Generator torque control after grid events

Abstract

Generator torque control after a grid event. A method for operating a wind turbine including a generator during a voltage sag is disclosed. The method includes detecting the end of the voltage sag; determining an acceleration of a rotor of the generator; and increasing the torque of the generator when the end of the voltage sag is detected according to a selected torque curve. The selected torque curve is selected from a plurality of predetermined torque curves, wherein the predetermined torque curves depict torque as a function of time. The selected torque curve is selected based on the determined acceleration of the rotor of the generator. A wind turbine configured for use with this method is also provided.

Description

Technical Field

[0001] The present disclosure relates to methods for operating a wind turbine generator after a grid event. More particularly, the present disclosure relates to methods for operating a wind turbine generator after a low voltage ride-through (LVRT) or zero voltage ride-through (ZVRT). The present disclosure also relates to wind turbines and generators configured for use in methods of recovering from LVRT or ZVRT. Background Art

[0002] Modern wind turbines are commonly used to supply electricity to the electrical grid. This type of wind turbine typically includes a tower and a rotor mounted on the tower. The rotor, typically consisting of a hub and a plurality of blades, rotates under the influence of the wind. This rotation generates torque that is typically transmitted to a generator via the rotor shaft, either directly ("direct drive") or through the use of a gearbox. The generator then generates electricity that can be supplied to the electrical grid.

[0003] The generator may be connected to the power grid via a power converter. Such a power converter may include a line-side converter connected to the grid, a machine-side converter connected to the rotor of the generator, and a DC-link between the line-side converter and the machine-side converter.

[0004] The power converter regulates the power output from the generator to the grid and can control the torque applied to the generator stator. During normal operation of a wind turbine, the active power generated by the generator is injected into the grid. The power that can be generated by the generator depends on the prevailing wind speed, but also on the torque applied to the generator stator. Wind turbine control typically depends on the prevailing wind speed, and the torque applied to the stator and the pitch angle of the blades are typically selected to maximize power generation and injection into the grid.

[0005] Grid codes define the requirements for all electrical machines (including wind turbines) connected to the electricity grid. Different grid codes apply to different grids in different countries.

[0006] Specifically, grid regulations may specify the ability of wind turbine generators to respond to events or anomalies on the power grid. One such grid event may be a voltage dip. A voltage dip (or "voltage sag") is a sudden decrease in the voltage of the power grid. During a voltage dip, the voltage may drop by 10%-100% below its nominal value. A voltage dip may last for up to several seconds.

[0007] Grid regulations may stipulate that wind turbines connected to the grid should not be disconnected from the grid and should not interrupt operation during such events. Instead, grid regulations may stipulate that wind turbines connected to the grid should be able to "ride through" such low-voltage or zero-voltage events. This behavior is sometimes described as low-voltage ride-through (LVRT) or zero-voltage ride-through (ZVRT). Specifically, the wind turbine may be required to rapidly reduce the power supplied to the grid. This is accomplished by reducing generator torque. This reduction in generator torque will result in a reduction in active power output. However, this will also increase the generator's rotational speed and may cause torsional oscillations in drive train components.

[0008] As used herein, a drive train can be considered to be all the components that operatively couple the wind turbine rotor to the generator. The drive train may include, for example, a low-speed shaft, a gearbox, and a high-speed shaft. The high-speed shaft exiting the gearbox may be connected to the rotor shaft of the generator via an elastic or flexible coupling. The coupling is typically designed to transmit torsional loads but limit the transmission of bending loads.

[0009] Grid regulations may also specify the behavior of wind turbines or minimum requirements for the behavior of wind turbines after a grid event has been resolved. For example, grid regulations may require that normal power generation be restored as quickly as possible after a grid event has been resolved.

[0010] The present disclosure relates to methods and systems designed to cope with such grid conditions and comply with grid regulations. Summary of the Invention

[0011] In one aspect, a method for operating a wind turbine including a generator during a voltage sag is provided. The method includes detecting the end of the voltage sag; determining an acceleration of a rotor of the generator; and increasing the torque of the generator when the end of the voltage sag is detected according to a selected torque profile. The selected torque profile is selected from a plurality of predetermined torque profiles, wherein the predetermined torque profiles depict torque as a function of time. The selected torque is selected based on the determined acceleration of the rotor of the generator.

