Controlling wind turbine during low voltage event

Abstract

There is described a method of controlling an asynchronous generator system (3) connected to a utility grid (47), in particular an asynchronous generator system (3) of a wind turbine (1), the method comprising setting a rotational speed reference (n) of the asynchronous generator system (3) depending on a utility grid voltage (Ug).

Description

Technical Field

[0001] The present invention relates to a method and a controller for controlling a doubly-fed induction generator system connected to a utility grid, in particular a doubly-fed induction generator system of a wind turbine. Furthermore, the present invention relates to a wind turbine comprising a controller. Background Art

[0002] A plurality of wind turbines or wind farms and a plurality of other power production facilities supply electricity to a utility grid. A plurality of consumers receive electrical power from the utility grid. The utility grid may be designed to operate at a specific nominal voltage and / or a specific nominal frequency. Depending on the balance of supply and extraction of active / reactive power from the utility grid, grid stability may be compromised because the nominal grid values ​​are not met.

[0003] Given the high introduction of wind power into electrical systems, grid operators worldwide are developing rigid regulations in an attempt to ensure grid stability.

[0004] One of the key requirements specified in grid connection codes around the world is related to the grid voltage range for operation of wind turbines, fixing the necessity to remain connected to the grid when voltage excursions of up to ±10-12% (from the nominal voltage value) occur, as well as other power control needs during those events.

[0005] Undervoltage (grid voltage below nominal grid voltage) causes higher currents on the stator windings of the induction generator, increasing losses and therefore component temperatures, and creating the necessity to overspecify the generator to support such events, since conventionally during those events the wind turbine control actually maintains a reference speed.

[0006] Conventionally, wind turbine components may have been oversized to be able to operate at currents higher than the nominal current. Thus, conventionally, components may have been overspecified to be able to meet the higher current requirements observed during an undervoltage event. Thus, conventionally, costs have been increased.

[0007] Other conventional approaches for solving the above mentioned problems may bear the risk of overheating; in conventional approaches, in the case of those extreme scenarios, the impact on wind turbine availability or component damage may persist for a long period of time.

[0008] Therefore, there may be a need for a method and controller for controlling a doubly fed induction generator (DFIG) system, or an induction generator system connected to a utility grid, wherein oversizing of components may be mitigated or even avoided, and wherein component loading is mitigated and safe and reliable operation is provided, particularly in undervoltage events.

[0009] Furthermore, there may be a need for a method and controller that can avoid costly oversizing of components primarily with respect to induction generators.In addition, wind turbine power derating due to overheating of components during timely maintenance voltage excursions can be addressed. Summary of the invention

[0010] This need is met by the subject matter according to the independent claim. Advantageous embodiments of the invention are described by the dependent claims.

[0011] According to one embodiment of the present invention, a method for controlling a doubly fed induction generator (DFIG) system (induction generator system) connected to a utility grid (particularly a doubly fed induction generator (DFIG) system of a wind turbine) is provided, the method comprising: setting a rotation speed reference of the asynchronous generator system depending on the utility grid voltage.

[0012] The method may be performed, for example, by a controller of an induction generator, which controller may be part of a wind turbine controller, for example.The method may be implemented in software and / or hardware.

[0013] Induction generator is used as a synonym for asynchronous generator.

[0014] An induction generator is an asynchronous generator that generates an alternating current when its rotor shaft rotates relative to the stator windings. The rotor needs to rotate faster than the synchronous speed in order to output a rotor power with a desired frequency. The so-called synchronous speed corresponds to the frequency of the AC power to be supplied to the utility grid. In order to operate the induction generator, an excitation signal or excitation power (also called a control signal) needs to be provided to the rotor windings. The relative difference between the synchronous speed and the operating speed is called slip, which is usually expressed as a percentage of the synchronous speed. When used in a wind turbine, the secondary shaft of the wind turbine, for example, coupled to the main shaft of the turbine (where a plurality of rotor blades are connected) via a gearbox, drives the rotor of the induction generator to rotate at a high speed above the synchronous speed. Power is generated in the stator windings as well as in the rotor windings.

[0015] Induction generators can be implemented in different ways. An example of an induction generator (or asynchronous generator) is a doubly fed machine. A doubly fed machine has two sets of windings (e.g. three-phase windings), one set is stationary (i.e. the stator windings) and one set is rotating (i.e. the rotor windings). The stator windings can be directly connected to the output terminals of the generator. The rotor windings (which are also traditionally called field windings) can be connected to a converter that provides AC-power at a variable frequency that can be adjusted as needed.

