capable, or carrying out such improved methods.SUMMARY

[0013] In an aspect of the present disclosure, a method for operating a wind turbine connected to a grid during a grid event is provided. The wind turbine comprises a generator and a power converter. The power converter includes a generator side converter, a grid side converter, a DC-link between the generator side converter and the grid side converter, and an energy dissipating unit arranged in the DC-link. The method comprises detecting the grid event and reducing a power generated by the generator and a power injected into the grid in response to detecting the grid event. Furthermore, the method comprises detecting an end of the grid event. The power injected into the grid is increased with a first rate in response to detecting the end of the grid event. The method also comprises increasing the power generated by the generator in response to a difference between the power generated by the generator and the power injected into the grid reaching a predetermined threshold.

[0014] According to this aspect of the disclosure, an improved response to grid events can be provided without requiring (hardware) modifications on the wind turbine components. Gridevents impose a restriction in the power that can be injected into the grid. According to the present disclosure, by not increasing the power generated by the generator until a predetermined threshold is reached, a reduced energy surplus is generated during the grid event and during the subsequent recovery phase. Hence, unlike in conventional methods, the power generated by the generator is not increased in response to the grid event being cleared.

[0015] As known to the skilled person, the rate at which power from the generator can be reduced exhibits some limitations. In particular, the torque on the generator cannot be reduced at very high rates as this poses a risk of overspeed of the wind turbine rotor and of overloading of wind turbine components. For this reason, it is known that a power surplus is typically present during grid events as the power generated by the generator, even if reduced, still exceeds the power injected into the grid, which is very quickly reduced during grid events. In order to account for such power surplus, power converters commonly include an energy dissipating unit, wherein imbalance power can be dissipated.

[0016] Unlike in prior-art methods, in the present disclosure the power generated by the generator is not increased immediately, or as soon as possible, after the grid event is cleared. Although this may seem counterintuitive, inventors have found that a delayed response of the electrical generator, i.e. a delayed increase of the generated power after a grid event or fault is cleared, allows for a more optimum usage of the wind turbine equipment. This is especially the case for wind turbines connected to electrical grids wherein grid codes demand a relatively slow rate for the increase in the grid injected power during the recovery phase. Hence, an overall reduced imbalance between the power generated by the generator and the power injected into the grid arises from this method when taking into account not only the duration of the grid event but also the recovery phase of the same. As a result, the dimensioning of the energy dissipating unit can be optimized.

[0017] In an example, upon detecting the end of the grid event, the method may comprise continue reducing the power generated by the generator until a difference between the power generated by the generator and the power injected into the grid reaches the predetermined threshold. Accordingly, in this example, it is not only that the power from the generator is not increased after the end of the grid event, but the power is actually decreased even further. Consequently, as shown in more detail below, the energy surplus dissipated in an energy dissipating unit may be further optimized or reduced. Specifically, continue reducing may comprise continue decreasing the generated power with substantially the same slope or rate of reduction.

[0018] In another example, upon detecting the end of the grid event, the method may comprise holding the power generated by the generator until a difference between the powerfrom the generator and the power injected into the grid reaches the predetermined threshold. In this variant, the power from the generator may not be decreased further after detection of the end of the grid event as in the previous example but, in any case, unlike in conventional prior art methods, it may not be increased either.

[0019] The present disclosure allows an independent regulation of both the power generated by the electrical generator and the power injected into the grid. Conventional methods comprise a single setpoint, which is typically used to control the power generated at the generator. No independent regulation of the power injected into the grid is provided by such conventional methods. On the contrary, in the present disclosure, individualized control is provided so that, upon detection of the end of the grid event, only the power injected into the grid is controlled and increased whereas the power generated by the generator is not decreased and, in some cases, continues to drop in a controlled manner, until a certain condition is satisfied. Overall, by providing individualized controls, a divergence, even including different trends, is allowed. Therefore, operational problems of the wind turbine, e.g. overspeed of the wind turbine rotor or overloading of the energy dissipating unit, are mitigated while fulfilling the increasingly strict grid integration requirements.

[0020] Furthermore, the method of the present disclosure can also be applied in wind turbines already in the field. Specifically, new or future grid code requirements may impose more strict requirements in the characteristics of the power injected into the grid during and after a grid event. Hence, a grid event may require a quick reduction in the amount of active power injected into the grid during the grid event itself. Upon clearing of the grid event, some grid codes may specify a fast ramp up of the power injected into the grid. However, other grid codes may impose certain constraints on the rate at which injected power is to be ramped up. In other words, a wind turbine may be required to recover injected power slowly after a grid event, e.g. a voltage dip or LV event. In these cases, a slow recovery on the power injected into the grid may result in an increased and prolonged imbalance between the power generated by the generator and the power that the wind turbine is allowed to inject into the grid. In other words, an increasing power surplus may originate in the power converter.

[0021] Accordingly, a conventional wind turbine, employing a standard energy dissipating unit, and operating under a conventional control strategy, i.e. a control strategy aimed at increasing generator power immediately after clearing of the grid event, may result in an overloading of the heat dissipating unit. On the contrary, based on the present aspect of the disclosure, a more efficient use of the energy dissipating unit is achieved.

[0022] In another aspect of the present disclosure, a method for operating a wind turbine connected to a grid during a low voltage event is provided. The wind turbine comprises agenerator and a power converter. The power converter includes a generator side converter, a grid side converter, a DC-link between the generator side converter and the grid side converter, and an energy dissipating unit arranged in the DC-link. The method comprises detecting a low voltage event and independently reducing a power generated by the generator and a power injected into the grid. Both powers are reduced at their respective maximum rates. The method also comprises detecting an end of the low voltage event. Upon detection of the end of the low voltage event, the power injected into the grid is increased with a first predetermined rate. Besides, the method comprises continuing reducing the power generated by the generator until a difference between the power generated by the generator and the power injected into the grid reaches a predetermined threshold. The method subsequentially comprises increasing the power generated by the generator with the same first predetermined rate in response to reaching the predetermined threshold.