[0012] According to this aspect, a method for operating a wind turbine to restore power production after a voltage sag is provided that can comply with strict grid regulations. Furthermore, the torque on drive train components can be kept below the limit at which the drive train components could be damaged. Peak generator speed and electrical power can also be reduced.

[0013] It has been found that the speed and timing of restoring active power after a fault has been cleared can significantly impact the torque levels observed in the main shaft of a wind turbine. Managing torque levels, as well as peaks in generator speed and electrical power, is critical to the integrity and life of the drive train components and to the ability of the wind turbine to ride through an LVRT event without tripping.

[0014] By selecting a suitable torque curve, torque peaks in the drive train can be avoided. Such peaks have been shown to damage, for example, the flexible coupling between the high-speed shaft and the generator rotor shaft. They can also be caused by applying high torque at the wrong moment.

[0015] Furthermore, according to this aspect, there is no need to wait for a specific instant to apply torque. Therefore, restoration of power generation is not delayed, and grid code requirements in this regard can be complied with. Certain grid codes limit the ability to independently determine the timing at which active power is restored, as the required power restoration time may necessitate generator torque restoration starting as soon as the LVRT or ZVRT event ends.

[0016] Finally, by providing a plurality of predetermined torque curves that can be tied to specific conditions of the generator rotor, there is no need to determine a torque curve in response to every possible grid event.

[0017] In another aspect, a wind turbine is provided. The wind turbine includes a wind turbine rotor having a plurality of blades and a generator including a generator rotor and a generator stator, wherein the generator rotor is operably coupled to the wind turbine rotor. The wind turbine also includes a controller for controlling the torque of the generator. The controller is configured to: determine the end of a voltage sag on a power grid connected to the generator; determine an acceleration of the generator rotor at the end of the voltage sag; and apply a torque to the generator according to a predetermined torque curve selected from a plurality of torque curves. The predetermined torque curve is selected based on the determined acceleration of the generator rotor at the end of the voltage sag.

[0018] In yet another aspect, a method of operating a wind turbine including a generator connected to a power grid is provided. The method includes determining a voltage event on the power grid and reducing torque of the generator during the voltage event. The method also includes determining restoration of the power grid after the voltage event and increasing the torque of the generator substantially to a level prior to the voltage event. The torque may be increased at a first substantially constant torque rate or at a second substantially constant torque rate, wherein the first torque rate or the second torque rate is selected based on acceleration of a rotor of the generator at the moment of restoration.

[0019] The term "torque rate," as used throughout this disclosure, indicates the speed at which torque increases. The rate or "ramp" of torque may be defined as the first time derivative of a torque curve depicting torque as a function of time.

[0020] The present invention provides a set of technical solutions.

[0021] Technical Solution 1. A method of operating a wind turbine (1) comprising a generator (10) in the event of a voltage sag, the method comprising:

[0022] detecting an end of the voltage sag;

[0023] determining an acceleration of a rotor (16) of the generator (10);

[0024] When the end of the voltage dip is detected according to the selected torque curve, the torque of the generator (10) is increased, wherein

[0025] The selected torque curve is selected from a plurality of predetermined torque curves, wherein the predetermined torque curves depict torque as a function of time; and wherein

[0026] The selected torque curve is selected based on the determined acceleration of the rotor (16) of the generator (10).

[0027] Technical Solution 2. The method according to Technical Solution 1, wherein the plurality of predetermined torque curves includes only two predetermined torque curves.

[0028] Technical Solution 3. The method according to Technical Solution 1 or 2, wherein the predetermined torque curve includes a first torque curve having a first torque recovery rate and a second torque curve having a second torque recovery rate, wherein the first torque recovery rate is higher than the second torque recovery rate.

[0029] Technical Solution 4. The method according to Technical Solution 3, wherein the first torque curve is selected when the acceleration of the rotor of the generator is higher than the acceleration threshold.

[0030] Technical Solution 5. The method according to Technical Solution 4, wherein the second torque curve is selected when the acceleration of the rotor of the generator is lower than the threshold value of the acceleration.