[0016] A converter may be connected between the output of the rotor winding and the stator winding to convert AC to DC and DC to AC. The converter may be bidirectional and may thereby enable power to be transferred in both directions. Thus, the converter may be used to actually supply control signals (or excitation signals) to the rotor winding, but simultaneously or concurrently also receive rotor power from the rotor winding, convert it to the desired grid frequency and output this rotor power to the utility grid. Because the output of the converter may be connected to the output of the stator winding, in addition to the rotor power, the stator power as output by the stator winding may also be supplied to the utility grid.

[0017] The doubly-fed induction generator may include a multi-phase wound rotor and a multi-phase slip ring assembly having brushes for accessing the rotor winding. According to an embodiment, it is possible to avoid the multi-phase slip ring assembly, but there may be problems with efficiency, cost and size. According to other embodiments, a brushless wound rotor doubly-fed machine may be utilized as an induction generator. Therefore, the stator winding may be connected to a utility grid, and the rotor winding may be connected to a converter via a slip ring and a back-to-back voltage source converter, which may control both the rotor and the grid current. Thus, it is possible to freely set the rotor frequency to a rotor winding reference frequency that varies with the actual grid frequency. In addition, by using a converter to control the rotor current, it is possible to adjust the active power and reactive power fed from the stator to the utility grid independently of the rotational speed of the generator. The control principle applied may, for example, involve dual-axis vector control or direct torque control.

[0018] Another example of an induction generator (or asynchronous generator) is a squirrel cage generator.

[0019] The utility grid voltage may be obtained, for example, from a measuring device capable of measuring, for example, the voltage at a point of common coupling to which a plurality of wind turbines are connected. Conventionally, the voltage is measured at the stator terminals. The measuring device may be configured to measure, for example, the grid voltage at the low voltage side or the high voltage side of a wind farm transformer, which may be connected between the point of common coupling and the utility grid. The utility grid voltage may be received, for example, at a wind farm or a wind farm pilot and may be communicated to the individual wind turbines.

[0020] The rotation speed reference may be or may comprise a desired rotation speed at which the rotor of the asynchronous generator system is intended to rotate. Depending on the specific configuration and application and / or external conditions, setting the rotation speed reference may be performed in different ways. The rotation speed reference is set dependent on the utility grid voltage insofar as the rotation speed reference may be changed when the utility grid voltage changes. A change of the rotation speed reference may for example only be performed if, for example, the utility grid voltage deviates from the nominal grid voltage by more than a threshold value and / or when the deviation is longer than a predetermined time interval.

[0021] In order to set the rotational speed reference, one or more components of the generator system, or for example one or more components of the drive system, can be controlled. If the asynchronous generator system is, for example, a generator system of a wind turbine, the rotor blade pitch angle can be set based on the rotational speed reference, such as to accelerate or decelerate the rotor due to wind impact on the rotor blades. Alternatively and / or additionally, based on the rotational speed reference, a counter-torque of the asynchronous generator system can be set or defined, which then alternatively or additionally causes a deceleration or acceleration of the rotor. Additionally or alternatively, airflow influencing devices, which can optionally be mounted at the rotor blades, can also be controlled depending on the rotational speed reference.

[0022] According to one embodiment of the invention, the rotation speed reference is set as a function of the grid voltage deviation (undervoltage condition), ie the deviation of the actual grid voltage from the nominal grid voltage. The rotation speed reference may also be referred to as a generator speed reference.

[0023] The method can be applied in any steady-state situation where a grid undervoltage causes the stator current to reach a value exceeding the rated value (P, Qmax, Vn).

[0024] Conventionally, in the worst undervoltage scenario, which may include a scenario where the utility grid voltage is 5% or even up to 10% below the nominal grid voltage, the asynchronous generator may be oversized for permanent operation. In this case, the stator current of the asynchronous generator system (DFIG) may conventionally be higher than the corresponding rated value (i.e., the current at the rated or nominal utility grid voltage). The higher current may conventionally cause higher copper losses and, therefore, higher winding temperatures.