[0023] According to this another aspect of the disclosure, an improved response in front of low voltage events is achieved. In particular, a very fast reduction in the power injected into the grid is required upon the detection of the low voltage event. To this end, not only injected power, but also generated power is reduced. Hence, the electrical generator and other components, e.g. the wind turbine blades, are controlled so as to reduce the input power into the wind turbine at the maximum rate.

[0024] However, due to the dynamics of the different systems, the maximum rate at which power generated in the generator can be reduced is less than the maximum rate at which the injected power into the grid is reduced. Furthermore, delays in the communication systems and controllers may also affect the reaction time to reduce the generated power. In any case, according to this aspect of the disclosure, not only the injected power but also the generated power is reduced with the maximum available rate, i.e. a rate not compromising the stability and integrity of the wind turbine. Such a fast reaction may be particularly relevant in cases comprising short and deep low voltage events, for which a significant imbalance can arise between the generated power and the injected power.

[0025] Moreover, as with the first aspect of the disclosure, in order to account for such a power imbalance, the power generated by the generator is not immediately increased upon clearing of the low voltage event as in the prior-art methods. On the contrary, the power coming from the generator is reduced further even if the low voltage fault is no longer present in the grid. In this manner, the different dynamics of the power injected into the grid and the power generated by the generator can be compensated and the excess generated energy is reduced.

[0026] Only after a difference between the generated power and the injected power reaches a certain threshold, is the generator power allowed to increase again during a recoveryphase. Furthermore, in order to maintain an optimum use of the energy dissipating unit, the power generated by the generator is increased with the same rate as the power injected into the grid. Consequently, once the threshold is reached, a constant and controlled imbalance is maintained. The threshold can then be predetermined such that this imbalance does not impose any significant burden on the energy dissipating unit.

[0027] In still another aspect of the present disclosure, a wind turbine is provided. The wind turbine comprises a generator and a power converter electrically connecting the generator to a grid. The power converter includes a generator side converter, a grid side converter, a Delink between the generator side converter and the grid side converter, and an energy dissipating unit arranged in the DC-link. The wind turbine also comprises a controller. The controller is configured to detect a grid event, and for determining first setpoints for reducing a power generated by the generator in response to detecting the grid event. The controller is also configured to determine second setpoints for reducing a power injected into the grid in response to detecting the grid event. Furthermore, the controller is configured for detecting an end of the grid event. Upon detection of the end of the grid event, the controller is configured for determining third setpoints for increasing the power injected into the grid with a first rate. The controller is also configured for determining a difference between the power generated by the generator and the power injected into the grid, and for detecting when such difference reaches a threshold. Then, the controller is further configured for determining fourth setpoints for increasing the power generated by the generator in response to detecting that the threshold is reached.

[0028] According to this aspect of the disclosure, a wind turbine with an improved capability to adapt to varying grid integration requirements is provided. In particular, the wind turbine is adapted to handle grid events, e.g. low voltage events, in such a manner that operational limits, e.g. structural loads on the wind turbine rotor or thermal load on an energy dissipating unit, are maintained while fulfilling grid code.

[0029] The wind turbine, according to this aspect of the disclosure, may either be a newly installed wind turbine or an already installed wind turbine with upgraded control functions. Hence, by implementing such a controller, even an already installed wind turbine may be able to cope with changing and more strict grid integration requirements without the need for any significant modifications in the already existing equipment. In particular, some grid codes may require a relatively slow recovery of the power injected into the grid after a grid event. In those cases, a significant power imbalance may build up in the power converter between the power generated by the generator and the power injected into the grid. A wind turbine, according to this aspect of the disclosure, is configured to contain the surplus energy arising from suchimbalance within certain limits, such that conventional equipment, e.g. energy dissipating units, can be employed.

[0030] Throughout this disclosure, a grid event is understood as a disturbance or abnormal condition on the electrical grid that can affect the stability, reliability, or the performance of connected equipment. In particular, a low voltage event, also known as a voltage sag or a voltage dip, is understood as a sudden and temporary reduction in grid voltage. This may be caused by, e.g. short circuits or equipment failures. The voltage in such low voltage events may be reduced to, e.g. 90% or lower percentage of the nominal voltage. In particular, in a low voltage event, a grid voltage may be reduced to 30%, 20%, 10% of the nominal voltage, and may even reach 0V, thus resulting in a zero voltage event. A duration of a low voltage event may be very short, i.e. in the range of milliseconds, but it can also reach a few seconds. Specific grid events may be defined in a grid code.

[0031] Grid codes typically prescribe that, in such conditions, the wind turbine must stay connected to the grid, i.e. it must ride through the low voltage event. The grid codes may also prescribe a quick reduction of the power injected into the grid during the grid event, as well as define a specific strategy for recovery after clearing of the grid event.BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Non-limiting examples of the present disclosure will be described in the following, with reference to the drawings, in whichFigure 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 the Figure 1 ;Figure 3 schematically illustrates the main electrical components, including a power converter, of a wind turbine according to an example;Figure 4 shows a flowchart of an example of a method for operating a wind turbine;Figures 5A - 5B schematically illustrate an evolution of the power generated by the generator of a wind turbine and the power injected into the grid according to an example of the disclosure (Fig. 5A), and to an example of the prior-art (Fig. 5B);Figures 6A - 6B show simulation results obtained when controlling a wind turbine with a method according to an example of the disclosure (Fig. 6B), and according to a method of the prior-art (Fig. 6B); andFigures 7 shows a flowchart of another example of a method for operating a wind turbine.DETAILED DESCRIPTION OF EXAMPLES

[0033] 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.

[0034] 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.

[0035] 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 a blade 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.