[0031] Technical Solution 6. The method according to Technical Solution 5, wherein the threshold is zero.

[0032] Technical Solution 7. A method according to any one of Technical Solutions 1-6, wherein when the voltage sag occurs, the generator has a first power output level, and wherein the first torque recovery rate and the second torque recovery rate are substantially constant until the power output of the generator reaches the first power level.

[0033] Technical Solution 8. The method according to any one of Technical Solutions 1 to 7, wherein determining the acceleration of the rotor of the generator of the wind turbine comprises measuring a rotational speed of the rotor.

[0034] Technical Solution 9. A wind turbine (1), comprising:

[0035] A wind turbine rotor (5), the wind turbine rotor (5) having a plurality of blades (7);

[0036] a generator (10), the generator (10) comprising a generator rotor (16) and a generator stator (18), wherein the generator rotor (16) is operatively coupled to the wind turbine rotor (5);

[0037] A controller (90) for controlling the torque of the generator, wherein the controller is further configured to:

[0038] determining the end of a voltage dip on a grid (80) connected to the generator (10);

[0039] determining an acceleration of the generator rotor (16) at the end of the voltage dip; and

[0040] A torque is applied to the generator (10) according to a predetermined torque curve selected from a plurality of torque curves, wherein

[0041] The predetermined torque curve is selected based on a determined acceleration of the generator rotor (16) at the end of the voltage dip.

[0042] Technical Solution 10. The wind turbine according to Technical Solution 9 further includes a gearbox (9) operably connecting the low-speed shaft (8) to the high-speed shaft (17), wherein the high-speed shaft (17) is connected to the shaft (11) of the generator rotor via a flexible coupling.

[0043] Technical Solution 11. The wind turbine according to Technical Solution 10, wherein the plurality of torque curves are such that the load on the flexible coupling does not exceed a threshold load.

[0044] Technical Solution 12. The wind turbine according to any one of Technical Solutions 9 to 11 further includes a power converter (60), wherein the power converter (60) is configured to apply torque to the generator (10) based on a signal received from the controller (90).

[0045] Technical Solution 13. A wind turbine according to any one of Technical Solutions 9-12, wherein the predetermined torque curve includes a first torque curve having a first torque recovery rate and a second torque curve having a second torque recovery rate, wherein the first torque recovery rate is higher than the second torque recovery rate.

[0046] Technical Solution 14. A wind turbine according to Technical Solution 13, wherein the controller is configured to select the first torque curve when the acceleration of the generator rotor (16) is positive, and is configured to select the second torque curve when the acceleration of the generator rotor (16) is negative.

[0047] Technical Solution 15. A wind turbine according to Technical Solution 13 or 14, wherein when the voltage sag occurs, the generator (10) has a first power output level, and wherein the first torque recovery rate and the second torque recovery rate are substantially constant until the power output of the generator reaches the first power level. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Non-limiting examples of the present disclosure will now be described with reference to the accompanying drawings, in which:

[0049] Figure 1 shows a perspective view of a wind turbine according to one example of the present disclosure;

[0050] Figure 2 shows a simplified interior view of a nacelle of a wind turbine according to one example of the present disclosure;

[0051] Figure 3 An example of a wind turbine is schematically shown;

[0052] Figure 4 Schematically illustrates a method for power recovery after a voltage dip applying two different torque curves;

[0053] Figure 5 schematically illustrates the maximum torque on the low-speed shaft of a wind turbine after a voltage sag when applying different torque curves as a function of the duration of the voltage sag;

[0054] Figure 6schematically illustrates the time until power recovery after a voltage sag when different torque curves are applied as a function of the duration of the voltage sag;

[0055] Figure 7 and Figure 8 schematically illustrates the acceleration of the generator rotor and the torque load on the low speed shaft during and after a voltage dip applying two different torque curves; and

[0056] Figure 9 and Figure 10 Schematically illustrated are the acceleration of the generator rotor and the torque load on the low-speed shaft during and after a voltage dip applying two different torque curves. DETAILED DESCRIPTION

[0057] In these figures, the same reference numerals have been used to designate matching elements.