[0025] According to one embodiment of the invention, in case the utility grid voltage drops below a rated voltage value (in particular drops by at least a threshold value), the rotor current and / or the stator current remain substantially at their rated or nominal values ​​(those values ​​which the rotor current and / or the stator current adopt at the rated or nominal grid voltage).

[0026] Advantages of this embodiment may include that the size of the generator system or components of the generator system may be reduced, or the cooling system may be designed for lower capacity (including, for example, self-ventilation). Thus, the cost and / or efficiency of the system may be improved.

[0027] According to one embodiment of the invention, the method is performed in an undervoltage situation, wherein the utility grid voltage drops below the nominal grid voltage or the utility grid voltage is below the nominal grid voltage, in particular between 0.1% and 10% below the nominal grid voltage. Conventionally, in such an undervoltage situation, the rotational speed of the system may have been kept substantially constant, for example at the nominal rotational speed. However, according to one embodiment of the invention, in an undervoltage situation, the rotational speed reference is increased compared to a situation where the utility grid adopts the nominal utility grid voltage.

[0028] For example, an undervoltage condition may be characterized by the actual grid voltage being up to 5% or up to even 10% lower than the nominal utility grid voltage.

[0029] According to an embodiment of the invention, setting the rotational speed reference comprises increasing the rotational speed reference if the utility grid voltage decreases (particularly decreases by at least a threshold value) for at least a minimum time interval. Increasing the rotational speed reference may result in the generator system being operated with an increased actual rotational speed. As a result, the rotor current and / or the stator current of the generator system may not substantially increase or may even remain substantially constant. As a result, loads on components, in particular copper losses or overheating, may be reduced or even avoided.

[0030] According to one embodiment of the invention, the rotation speed reference is set depending on the ratio between the nominal utility grid voltage and the (actual) utility grid voltage.

[0031] When the rotation speed reference is set depending on the ratio at which the utility grid voltage decreases below the nominal voltage, the rotation speed reference may be set to be proportional to the nominal grid voltage divided by the actual (i.e., decreased) utility grid voltage. Thus, excessively increasing or completely increasing the rotor current and / or stator current of the generator system may be effectively avoided.

[0032] According to one embodiment of the invention, the rotation speed reference is arranged such that the stator current and / or the rotor current of the asynchronous generator system (DFIG) remains substantially constant when the utility grid voltage varies (in particular decreases).

[0033] When the stator current and / or the rotor current is kept substantially constant, it may mean keeping the stator current and / or the rotor current within limits, which may be, for example, up to 1% to 5% above or below the corresponding nominal current. The limits may be set according to specific requirements and may, for example, be set at 0.5% or 1% of the corresponding nominal current. Thus, wind turbine components or generator system components may operate reliably, avoiding risks of, for example, overheating or excessive copper losses.

[0034] According to one embodiment of the invention, the rotational speed reference is arranged such that the product of the (actual) utility grid voltage and the rotational speed reference remains substantially constant (in particular equal to the product of the nominal utility grid voltage and the nominal rotational speed) when the utility grid voltage changes (in particular decreases).

[0035] When the product of the grid voltage and the rotational speed reference remains substantially constant as the grid voltage changes, it may mean that this product stays within limits of up to 0.5% or 1% or 2% or 3% above or below the product of the nominal grid voltage and the nominal rotational speed reference.

[0036] Embodiments of the invention may involve changing the rotational speed reference to be different from a nominal rotational speed reference. A nominal rotational speed reference may be set for a generator system, for example, in order to optimize efficiency or performance. For example, the nominal rotational speed reference may also be set additionally and / or alternatively depending on the size of the mechanical system and / or the drive system and / or, for example, the wind turbine components.

[0037] According to one embodiment of the invention, the method further comprises measuring the utility grid voltage, in particular measuring the utility grid voltage at a common coupling point to which a plurality of wind turbines are connected. The utility grid voltage may be measured and / or derived, for example, from a measurement location at the high voltage side of a wind turbine transformer (which may couple the generator output terminals to the common coupling point), or may be measured at the low voltage side or the high voltage side of a wind farm transformer, which may connect the common coupling point to the utility grid. In other embodiments, the utility grid voltage may be measured at a plurality of locations.