[0036] 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 91m. 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 readable medium (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.

[0041] 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 a 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 alongitudinal 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, and transformer voltages may for instance be between 30 kV and 70 kV. Said electrical energy is conducted via power cables from the nacelle 16 into the tower 15.

[0042] 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.

[0043] 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, main frame 52, and forward support bearing 60 and aft support bearing 62, are sometimes referred to as a drive train 64.

[0044] 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.

[0045] The nacelle 16 may also include a yaw system 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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 electrical 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 energy extracted 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.

[0050] 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 windturbine 10, the wind turbine controller 36 may control the pitch drive system 68 to adjust a pitch angle of rotor blades 22.

[0051] 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 electrical power loss event of the wind turbine 10. The electrical 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 electrical power loss event, the power generator 84 operates to provide electrical power to the pitch assembly 66 such that pitch assembly 66 can operate during the electrical power loss event.

[0052] 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.

[0053] 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.

[0054] Figure 3 schematically illustrates the main electrical components, including a power converter 61 , of a wind turbine 10 according to an example. The generator 42 of a corresponding wind turbine 10 is configured to convert mechanical power into electrical power having AC (alternating current) voltage and current. The generated power may exhibit variable frequency due to, e.g. varying wind conditions, and it may be fed to the power converter 61. Hence, the power converter 61 may be provided for adapting the power output from the generator 42 to the requirements of the electrical grid, e.g. to AC power having fixed frequency. The power converter 61 may comprise a generator side converter 63, a grid side converter 65, and a DC (direct current) link connecting the generator side converter 63 and the grid side converter 65. The DC-link may comprise a capacitor 67. The generator side converter 63 may be configured to convert or rectify the received AC voltage to DC voltage, the DC voltage then being delivered to the DC-link. The grid side converter 65 may be configured to convert the DC voltage from the DC-link into a fixed frequency AC voltage.

[0055] The generator 42 of the wind turbine 10 may be a permanent magnet generator comprising a generator rotor carrying a plurality of permanent magnets and a stator in some examples. The permanent magnet generator may be directly driven by the wind turbine rotor 18. The stator of the generator 42 may be connected to the generator side converter 63.

[0056] The grid side converter 65 may be connected to a conductor or to the grid 95 through a transformer 90. The transformer 90 may be configured to step-up the voltage delivered by the power converter 61 , e.g. to 33 kV. The transformer 90 may be installed within the nacelle 16 or the tower 15 of the wind turbine 10 in some examples.

[0057] Figure 4 shows a flowchart of a method 100 for operating a wind turbine 10 that is connected to a grid 95 during a grid event according to an example of the disclosure. As already shown in Figure s, the wind turbine 10 comprises a generator 42 and a power converter 61. The power converter 61 includes a generator side converter 63, a grid side converter 65, a DC-link between the generator side converter 63 and the grid side converter 65, and an energy dissipating unit 69 arranged in the DC-link. The method 100 comprises, in block 110, detecting the grid event. Subsequently, block 120 of the method comprises reducing a power generated by the generator 42 and a power injected into the grid 95 in response to detecting the grid event. Furthermore, block 130 comprises detecting an end of the grid event. Then, the power injected into the grid is increased with a first rate in block 140 in response to detecting the end of the grid event. The method 100 also comprises, in block 150, increasing the power generated by the generator 42 in response to a difference between the power generated by the generator 42 and the power injected into the grid 95 reaching a predetermined threshold.

[0058] The method 100 described in Figure 4 allows an improved response to grid events such as low voltage events. In particular, the method 100 comprises not increasing the power generated by the generator 42 immediately after the grid event has been already cleared. Although this may sound counterintuitive, inventors have found that such a delayed response exhibits beneficial effects in certain scenarios.

[0059] Specifically, in an example, the method 100 may comprise, upon detecting the end of the grid event in block 130, and while increasing the power injected into the grid in block 140, continue reducing the power generated by the generator 42 until a difference between the power generated by the generator 42 and the power injected into the grid 95 reaches the above-mentioned predetermined threshold. Specifically, continue reducing the power generated by the generator 42 comprises continue reducing with the same slope as the one used during the grid event.

[0060] By separately regulating the power from the generator 42 and the power injected into the grid 95, a more flexible method 100 is achieved. More particularly, the amount of surplus energy that needs to be dissipated by the energy dissipating unit 69 can be optimized. This is especially the case when dealing with grid codes demanding a relatively slow recovery in the power injected into the grid 95 after a grid event. Accordingly, the method 100 allows fulfilling grid integration requirements, e.g. imposing an upper limit on the ramp for active power injection into the grid 95 during a recovery phase after a grid event, while also satisfying the operational limits of the wind turbine 10.

[0061] In an example, reducing the power generated by the generator 42 in response to detecting the grid event in block 120, and increasing the power generated by the generator 42 in response to detecting reaching of the predetermined threshold in block 150, may comprise determining a generator power setpoint 631 (see Figure 3) for the generator side converter 63 and adjusting the generator 42 power according to the generator power setpoint 631.

[0062] According to this example, a first specific setpoint may be provided to individually control the generator 42. Such generator power setpoint 631 , and its evolution over time, may be optimized in accordance with existing conditions. In particular, the generator power setpoint 631 may be reduced during a grid event requiring a fast reduction in the active power of the wind turbine 10.

[0063] In examples of the disclosure, the power generated by the generator 42 may be controlled by the generator side converter 63 by controlling an electrical torque of the generator 42. Hence, either the generator power setpoint 631 may be specified in the form of a torque, or the generator side converter 63 may be configured to convert the received generator power setpoint 631 into a torque setpoint for the generator 42. In particular, the skilled person is well aware of the relationship between the power and the torque of a generator 42. Accordingly, any of the two magnitudes may be used interchangeably to control a generator 42 during the grid event.