[0058] Figure 1 A perspective view of one example of a wind turbine 1 is shown. As shown, the wind turbine 1 includes a tower 2 extending from a support surface 3, a nacelle 4 mounted on the tower 2, and a rotor 5 coupled to the nacelle 4 at a front region. The rotor 5 includes a rotatable hub 6 and at least one rotor blade 7 coupled to the hub 6 and extending outwardly from the hub 6. For example, in the example shown, the rotor 5 includes three rotor blades 7. However, in alternative embodiments, the rotor 5 may include more or less than three rotor blades 7. Each rotor blade 7 may be spaced apart from the hub 6 to facilitate rotating the rotor 5 so that kinetic energy can be converted from the wind into usable mechanical energy and subsequently into electrical energy. For example, the hub 6 may be rotatably coupled to a generator 10 ( Figure 2 ), to allow the generation of electrical energy. The rotation of the rotor may be transmitted directly to a generator, for example in a direct drive wind turbine or by using a gearbox.

[0059] Figure 2 Shown Figure 11 is a simplified interior view of an example of a nacelle 4 of a wind turbine 1. As shown, a generator 10 can be deployed within the nacelle 4. Generally, the generator 10 can be coupled to the rotor 5 of the wind turbine 1 for generating electrical power based on the rotational energy generated by the rotor 5. For example, the rotor 5 can include a main rotor shaft 8 that is coupled to the hub 6 for rotation therewith. The generator 10 can then be coupled to the rotor shaft 8 such that rotation of the rotor shaft 8 drives the generator 10. For example, in the embodiment shown, the generator 10 includes a generator shaft 11 that is rotatably coupled to the rotor shaft 8 via a gearbox 9. In an alternative example, the hub can be directly coupled to the rotor of the generator, and rotation of the hub can thereby drive the rotor of the generator.

[0060] The generator 10 may be electrically coupled to a converter. The wind turbine converter may adapt the output electrical power of the generator to the requirements of the power grid.

[0061] It will be appreciated that the rotor shaft 8 , gearbox 9 , and generator 10 may typically be supported within the nacelle 4 by a bedplate or support frame 12 placed atop the wind turbine tower 2 .

[0062] The nacelle 4 is rotatably coupled to the tower 2 via a yaw system 20. The yaw system includes a yaw bearing (in Figure 2 (not visible in the figure), the yaw bearing has two bearing assemblies configured to rotate relative to the other. Tower 2 is coupled to one of the bearing assemblies, and the bedplate or support frame 12 of nacelle 4 is coupled to the other bearing assembly. A yaw system 20 includes a ring gear 21 and a plurality of yaw drives 22 having a motor 23, a gearbox 24, and pinions 25 for meshing with the ring gear to rotate one of the bearing assemblies relative to the other.

[0063] The nacelle 4 also includes a cover structure 50 for housing wind turbine components. In this example, the wind turbine components housed in or surrounded by the cover structure 50 include a generator 10, a converter, a gearbox 9, and a shaft 8. In other examples, the wind turbine components arranged in the nacelle may refer to a converter and a generator.

[0064] Figure 3 A method of operating a wind turbine and a power converter according to an example of the prior art is schematically shown. Figure 3 In the example of FIG, a wind turbine includes a generator 10 having a generator rotor 16 and a generator stator 18. The rotor 5 drives a low-speed shaft 8. The slow rotation of the shaft 8 is converted to a high-speed rotation of a high-speed shaft 17 by a gearbox 9. The high-speed shaft 17 can be connected to the shaft 11 of the generator rotor 16 by a flexible coupling.

[0065] Generator stator 18 is directly connected to power grid 80 and supplies electrical power to the grid according to the grid frequency. Power converter 60 electrically connects the windings on the generator rotor to power grid 80. In this particular example, rotor 16 of generator 10 is connected to machine-side converter 62. The machine-side converter is connected to line-side converter 66 via DC link 64. The generator in this particular example is a doubly-fed induction generator (DFIG). Other generator topologies and converter configurations may be used within the scope of the present disclosure.