[0038] The utility grid voltage may be available, for example, to or at a wind farm controller or wind farm director and may be communicated, for example, to the individual wind turbines. The utility grid voltage may be received, for example, at a wind turbine controller (e.g., which includes a controller for controlling an asynchronous generator system according to one embodiment of the invention, as described below). In other embodiments, the utility grid voltage may be estimated or derived from electrical or mechanical quantities other than the grid voltage itself.

[0039] According to one embodiment of the invention, the method further comprises at least one of the following: low-pass filtering the measured grid voltage; averaging the measured grid voltage over a moving time window, the time window particularly having a width between 1 minute and 20 minutes, further particularly between 3 minutes and 10 minutes.

[0040] One or more of the above-mentioned method steps may ensure that the rotation speed reference does not change too quickly. In particular, the rotation speed reference may be set based on a low-pass filtered grid voltage and / or based on an averaged measured grid voltage. Thus, too fast a change of the rotation speed reference and, therefore, also a too fast change of the actual rotation speed of the generator system may be avoided.

[0041] According to one embodiment of the invention, the method further comprises controlling the generator system according to a rotational speed reference. In particular, the generator system may be controlled with respect to a reference torque and / or a reference active power and / or a reference reactive power of the generator system. Thereby, in particular, a generator or a converter that may be included in the generator system may be controlled.

[0042] However, it should be understood that additionally or alternatively, other components of the wind turbine, such as a drive system including rotor blades, may be controlled, for example.

[0043] The generator system and / or the drive system may be controlled based on a rotational speed reference, which is set in dependence on the utility grid voltage.

[0044] According to one embodiment of the invention, an asynchronous generator system is driven by a rotor of a wind turbine at which a plurality of rotor blades are mounted, the wind turbine comprising a blade pitch adjustment system for adapting at least one blade pitch angle, wherein controlling the generator according to a rotational speed reference comprises: determining a blade pitch adjustment system reference based on the rotational speed reference, in particular comprising at least one of the following: a blade pitch angle reference and / or a mechanical power reference and / or a mechanical torque reference; supplying the blade pitch adjustment system reference to the blade pitch adjustment system.

[0045] The blade pitch adjustment system is capable of setting the blade pitch angle of the blades of the wind turbine. Setting the blade pitch angle may involve rotating the corresponding blade around the longitudinal axis of the rotor blade. Adapting the blade pitch angle may change the lift of the airflow across the surface of the rotor blade. Thus, the drive torque may be changed. For example, in order to increase the rotational speed (when the rotational speed reference is, for example, higher than the nominal rotational speed reference), the lift of the rotor blade may be increased due to the changed blade pitch angle, so that the drive torque is increased, resulting in a higher actual rotational speed.

[0046] The control signal supplied to the blade pitch adjustment system may be selected from a variety of different signal types, which may be selected based on a particular application. A simple option may be to supply a blade pitch angle reference to the blade pitch adjustment system. A corresponding blade pitch angle reference may be derived or calculated from a rotational speed reference. In other embodiments, the blade pitch angle reference may be calculated directly from the utility grid voltage.

[0047] The blade pitch adjustment system may then set the corresponding blade pitch angle(s) according to the blade pitch adjustment system reference.

[0048] According to one embodiment of the invention, the asynchronous generator system comprises: an asynchronous generator having a multi-phase stator winding and having a multi-phase rotor winding; a converter connected to the rotor winding and configured to convert a variable frequency rotor power flow into a fixed frequency rotor power flow at a converter output terminal connected to the utility grid; a stator winding connected to the utility grid, wherein controlling the generator according to the rotational speed reference comprises: determining a converter reference based on the rotational speed reference, the converter reference in particular comprising at least one of the following: a converter power reference; a converter torque reference, the converter reference being supplied to the converter.

[0049] The term "asynchronous generator" is used in this application as a synonym for the term "induction generator".

[0050] The converter may include an AC-DC part, a DC link and a DC-AC part. The AC-DC part and the DC-AC part may include a plurality of controllable switches, such as IGBTs, which are controlled by supplying corresponding gate driver signals to the gates of the controllable switches.

[0051] The converter reference may be supplied to the converter for controlling the converter. Depending on the specific application, the converter reference may be provided as, for example, a converter power reference indicating a desired power that the converter should output, or the converter reference may be provided as a converter torque reference indicating a desired torque that the converter should produce or defining a counter torque that the generator should apply to the rotating rotor.