[0064] Another example of the method 100 may be conceived in which reducing the power injected into the grid 95 in response to detecting the grid event in block 120, and increasing the power injected into the grid 95 in response to detecting the end of the grid event in block 140, may comprise determining a grid power setpoint 651 for the grid side converter 65 and adjusting the grid power according the grid power setpoint 651.

[0065] Similarly to the previous example, an improved control over the corresponding power may be provided in this case. Indeed, by determining explicit power setpoints, an improved capability of the wind turbine 10 to fulfill grid integration requirements may beachieved. Specifically, conventional control methods rely on a single power setpoint, which is typically provided to the generator 42. Accordingly, such conventional methods lack the flexibility of the method 100 of the present disclosure, which allows direct control of the active power injected into the grid 95 by controlling the grid side converter 65, i.e. by controlling the portion of the power converter 61 that is connected to the grid 95.

[0066] Furthermore, both mentioned examples can be combined in still another example. Hence, a variant of the method 100 may be envisaged in which a generator power setpoint 631 is used by the generator side converter 31 to control the generator 42, whereas an independently generated grid power setpoint 651 may be used so that the grid side converter 65 controls the power output from the wind turbine 10, i.e. the power injected into the grid 95.

[0067] This enhanced flexibility allows, for instance, an efficient implementation of the different behaviors observed for the power generated by the generator 42 and the power injected into the grid 95. Hence, upon detection of the end of the grid event in block 130, a power injected into the grid 95 is increased (block 140), whereas the power generated by the generator 42 is not increased until a predetermined threshold is reached in block 150. In particular, in an example, the power generated by the generator 42 keeps decreasing even after the end of the grid event is detected. Such divergent behavior is facilitated by the present example, in which explicit distinct setpoints 631 , 651 are determined for each of those two powers.

[0068] More particularly, the grid power setpoint 651 for the grid side converter 65 may be determined with a priority to comply with grid requirements during grid events, e.g. to comply with a requirement comprising a very quick reduction of the power injected into the grid 95. Thus, the control of the grid side converter 65, which is directly connected to the grid 95, may be particularly adequate for this purpose. On the other hand, a different setpoint, i.e. a generator power setpoint 631 , may be chosen for the generator side converter 63. As shown in Figure 3, the generator side converter 63 may be directly electrically connected to the generator 42 so that control of the generator 42 may be optimized. In this case, the generator power setpoint 631 may be selected by prioritizing the avoidance of any operational problems of the generator 42, the wind turbine rotor 18 (including the blades 22), or some other components such as the energy dissipating unit 69 of the power converter 61.

[0069] In any case, the values and evolutions of the two different setpoints 631 , 651 may be determined in response to the detection of the grid event in block 110 of the method 100. Accordingly, even if the injected grid power setpoint 651 may be identified as the one having a more direct influence on the performance of the wind turbine 10 with respect to the fulfillment of grid codes, the generator power setpoint 631 may also be determined on the basis of thedetected grid event so that the wind turbine 10 itself is capable of supporting the grid in an efficient manner, e.g. without overloading of any critical components of the wind turbine 10.

[0070] In particular, in some examples of the method 100, the grid event may comprise a low voltage event. A low voltage event may correspond to a disturbance in the grid 95 leading to a sudden and temporary drop in the value of the voltage with respect to a nominal voltage. In more extreme examples, the grid event may even comprise a zero voltage event, i.e. a disturbance in which the voltage in the grid 95 is completely collapsed and reduced to zero Volts.

[0071] During such low voltage events (including zero voltage events), the wind turbine 10 may be required to remain connected to the grid 95, i.e. a low voltage ride through (LVRT) or zero voltage ride through (ZVRT) may be requested. Furthermore, these events can typically exhibit a very short duration, so that a fast response is critical. For this reason, the power injected into the grid 95 by the wind turbine 10 may be required to ramp down at a very fast rate. In particular, such rate may be much faster than the rate at which generated power from the generator 42 can be reduced. Accordingly, the method 100 according to the present disclosure may facilitate riding through such events by separately controlling the power injected into the grid 95 and the power generated by the generator 42.

[0072] During the recovery phase of such a low voltage event, the injected power from the wind turbine 10 may be required to recover a value, e.g. a value present before the occurrence of the low voltage event. However, certain grid codes may prescribe a relatively slow rate for the recovery of the power injected into the grid 95. In that sense, a risk may exist that the power from the generator 42 remains at relatively high values with respect to the power injected into the grid 95 for a prolonged period of time, thus resulting in an energy surplus potentially exceeding the dissipation limits of the energy dissipating unit 69.

[0073] With a method 100 according to the present disclosure, the power generated by the generator 42 does not increase in response to detecting the end of the grid event. Actually, as already described, in an example such power decreases even further after the low voltage event has been cleared, thus reducing the amount of power that needs to be dissipated. In order to further describe the benefits of the present disclosure, Figures 5A and 5B schematically illustrate an evolution of the power generated by the generator 42 and the power injected into the grid 95 with a method according to an example of the present disclosure (Fig. 5A), and with a method according to the prior art (Fig. 5B).

[0074] As shown in both Figure 5A and Figure 5B, normal operation exists until time t1 , when a low voltage event occurs at the grid 95 as shown in the middle graph, which shows theevolution of the grid voltage 206, 206p. Hence, the grid voltage drops abruptly from a normal value Vo to a low voltage VdiPduring the low voltage dip. The value of the low voltage VdiPmay correspond to a percentage of the normal voltage Vo. Such a grid event results in the activation of a signal LVRT ON 207, 207p, which transitions from a low or 0 value to a high or 1 value. The evolution of the LVRT ON signal is depicted int the corresponding lower graphs. In some examples, the LVRT ON signal 207, 207p may be generated by a controller of the power converter 61. Upon the detection of a grid event, a grid event strategy is followed. Both the power injected into the grid 95 (as depicted in curves 202, 202p) and the power generated by the generator 42 (as depicted in curves 201 , 201 p) are reduced after the activation of the signal LVRT ON, i.e. after a time t1. Specifically, the power 202, 202p injected into the grid 95 is reduced with a very fast, ideally almost instantaneous, response. On the contrary, due to limited dynamic capabilities of the different wind turbine 10 components, the power 201 , 201 p generated by the generator 42 is reduced with a lower ramp rate.