[0066] According to this example, power converter 60 can receive a setpoint 92 from a wind turbine controller 90 to adapt the generator torque. Setpoint 92 can be based on optimal operation of the wind turbine according to prevailing meteorological conditions. By controlling the generator torque, the rotational speed of the generator can be controlled. The rotational speed of the generator, in turn, determines the rotational speed of the wind turbine rotor 5. The rotational speed can be selected according to a predefined operation schedule. In particular, it is known to control a wind turbine differently in different wind speed ranges. In wind speed ranges lower than the nominal wind speed, the rotational speed can be selected so that the wind impinges on the rotor blades at an optimal angle of attack. This operating method can be maintained until the maximum rotational speed is reached.

[0067] At higher wind speeds, and particularly above nominal wind speed, the rotational speed can be controlled to remain constant. Maximum torque can be applied to the stator, and the blades can be pitched to ensure constant rotational speed. Variations on this optimized operation are possible.

[0068] According to predefined operation, the torque signal 92 can be sent by the wind turbine controller to the machine-side converter. The resultant active power output from the stator 18 is fed into the grid. The wind turbine controller 90 can be a local wind turbine controller or, for example, a wind farm controller.

[0069] A voltage event as described herein may be considered to be an event in which the voltage level on the power grid deviates from a normal level. A voltage sag is an example of such an event.

[0070] A voltage sag or voltage dip is a short-duration decrease in voltage on an electrical grid. Throughout this disclosure, the terms voltage sag and voltage dip are used interchangeably. During such a sag, the voltage level on the grid can drop to less than 90% of its nominal value and can reach, for example, 30%, 20%, or 10% of its nominal value. During such a sag, the voltage level can even reach zero. A voltage sag can last from less than one second to several seconds.

[0071] Today, many grid regulations require that wind turbines connected to the grid remain connected during such voltage dips and that power generation begin within a very short time frame or immediately after the voltage dip has ended.

[0072] Controller 90 can determine various electrical parameters of the grid, including, for example, voltage, phase angle, and frequency. Controller 90 can measure these variables or receive parameter values ​​from, for example, a higher-level controller. For example, controller 90 can be a local wind turbine controller, and it can receive values ​​from a wind farm controller.

[0073] When controller 90 determines the presence of a voltage sag, it can adjust control signals 92 to the converter. This can reduce the torque on the rotor, and as a result, the generator rotor speed can increase. The rotational speed of all drivetrain components can therefore increase. Furthermore, torsional oscillations can occur in all drivetrain components and the generator.

[0074] Controller 90 may determine that the voltage sag on the grid has ended and send appropriate control signals to increase generator torque to increase power production. Figure 4 Two different torque curves are shown schematically. A torque curve can be understood as a function that depicts the generator torque as a function of time. In one case, the torque curve is steep, with a fast recovery rate. In the other case, the torque curve is less steep, with a slower recovery rate.

[0075] It has been found that, depending on the voltage sag environment and the characteristics of the wind turbine, applying high torque rates can result in high torsional loads that can damage the flexible coupling between the high-speed shaft 17 and the generator rotor shaft 11. In addition, when applying high torque rates, the instantaneous maximum generator speed and maximum electrical power can be very high.

[0076] On the other hand, a slow restoration rate may not comply with grid regulations because restoration of power generation may take too long.

[0077] Figure 5 The maximum torque on the low-speed shaft of a wind turbine is schematically shown as a function of the duration of a voltage sag for four different torque curves. Each of the torque curves has a constant but different torque rate. In all cases, a wind speed of 13 m / s and a voltage sag of 20% of nominal are assumed (i.e., the voltage level reached during the sag on the power grid is approximately 20% of its nominal value).

[0078] Figure 6 The time to recovery to "normal" power output is schematically shown for the same four different torque curves as a function of the duration of the voltage sag.

[0079] Paradoxically, depending on the characteristics of the voltage sag, Figure 5 It can be seen that for the torque curve with the highest torque rate, the highest torque load does not necessarily occur. Figure 6 It can also be seen in that for a given torque rate, the power recovery may vary depending on the duration of the voltage sag.