[0052] By determining and supplying a converter reference, the entire generator system can be controlled with respect to the counter-torque reacting against the rotating rotor.Thus, the rotational speed of the rotor can be controlled so as to be substantially equal to the rotational speed reference as determined based on the utility grid voltage.

[0053] It should be understood that the features described, explained, provided or applied, alone or in any combination, for the method of controlling an asynchronous generator system may also be applied or provided, alone or in any combination, to a controller for controlling an asynchronous generator system (DFIG) according to an embodiment of the present invention, and vice versa.

[0054] According to one embodiment of the invention, a controller for controlling an asynchronous generator system connected to a utility grid, in particular an asynchronous generator system of a wind turbine, is provided, the controller being configured to set a rotation speed reference of the asynchronous generator system depending on the utility grid voltage, the controller being in particular configured to control or execute a method according to one of the aforementioned embodiments.

[0055] The controller may for example be part of a wind turbine controller.The controller may be implemented in software and / or hardware.

[0056] According to one embodiment of the present invention, there is provided a wind turbine, comprising: an asynchronous generator system (DFIG); and a controller according to the aforementioned embodiment.

[0057] The wind turbine may include a rotor at which a plurality of rotor blades are connected. The rotor may be configured to rotate relative to a stator of an asynchronous generator. A rotor winding may be mounted at the rotor.

[0058] Embodiments of the present invention may particularly support multi-phase asynchronous generators, particularly three-phase asynchronous generators.

[0059] According to the example of embodiment to be described hereinafter, the above-defined aspects of the present invention and other aspects are apparent, and the above-defined aspects of the present invention and other aspects are explained with reference to the example of embodiment. The present invention will be described in more detail hereinafter with reference to the example of embodiment, but the present invention is not limited to the example of the embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] Embodiments of the present invention will now be described with reference to the accompanying drawings. The present invention is not limited to the embodiments shown or described.

[0061] The figure schematically illustrates a wind turbine according to an embodiment of the invention. DETAILED DESCRIPTION

[0062] The wind turbine 1 according to an embodiment of the invention schematically illustrated in the figure comprises a double-fed induction generator 3 (corresponding to the implementation of an asynchronous generator). The induction generator 3 is driven by a secondary shaft 5 coupled to an (optional) gearbox 7. The gearbox 7 converts the relatively low rotational speed of the main shaft 9 of the wind turbine into a relatively high rotational speed of the secondary shaft 5. A plurality of rotor blades 11 are connected at the main shaft 9, which are driven by the impinging wind 13. A nacelle not shown mounted on top of the wind turbine power source harbours the generator 3, the shafts 9, 5, the gearbox 7 and the converter 17.

[0063] The generator 3 comprises a stator winding 13 and a rotor winding 15, both of which are only schematically illustrated. The stator winding 13 may, for example, comprise a three-phase winding or coil wound around the teeth of a ferromagnetic stator yaw. The rotor winding 15 may also comprise, for example, a three-phase rotor winding wound around a ferromagnetic core.

[0064] The three-phase rotor windings 15a, 15b, 15c are connected to a converter 17, which includes an AC-DC converter portion 19, a DC link 21, and a DC-AC converter portion 23. The converter 17 converts AC power into AC power of different frequencies in two directions. The converter 17 receives (e.g., three-phase) rotor power 25a, 25b, 25c (P_rotor) from the rotor winding 15.

[0065] According to one embodiment of the invention, a controller 30 for controlling an induction generator 3 connected to a utility grid 47 is provided.

[0066] The induction generator 3 may be controlled, for example, via a converter 17. In order to control the converter 17, the converter 17 receives a converter reference 27 from a controller 30, which in addition to the converter 17 may also control other parts of the wind turbine.

[0067] Output terminals 43a, 43b, 43c of the converter 17 are connected to output terminals 45a, 45b, 45c of the stator winding 13. The stator power P_stator and the rotor power P_rotor are received by a step-up transformer (not shown) and this step-up transformer is connected to a utility grid 47, where other wind turbines are also connected.

[0068] The utility grid 47 is intended to operate at a nominal frequency and voltage. However, due to disturbances or imbalances between power production and power consumption, the voltage of the utility grid may occasionally deviate from the nominal voltage.

[0069] Conventionally, the rotor winding reference frequency may not have been set as a function of the grid voltage U_grid.