[0075] As also shown in both Figure 5A and Figure 5B, the low voltage event has a limited duration. Indeed, the grid voltage is recovered at a time t2 or, in other words, the grid event has a duration of t2-t 1. Clearly different strategies are followed after clearing of the low voltage event, i.e. during the power recovery phase.

[0076] Hence, in the prior art method, the clearing of the low voltage event is handled by deactivating the LVRT ON signal 207p as soon as possible (see transition of LVRT ON signal from 1 to 0 at t2 in the lower graph of Figure 5B). Consequently, as shown in the upper graph of Figure 5B, the power 201 p generated by the generator 42 is recovered as quickly as possible. On the other hand, the power 202p injected into the grid 95 is recovered with a certain ramp-up rate 204.

[0077] A power surplus arises in the power converter 61. Indeed, as shown in the upper graph, a significant gap exists between the power 201 p generated by the generator 42 and the power 202p injected into the grid 95. Such a gap is present, not only during the low voltage event, i.e. between t1 and t2, but also during the recovery phase, i.e. until a time t4. The area 205p between the curves of the generator power 201 p and the power injected into the grid 202p over the duration of the complete grid event strategy, i.e. the duration of the low voltage event plus the duration of the recovery phase, corresponds to the amount of energy that needs to be dissipated in the energy dissipating unit 69 of the power converter 61.

[0078] A clearly different strategy is implemented in the example according to the present disclosure. Indeed, as shown in Figure 5A, the same behavior may be observed for the power injected into the grid 95 (see curve 202). Hence, the curve 202 may also ramp with a rate 204, which may be determined by the requirements of applicable grid codes. Accordingly, thecurves 202 and 202p may be substantially identical. In other words, the power injected into the grid 95 may exhibit the same behavior so that, from the point of view of the grid operator, equivalent performances may be obtained. Nevertheless, as shown in the lower graph of Figure 5A, the signal LVRT ON may be maintained in an activated state, i.e. a high or 1 level, even after an end of the grid event at t2 and until a later time t3. Accordingly, as clearly shown in the upper graph, the power 201 generated by the generator 42 may not increase even after the low voltage event has been cleared. Actually, in the example shown in Figure 5A, the power generated by the generator 42 may keep decreasing after the low voltage event is cleared at t2 as shown by the curve 206 in the middle graph.

[0079] Although this may seem counterintuitive, it is important to emphasize that the power injected into the grid 95 may remain substantially the same as in the prior art method. So, the power 201 generated by the generator 42 may decrease, or at least not increase, until a difference with the power injected 202 into the grid 95 reaches a certain predetermined threshold 203 at time t3. At this time, the LVRT ON signal in the lower graph may be deactivated, i.e. it may transition from H or 1 to L or 0, and the power 201 generated by the generator 42 may be increased.

[0080] As in the prior art, a gap may also exist between the power generated by the generator 42 and the power injected into the grid 95, i.e. between curves 201 and 202 in the upper graph. Such gap may be substantially equivalent to the one observed in the prior art during the duration of the low voltage event, i.e. from the time period between t1 and t2. Nevertheless, the gap observed during the recovery phase, i.e. after t2 and until t4, can be much lower when implementing a method according to the present disclosure, especially in examples comprising a continued reduction of the power generated by the generator 42 until the predetermined threshold 203 is reached. Consequently, an area 205 between the two curves 201 , 202 may be significantly smaller than the area between the equivalent curves 201 p, 202p in the prior art method. Bearing in mind that the area 205 represents the amount of energy that needs to be dissipated in the energy dissipating unit 69, a much more efficient performance can be observed for the implementation of Figure 5A, in which such an amount is significantly reduced.

[0081] Overall, a comparison between the behaviors observed in Figure 5A and Figure 5B shows that both methods may be capable of fulfilling the grid integration requirements, i.e. both methods may exhibit substantially the same behavior for the power injected into the grid 95 in curves 202, 202p. However, the method according to the present disclosure achieves such a result with significantly reduced energy dissipation needs in the energy dissipating unit 69 of the power converter 61. Furthermore, such lower dissipation needs is not achieved bychanging any of the components of the wind turbine 10, i.e. the generator 42 or the blades 22, but by allowing a somewhat delayed response of the power 201 generated by the generator 42.

[0082] As already discussed, a power surplus between the power generated by the generator 42 and the power injected into the grid 95 may be dissipated in the energy dissipating unit 69. Specifically, in an example, the energy dissipated in the energy dissipating unit 69 during the grid event and a subsequent recovery phase may be less than a predetermined percentage of a rated value for the energy dissipating unit 69. As an example, a percentage of 50% of the rated value may be predetermined. Hence, energy dissipating units 69 are typically designed so that they can dissipate a predetermined amount of energy. In particular, such rated value may correspond to an amount of energy to be dissipated while employing a method according to the prior art. Above such rated capability, overheating of the component of the heat dissipating unit 69 may prevent further energy dissipation.

[0083] According to the present disclosure, a more efficient use of the heat dissipating unit 69 is provided so that overloading of the same is prevented. Accordingly, during a duration of a grid event, and also during the recovery phase in which an imbalance still exists between the power generated by the generator 42 and the power injected into the grid 95, an energy dissipated in the energy dissipating unit 69 may monotonously increase.