[0080] The inventors have discovered that an appropriate torque curve can be selected based on the acceleration of the generator rotor at the end of a voltage sag. Accordingly, a method for operating a wind turbine including a generator during a voltage sag is provided. The method comprises: detecting the end of the voltage sag; determining the acceleration of the generator rotor; and increasing the torque of the generator when the end of the voltage sag is detected according to a selected torque curve. The selected torque curve is selected from a plurality of predetermined torque curves, wherein the predetermined torque curves depict torque as a function of time. The selected torque is selected based on the determined acceleration of the generator rotor.

[0081] The operating conditions when applying torque are relevant to the selection of the most suitable curve. Choosing the most suitable curve means that recovery can start immediately or as soon as possible after the voltage dip has ended. Torque may increase immediately after recovery.

[0082] In some examples, the plurality of predetermined torque curves includes only two predetermined torque curves. In other examples, three or more torque curves may be used.

[0083] In some examples, the predetermined torque curve includes a first torque curve having a first torque restoration rate and a second torque curve having a second torque restoration rate, wherein the first torque restoration rate is higher than the second torque restoration rate.

[0084] In some examples, the first torque rate may be greater than 10 power units per second, and the second torque rate may be less than 5 power units per second.

[0085] The term "power unit" refers to the amount of power to be restored after a grid event and can therefore be defined as the power generated before the grid event minus the power generated at the end of the grid event. As used herein, the term "power units per second" indicates the rate at which power is restored, and therefore indirectly indicates the torque rate. One power unit per second means that the generated power will reach its pre-grid event level within one second.

[0086] exist Figure 7-10In the example of FIG, a first torque curve having a torque rate of 20 power units per second is provided. This means that the power level before the voltage sag is reached within 0.05 seconds. A second torque curve having a torque rate of 1.5 power units per second is provided.

[0087] In these examples, the first torque restoration rate and the second torque restoration rate may be substantially constant until the power output of the generator reaches the first power level before the voltage sag occurs.

[0088] In some examples, a first torque curve may be selected when the rotor's acceleration is above a threshold, and a second torque curve may be selected when the rotor's acceleration is below the same threshold. For example, the threshold may be zero. The first torque curve may be selected when the rotor's acceleration is positive. Positive acceleration may be understood as acceleration in the direction of rotation during normal operation. Negative acceleration may be understood as acceleration in the opposite direction. The second torque curve may be selected when the rotor's acceleration is negative.

[0089] Note that the threshold for selecting the first torque rate or the second torque curve does not need to be zero, and may be set to be (slightly) positive or (slightly) negative.

[0090] exist Figure 7 and Figure 8 In FIG, , the voltage dip ends when the acceleration is positive. Applying the first torque curve with a high torque rate results in a lower torque load on the drive train than applying the second torque curve.

[0091] In some examples, determining the acceleration of a rotor in a wind turbine's generator can include measuring the rotor's rotational speed. The rotor's acceleration can be readily derived from the speed. The rotor's acceleration can be determined relatively easily using sensors already standard on wind turbines. The generator's acceleration is also a reliable and valid parameter, independent of the theoretical behavior of the drive train.

[0092] The torque curve is chosen so that the high speed shaft from the gearbox (in the case of this wind turbine configuration, see Figure 3 ) and the generator rotor shaft does not exceed the load threshold.

[0093] exist Figure 9 and Figure 10 In the figure, it can be seen that the torque curve that results in a lower torque load on the drive train when acceleration is negative is the second torque curve with a lower torque rate. By selecting this curve under these circumstances, the load can be controlled again, and the rotor speed can also be kept below dangerous levels. A torque curve suitable for these circumstances can be applied immediately after the voltage dip has ended.

[0094] This written description uses examples to disclose the invention, including preferred embodiments, and also to enable a person skilled in the art to practice the invention, including making and using any devices or systems and performing any combined methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to fall 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 that do not differ substantially from the literal language of the claims. Aspects from the various described embodiments, as well as other known equivalents of each such aspect, can be mixed and matched by a person of ordinary skill in the art to construct additional embodiments and techniques according to the principles of the present application. If reference marks relating to the drawings are placed in parentheses in the claims, they are used solely to attempt to increase the intelligibility of the claims and should not be construed as limiting the scope of the claims.