[0070] The controller 30 for controlling the asynchronous generator system 3 according to one embodiment of the present invention is configured to depend on the utility grid voltage U g To set the rotation speed reference n of the asynchronous generator system 3.

[0071] Therefore, the public grid voltage U g The voltage U is measured by, for example, a measuring device 50 which transmits the utility grid voltage of the utility grid 47. The controller 30 receives a corresponding voltage measurement signal 51. The controller 30 then determines the utility grid voltage U based on the utility grid voltage U g To determine the rotation speed reference n.

[0072] The rotational speed reference may be an internal variable or quantity used internally within the controller 30. Sometimes, the rotational speed reference itself may be utilized to control one or more components of the wind turbine.

[0073] According to a particular embodiment, the controller determines the converter reference 27 and / or the blade pitch adjustment system reference 52 supplied to the blade pitch adjustment system 53 based on a rotation speed reference or typically based on the utility grid voltage. The blade pitch adjustment system 53 is configured to set the blade pitch angle of at least one rotor blade 11 mounted at the main rotor 9 of the wind turbine 1 illustrated in the figure. The control signal 52 supplied from the controller to the rotor blade adjustment system 53 may be, for example, a blade pitch angle reference.

[0074] According to an embodiment of the invention, when the utility grid voltage drops below the rated utility grid voltage or the nominal utility grid voltage, the stator current and / or the rotor current is substantially maintained at a rated value.

[0075] In the following equation explaining how to determine the rotation speed reference, P s Represents stator power, P T represents the total power (i.e., the sum of the stator power and the rotor power), n represents the rotation speed reference, and n s Represents the synchronous speed which is proportional to the grid frequency. g It also represents the stator voltage U s The actual grid voltage.

[0076]

[0077] At the nominal or rated power P n , reactive power Q n and the nominal frequency f n However, in the undervoltage scenario, if the stator current I s Intent remains constant, then the following equation applies:

[0078] nU g=cte=n n U n

[0079] For example, if U g =0.95·U n :

[0080] n0.95U n =n n U n

[0081]

[0082] Therefore, when the stator voltage (utility grid voltage) decreases by 5%, the stator current will stay at the rated or nominal value if the rotation speed increases by approximately 5%.

[0083] In the following equation, P r represents the rotor power, I r Represents the rotor current, I s represents the stator current, fg represents the actual grid frequency, and rt represents the rotor-stator turns ratio.

[0084]

[0085] At rated active power, reactive power and utility grid frequency, but in an undervoltage scenario, if the intention is to maintain the rotor current I r is essentially constant, then the following equation applies:

[0086] nU g =cte=n n U n

[0087] For example, if U g =0.95U n , then the following equation applies:

[0088] n0.95U n =n n U n

[0089]

[0090] In order to reduce loads and mechanical elements, it is effective to apply, if necessary, for example a 1 minute, 3 minute or 10 minute moving average filter to the utility grid voltage in view of the high thermal inertia of the system.

[0091] As can be seen from the equations relating to keeping the stator current constant and keeping the rotor current constant, the same relationship is obtained for how the rotational speed reference is set. Thus, if the utility grid voltage decreases, the rotational speed reference increases.

[0092] In addition, depending on the ratio U n / U g The rotation speed reference is set depending on the ratio between the nominal voltage and the actual utility grid voltage.

[0093] When the rotation speed reference is set according to the above equation, the stator current and / or the rotor current of the asynchronous generator system 3 is at the utility grid voltage U g Remain essentially constant when changing.

[0094] To obtain the voltage measurement signal 51 , the voltage measurement signal may be low-pass filtered or averaged over a moving time window.

[0095] The wind turbine 1 illustrates that the controller 30 controls the generator system 3 via the converter 17 and additionally or alternatively controls the blade pitch adjustment system 53 .

[0096] According to an embodiment of the present invention, the controller 30 may exclusively control the blade pitch adjustment system 53 via the blade pitch adjustment system reference 52 without additionally controlling the generator system 3 or the converter 17. In still other embodiments, the controller 30 controls only the converter 17, without controlling the blade pitch adjustment system 53 either.

[0097] Conventionally, undervoltage conditions are considered a requirement that the generator system must comply with, as control strategies are not conventionally applied.