[0084] A comparison of the evolution in the energy dissipated in the energy dissipation unit 69 is presented in Figures 6A and 6B for a method according to an example of the present disclosure (Fig. 6A) and a method of the prior art (Fig. 6B). In this case, a comparison between simulations carried out when using the two different control strategies are presented. The simulated event comprises a 3-phase fault with a zero voltage, i.e. a zero voltage event. Furthermore, the injected power recovery rate is set at 20% of the wind turbine rated power per second.

[0085] Similarly to the already discussed Figures 5A and 5B, a similar behavior is observed during the zero voltage event, i.e. while the voltage grid V, as shown in the lower graphs, exhibits a low value. Furthermore, as in the previous example, the power injected into the grid 95, as depicted in curves 302, 302p exhibits substantially the same behavior in both implementations. Nevertheless, the continued ramp down of the power generated by the generator 42 (see curve 301) in the implementation according to an example of the present disclosure (upper graph in Figure 5A) results in a much smaller area 305 in the new method than in the prior art (see area 305p between the curves 301 p and 302p).

[0086] The evolution of the energy dissipated in the energy dissipating unit 69 is depicted with curves 303, 303p in the corresponding middle graph. As shown by the values in the vertical axis, the energy consumed in the heat dissipating unit 69 when implementing a method according to an example of the present disclosure amount to only 16% approximately of the rated capability of the energy dissipating unit 69. On the contrary, a value in excess of 50% is shown for the method according to the prior art. It is important to note that, in both simulations, the same grid event was considered and the same recovery rate was applied to the power injected into the grid 95. In particular, a same recovery rate with a ramp up corresponding to a 20% of the wind turbine rated power per second was selected as shown in the curves 302, 302p.

[0087] Accordingly, a much gentler operation of the energy dissipating unit 69 can be obtained with the method according to the present disclosure. This exhibits different benefits. On the one hand, a more reliable and durable operation may be expected for the energy dissipating unit 69. On the other hand, the more efficient use may allow for an optimized design of the energy dissipating unit 69, thus leading to a reduced size of the components, i.e. resistors 692 and switches 691 (see Figure 3), of the energy dissipating unit 69. Consequently, direct cost savings may also arise.

[0088] In an example, the first rate 204, i.e. the rate for increasing the power injected into the grid 95 in response to detecting the end of the grid event may be a predetermined rate of between 10% and 100% of a wind turbine rated power per second, i.e. 10%-100% Prated / s. Specifically, the recovery may be configured between 10% and 20% of the wind turbine rated power per second. In other words, the slope indicated as 204 in Figure 5A may be adjustable between such ranges. Accordingly, improved capabilities may be achieved by adapting to the different requirements of different grid codes, e.g. in different jurisdictions. Furthermore, the configuration of the first rate 204 may also allow tuning to different operating conditions on either the wind turbine 10 or the grid 95, which may result in different requirements for the recovery phase after a grid event. In particular, the present disclosure may be especially suitable for relatively slow recovery rates as those indicated by the mentioned range.

[0089] Furthermore, in some examples, more than one rates may be defined during the recovery phase. Accordingly, a first rate 204 may be defined as shown in the figures for the initial stage after the grid event is cleared. A second different rate may be implemented after such initial stage. In other words, although curves 202 and 302 comprise a straight line with a single slope during the recovery phase, a curve comprising multiple segments with different slopes may also be envisaged for the recovery phase in other examples.

[0090] In an example of the disclosure, the power generated by the generator 42 may be reduced at a maximum rate in response to detecting the grid event. Hence, as schematically illustrated in Figure 5A, the curve 201 may be reduced as quickly as possible after detecting the grid event, i.e. after activation of the LVRT ON signal 207. Accordingly, by reducing the generated power in the generator 42, an easier reduction of the power injected into the grid 95 may be achieved. In particular, the requirements for the energy dissipating unit 69 may be constrained as illustrated by the comparison of Figures 5A and 5B, or by the comparison of Figures 6A and 6B.

[0091] The maximum rate may be selected such that the stability and integrity of the wind turbine 10 may not be compromised. Such fast reaction may be particularly relevant in cases comprising short and deep low voltage events, for which a significant imbalance may arise between the generated power and the injected power.

[0092] In a variant of this example, the maximum rate may be controlled in real time and it may be determined to avoid an overspeed of a rotor 18 of the wind turbine 10. Hence, in this variant, an improved performance of the wind turbine 10 may be achieved by adjusting the response to the operating conditions of the wind turbine 10, and / or to the environmental conditions. Hence, the wind turbine 10 may be operating in very different conditions at the moment a grid event occurs. Accordingly, by real-time monitoring on the operating conditions, a more reliable and optimized response may be provided.

[0093] On the contrary, in other variants, the power generated by the generator may be reduced at a predetermined rate between 100% and 200% of the wind turbine rated power per second, Prated / s. In these variants, a maximum rate may be predefined based on previous experience and / or simulations. In particular, worst case scenarios may be taken into account when selecting such rates. Although these variants may provide reduced flexibility or adaptability when compared with previously mentioned variants involving real-time adjustment, a simpler and more reliable implementation may be provided when using predetermined values.

[0094] In an example of the disclosure, increasing the power generated by the generator 42 in response to reaching the predetermined threshold 203 may comprise increasing the power generated by the generator 42 with the first rate used for increasing the power injected into the grid 95. Such an example is illustrated in Figures 5A and 6A. The use of the same first rate may provide increased stability and control during the recovery phase. In particular, a constant or stable imbalance between the power generated by the generator 42 (curves 201 , 301) and the power injected into the grid 95 (curves 202, 302) may be obtained during the ramp up, i.e. during the recovery phase. This constant difference may permit a more stablecontrol of the energy dissipating unit 69. Hence, as shown in Figure 6A, a constant and moderate slope can be obtained for the energy dissipated 303 in the energy dissipating unit 69 after t3, i.e. after reaching the predetermined threshold 203.