Claims

1. A method of operating a wind turbine (1) comprising a generator (10) in the event of a voltage dip, the method comprising: detecting an end of the voltage sag; determining an acceleration of a rotor (16) of the generator (10); When the end of the voltage dip is detected according to the selected torque curve, the torque of the generator (10) is increased, wherein The selected torque curve is selected from a plurality of predetermined torque curves, wherein the predetermined torque curves depict torque as a function of time; and wherein The selected torque curve is selected based on a determined acceleration of the rotor (16) of the generator (10). 2 . The method of claim 1 , wherein the plurality of predetermined torque curves comprises only two predetermined torque curves.

3. The method of claim 1 , wherein the predetermined torque curve comprises a first torque curve having a first torque restoration rate and a second torque curve having a second torque restoration rate, wherein the first torque restoration rate is higher than the second torque restoration rate, and wherein the torque restoration rate indicates a speed at which torque increases.

4. The method of claim 2, wherein the predetermined torque curve includes a first torque curve having a first torque restoration rate and a second torque curve having a second torque restoration rate, wherein the first torque restoration rate is higher than the second torque restoration rate, and wherein the torque restoration rate indicates a speed at which torque increases. 5 . The method of claim 3 , wherein the first torque curve is selected when the acceleration of the rotor of the generator is above a threshold value of the acceleration. 6 . The method of claim 5 , wherein the second torque curve is selected when the acceleration of the rotor of the generator is below the threshold value of the acceleration. The method of claim 6 , wherein the threshold is zero.

8. The method of any one of claims 3-7, wherein the generator has a first power output level when the voltage sag occurs, and wherein the first torque recovery rate and the second torque recovery rate are substantially constant until the power output of the generator reaches the first power level.

9. The method according to any of claims 1-7, wherein determining the acceleration of the rotor of the generator of the wind turbine comprises measuring a rotational speed of the rotor.

10. A wind turbine (1) comprising: A wind turbine rotor (5) having a plurality of blades (7); a generator (10) comprising a generator rotor (16) and a generator stator (18), wherein the generator rotor (16) is operatively coupled to the wind turbine rotor (5); A controller (90) for controlling the torque of the generator, wherein the controller is further configured to: determining the end of a voltage dip on a power grid (80) connected to the generator (10); determining an acceleration of the generator rotor (16) at the end of the voltage dip; as well as Torque is applied to the generator (10) according to a predetermined torque curve selected from a plurality of torque curves, wherein the predetermined torque curve is selected based on the determined acceleration of the generator rotor (16) at the end of the voltage dip, wherein the predetermined torque curve depicts torque as a function of time.

11. The wind turbine of claim 10, further comprising a gearbox (9) operatively connecting the low speed shaft (8) to a high speed shaft (17), wherein the high speed shaft (17) is connected to the shaft (11) of the generator rotor via a flexible coupling.

12. The wind turbine of claim 11, wherein the plurality of torque curves are such that the load on the flexible coupling does not exceed a threshold load.

13. The wind turbine according to any one of claims 10-12, further comprising a power converter (60) configured to apply torque to the generator (10) based on a signal received from the controller (90).

14. The wind turbine according to any one of claims 10 to 12, wherein the predetermined torque curve comprises a first torque curve having a first torque restoration rate and a second torque curve having a second torque restoration rate, wherein the first torque restoration rate is higher than the second torque restoration rate, and wherein the torque restoration rate indicates a speed at which torque increases.

15. The wind turbine of claim 14, wherein the controller is configured to select the first torque curve when the acceleration of the generator rotor (16) is positive, and to select the second torque curve when the acceleration of the generator rotor (16) is negative.

16. The wind turbine of claim 14, wherein the generator (10) has a first power output level when the voltage sag occurs, and wherein the first torque recovery rate and the second torque recovery rate are substantially constant until the power output of the generator reaches the first power level.

17. The wind turbine of claim 15, wherein the generator (10) has a first power output level when the voltage sag occurs, and wherein the first torque recovery rate and the second torque recovery rate are substantially constant until the power output of the generator reaches the first power level.

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