[0098] Embodiments of the present invention may provide the following advantages:

[0099] Cost optimization

[0100] Avoid over-specification of systems and electrical components

[0101] Protect / increase the grid connection capabilities of wind turbines

[0102] Avoiding possible reductions in wind turbine availability

[0103] Increase product competitiveness.

[0104] It should be noted that the term "comprising" does not exclude other elements or steps, and "one" and "an" do not exclude a plurality. Elements described in conjunction with different embodiments may also be combined. It should also be noted that the reference symbols in the claims should not be interpreted as limiting the scope of the claims.

Claims

1. A method for controlling an asynchronous generator system (3) connected to a utility grid (47), in particular an asynchronous generator system (3) of a wind turbine (1), the method comprising: A rotation speed reference (n) of the asynchronous generator system (3) is set depending on the utility grid voltage (Ug).

2. The method according to the preceding claim, which is performed under undervoltage conditions, wherein: The utility grid voltage (Ug) is reduced to below the nominal grid voltage (Un) or below the nominal grid voltage (Un), in particular between 0.1% and 10% below the nominal grid voltage (Un).

3. The method according to one of the preceding claims, in, Setting the rotation speed reference (n) comprises: If the utility grid voltage (Ug) decreases, in particular decreases by at least a threshold value, for at least a minimum time interval, the rotational speed reference (n) is increased.

4. The method according to one of the preceding claims, in, The rotation speed reference (n) is set depending on the ratio between the nominal grid voltage (Un) and the utility grid voltage (Ug).

5. The method according to one of the preceding claims, in, The rotation speed reference (n) is arranged such that the stator current (Is) and / or the rotor current (Ir) of the asynchronous generator system remains substantially constant when the utility grid voltage changes, in particular decreases.

6. The method according to one of the preceding claims, in, The rotational speed reference (n) is arranged such that the product of the utility grid voltage (Ug) and the rotational speed reference (n) remains substantially constant when the utility grid voltage changes, in particular decreases.

7. The method according to one of the preceding claims, The method further comprises: The utility grid voltage (Ug) is measured, in particular at a point of common coupling to which a plurality of wind turbines are connected or directly at generator terminals.

8. The method according to the preceding claim, The method further comprises at least one of the following: Perform low-pass filtering on the measured grid voltage; The measured grid voltage is averaged over a moving time window, which in particular has a width between 1 minute and 20 minutes, further in particular has a width between 3 minutes and 10 minutes.

9. The method according to one of the preceding claims, further comprising: The generator system (3) is controlled according to the rotational speed reference.

10. The method according to the preceding claim, in, The asynchronous generator system (3) is driven by a rotor (9, 5) of a wind turbine (1), on which a plurality of rotor blades (11) are mounted, the wind turbine comprising a blade pitch adjustment system (53) for adapting at least one blade pitch angle, Wherein, controlling the generator (3) according to the rotation speed reference comprises: A blade pitch adjustment system reference (52) is determined based on the rotation speed reference (n), wherein the blade pitch adjustment system reference (52) particularly comprises at least one of the following: Blade pitch angle reference and / or Mechanical power reference and / or Mechanical torque benchmark; The blade pitch adjustment system reference (52) is supplied to the blade pitch adjustment system (53).

11. The method according to one of the two preceding claims, the asynchronous generator system (3) comprising: an asynchronous generator (13, 15) having a multi-phase stator winding and having a multi-phase rotor winding; a converter (17) connected to the rotor winding and configured to convert a variable frequency rotor power flow into a fixed frequency rotor power flow at a converter output terminal connected to the utility grid; stator windings, which are connected to the utility grid (47), Wherein, controlling the generator according to the rotation speed reference comprises: A converter reference (27) is determined based on the rotational speed reference, the converter reference (27) comprising in particular at least one of the following: Converter power reference; Converter torque reference; The converter reference is supplied to the converter.

12. The method according to the preceding claim, in, The asynchronous generator comprises at least one of the following: Doubly-fed induction generator (13, 15).

13. A controller (30) for controlling an asynchronous generator system (3) connected to a utility grid (47), in particular an asynchronous generator system (3) of a wind turbine, the controller being configured to: The rotation speed reference (n) of the asynchronous generator system is set depending on the utility grid voltage (Ug), the controller being in particular configured to control or perform the method according to one of the preceding claims.

14. A wind turbine (1) comprising: Asynchronous generator system (3); as well as A controller (30) as claimed in the preceding claim.

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