[0095] In an example of the disclosure, the threshold 203, i.e. the threshold between the power generated by the generator 42 and the power injected in the grid 95, may comprise a predefined value. A small threshold 203 may be selected to optimize the performance of the method. The value of this threshold may be selected depending on the electrical losses between the power generated by the generator 42 and the power injected in the grid 95 and also delays in the communication systems and controllers. Hence, as shown in Figure 5A, a small threshold 203 may be preferred to take full advantage of the disclosure, i.e. to reduce as much as possible the amount of energy to be dissipated in the energy dissipating unit 69. In particular, in a variant of this example, the threshold 203 is a value between 1%% and 25%, of the wind turbine rated power, specifically between 5% and 15% of the wind turbine rated power.

[0096] Figures 7 shows a flowchart of another example of a method 200 for operating a wind turbine 10 connected to a grid 95 during a low voltage event. The wind turbine comprises a generator 42 and a power converter 61 . The power converter 61 includes a generator side converter 63, a grid side converter 65, a DC-link between the generator side converter 63 and the grid side converter 65, and an energy dissipating unit 69 arranged in the DC-link. The method 200 comprises, in block 210, detecting a low voltage event. Block 220 of the method 200 comprises independently reducing a power generated by the generator 42 and a power injected into the grid 95. Both powers are reduced at their respective maximum rates. The method also comprises, in block 230, detecting an end of the low voltage event. Upon detection of the end of the low voltage event, block 240 comprises that the power injected into the grid 95 is increased with a first predetermined rate 204. Furthermore, the method 200 comprises, in block 250, continuing reducing the power generated by the generator 42 until a difference between the power generated by the generator 42 and the power injected into the grid 95 reaches a predetermined threshold 203. Subsequently, the method 200 comprises, in block 260, increasing the power generated by the generator 42 with the same first predetermined rate 204 in response to reaching the predetermined threshold 203.

[0097] The method 200 shown in Figure 7 provides an improved response in front of low voltage events. A fast reduction in the power injected into the grid 95 is desired upon the detection of the low voltage event. To this end, not only injected power, but also generated power, is reduced. Hence, the generator 42 and other components, e.g. the wind turbine blades 22, are controlled so as to reduce the input power into the wind turbine 10 at themaximum rate. The resulting fast reaction may be particularly relevant in cases comprising short and deep low voltage events, for which a significant imbalance can arise between the generated power and the injected power.

[0098] In order to maintain an optimum use of the energy dissipating unit 69, the power generated by the generator 42 is increased with the same first rate 204 as the power injected into the grid 95. Consequently, once the threshold 203 is reached, a constant and controlled imbalance is maintained. The threshold 203 can then be predetermined such that this imbalance does not impose any significant burden on the energy dissipating unit 69.

[0099] As already described with reference to Figure 3, a wind turbine 10 is also provided in the disclosure. The wind turbine 10 comprises a generator 42 and a power converter 61 electrically connecting the generator 42 to a grid 95. The power converter 61 includes a generator side converter 63, a grid side converter 65, a DC-link between the generator side converter 63 and the grid side converter 65, and an energy dissipating unit 69 arranged in the DC-link. The wind turbine 10 also comprises a controller 85. The controller 85 is configured to detect a grid event, and for determining first setpoints for reducing a power generated by the generator 42 in response to detecting the grid event. The controller 85 is also configured to determine second setpoints for reducing a power injected into the grid 95 in response to detecting the grid event. Furthermore, the controller 85 is configured for detecting an end of the grid event. Upon detection of the end of the grid event, the controller 85 is configured for determining third setpoints for increasing the power injected into the grid 95 with a first rate 204. The controller 85 is also configured for determining a difference between the power generated by the generator 42 and the power injected into the grid 95 and for detecting when such difference reaches a threshold 203. Then, the controller 85 is further configured for determining fourth setpoints for increasing the power generated by the generator 42 in response to detecting that the threshold 203 is reached

[0100] In an example, the controller 85 may be configured to send the first and the fourth setpoints to the generator side converter 63, and the generator side converter 63 may be configured to adjust a power generated by the generator 42 based on the first and fourth setpoints, specifically by adjusting a torque of the generator 42. The first and fourth setpoints are labelled as 631 in Figure 3.

[0101] Furthermore, in another example of the disclosure, the controller 85 may be configured to send the second and the third setpoints to the grid side converter 65. The grid side converter 65 may be configured to adjust the power injected into the grid 95 based on the second and third setpoints. Similarly to the previous case, the second and third setpoints are labelled as 651 in Figure 3.

[0102] As also indicated in Figure 3, the two previous examples may be combined. Hence, a first set of setpoints 631 may be sent from the controller 85 to the generator side converter 63 to control the generated power, i.e. the power generated by the generator 42, and another set of setpoints 651 may be sent to the grid side converter 65 to control the injected power, i.e. the power injected into the grid 95.

[0103] In some examples, the controller 85 may be configured to receive an indication from a grid operator. Furthermore, the controller 85 may be configured to monitor a state of the grid 95 and to detect a grid abnormality, such as low voltage event. In particular, the controller 85 may be configured to measure electrical variables, e.g. voltage, frequency, phase angle, at the grid 95. Based on such detection, the controller 85 may be configured to determine the setpoints 631 , 651. Specifically, the controller 85 may be configured to determine the setpoints 651 for the grid side converter 65 so as to control the power injected into the grid 95. Besides, in some examples, the controller 85 may be configured to receive data indicative of the operating and environmental conditions. Such data may be used to determine the setpoints 631 for the generator side converter 63 so as to modulate the power generated by the generator 42.

[0104] In an example, and as also shown in Figure 3, the energy dissipating unit 69 may comprise a resistive element 692 and a controllable switch 691. Besides, the energy dissipating unit 60 may be configured for dissipating a power surplus between a power generated by the generator 42 and a power injected into the grid 95 during the grid event. Specifically, the switch 691 may be controlled such that electrical current may be allowed to flow through the resistor 692, thus enabling the dissipation of surplus power.

[0105] The controller 85 depicted in Figure 3 is schematically represented as a single unit. However, in an example, the controller 85 may comprise a distributed controller 85 including at least a power converter controller (not shown) and a wind turbine controller 36. On the one hand, the power converter controller may be configured for detecting the grid event and for determining the second and third setpoints for controlling the power injected into the grid 95, i.e. for determining the injected power setpoints 651. On the other hand, the wind turbine controller 36 may be configured for determining the first and fourth setpoints for controlling the power generated by the generator 42.

[0106] By splitting the responsibilities between two separate control units, a more efficient and optimized response may be provided. In particular, the converter controller may be better suited to control the power injected into the grid 95, as this is basically flowing out of the grid side converter 65. On the other hand, the wind turbine controller 36 may be particularly suitable to control the power generated by the generator 42, as this may be influenced by the operationof actuators such as the pitch actuators, which may also be controlled with the same wind turbine controller 36.

[0107] 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 techniques in 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

27CLAIMS1. A method (100) for operating a wind turbine (10) connected to a grid (95) during a grid event, the wind turbine (10) comprising: a generator (42); and a power converter (61) including a generator side converter (63), a grid side converter (65), a DC-link between the generator side converter (63) and the grid side converter (65), and an energy dissipating unit (69) arranged in the DC-link; wherein the method (100) comprises: detecting the grid event; reducing a power generated by the generator (42) and reducing a power injected into the grid (95) in response to detecting the grid event; detecting an end of the grid event; increasing the power injected into the grid (95) with a first rate (204) in response to detecting the end of the grid event; and increasing the power generated by the generator (42) in response to a difference between the power generated by the generator (42) and the power injected into the grid (95) reaching a predetermined threshold (203).

2. The method of claim 1 , comprising, upon detecting the end of the grid event, continue reducing the power generated by the generator (42) until a difference between the power generated by the generator (42) and the power injected into the grid (95) reaches the predetermined threshold (203).

3. The method of any of claims 1 or 2, wherein reducing the power generated by the generator (42) in response to detecting the grid event, and increasing the power generated by the generator (42) in response to detecting reaching of the predetermined threshold (203), comprises determining a generator power setpoint (631) for the generator side converter (63) and adjusting the generator power according to the generator power setpoint (631).

4. The method of any previous claim, wherein reducing the power injected into the grid (95) in response to detecting the grid event, and increasing the power injected into the grid(95) in response to detecting the end of the grid event, comprises determining a grid power setpoint (651) for the grid side converter (65) and adjusting the grid power according the grid power setpoint (651).

5. The method of any previous claim, wherein the grid event is a low voltage event.

6. The method of any previous claim, wherein a power surplus between the power generated by the generator (42) and the power injected into the grid (95) is dissipated in the energy dissipating unit (69), and wherein an energy dissipated in the energy dissipating unit (69) during the grid event and a subsequent recovery phase is less than a predetermined percentage of a rated value for the energy dissipating unit (69).

7. The method of any previous claim, wherein the first rate (204) is a predetermined rate of between 10% and 100% of a wind turbine rated power per second.

8. The method of any previous claim, wherein the power generated by the generator (42) is reduced at a maximum rate in response to detecting the grid event, the maximum rate being controlled in real time and being determined to avoid an overspeed of a rotor (18) of the wind turbine (10).

9. The method of any of claims 1 to 7, wherein the power generated by the generator (42) is reduced at a predetermined rate between 100% and 200% of the wind turbine rated power per second.

10. The method of any previous claim, wherein increasing the power generated by the generator (42) in response to reaching the predetermined threshold (203) comprises increasing the power generated by the generator (42) with the first rate (204) used for increasing the power injected into the grid (95).

11. The method of any previous claim, wherein the predetermined threshold (203) is a value between 1% and 25% of the wind turbine rate power, specifically between 5% and 15% of the wind turbine rated power.

12. A wind turbine (10) comprising: a generator (42); a power converter (61) electrically connecting the generator (42) to a grid (95), the power converter (61) including a generator side converter (63), a grid side converter (65), a DC-link between the generator side converter (63) and the grid side converter (65), and an energy dissipating unit (69) arranged in the DC-link; and a controller (85), wherein the controller is configured to detect a grid event; determine first setpoints for reducing a power generated by the generator (42) in response to detecting the grid event; determine second setpoints for reducing a power injected into the grid (95) in response to detecting the grid event; detect an end of the grid event; determine third setpoints for increasing the power injected into the grid (95) with a first rate (204) in response to detecting the end of the grid event; determine a difference between the power generated by the generator (42) and the power injected into the grid (95) and detect when such difference reaches a threshold (203); determine fourth setpoints for increasing the power generated by the generator (42) in response to detecting that the threshold (203) is reached.

13. The wind turbine (10) of claim 12, wherein the controller (85) is configured to send the first and the fourth setpoints to the generator side converter (63), and the generator side converter (63) is configured to adjust a power generated by the generator (42) based on the first and fourth setpoints, specifically by adjusting a torque of the generator (42).

14. The wind turbine (10) of any of claims 12 or 13, wherein the controller (85) is configured to send the second and the third setpoints to the grid side converter (65), and the grid side converter (65) is configured to adjust the power injected into the grid (95) based on the second and third setpoints.

15. The wind turbine (10) of any of claims 12 to 14, the controller (85) comprising a distributed controller (85) including at least a power converter controller and a wind turbine controller (36), wherein the power converter controller is configured for detecting the grid event and for determining the second and third setpoints for controlling the power injected into the grid (95), and the wind turbine controller (36) is configured for determining the first and fourth setpoints for controlling the power generated by the generator (42).

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