Control of an induction generator of a wind turbine

Abstract

A method of controlling an induction generator (3) connected to a utility grid (47) is described, the method comprising: receiving an actual grid frequency (f); and controlling a rotor winding (15) of the generator (3) by rotor control signals (25a, b, c) having a rotor winding reference frequency (Ω_ref) set in dependence on the actual grid frequency (f).

Description

Technical Field

[0001] This invention relates to a method and apparatus for controlling an induction generator connected to a public power grid, particularly during changes in grid frequency. Furthermore, this invention relates to a wind turbine comprising an induction generator and the apparatus thereof. Background Technology

[0002] Wind turbines are connected to a public power grid that operates at a specific nominal frequency, such as 50Hz or 60Hz. However, due to imbalances in electricity production and consumption, the actual frequency of the public power grid can deviate from the nominal grid frequency. Grid operators are developing stringent specifications to ensure grid stability. Under specific grid connection specifications, wind turbines may be required to remain connected to the grid even if the actual grid frequency deviates from a specific range, such as 6-8% from the nominal grid frequency. Furthermore, when the actual frequency deviates from the nominal grid frequency, the wind turbine may need to perform additional control functions.

[0003] However, frequency deviations from the nominal grid frequency can affect the operation of wind turbine components, such as the generators and / or converters included in the wind turbine. In particular, the operation of induction generators can be affected by frequency variations. Conventionally, in order to allow operation within a specific grid frequency range near the nominal grid frequency, induction generators and / or converters connected to them can be designed to be oversized or over-specification. Through oversized design of components (e.g., in terms of rated power, rated current, or rated voltage), these components are able to operate at different frequencies. However, according to existing technology, this requires heavier and more expensive equipment. Conventionally, a demand envelope has been calculated to define the component ratings for a specific frequency range of expected variation from the nominal grid frequency. Here, conventionally applied methods and systems require complex, heavy equipment and are associated with high costs.

[0004] In addition, it has been routinely observed that certain components can overload during specific frequency events, such as those involving overheating risks, impacts on wind turbine availability, or even component damage.

[0005] Therefore, there may be a need for methods and apparatus for controlling induction generators connected to the public power grid, wherein oversized designs can be reduced or even avoided, and wherein heavy or expensive equipment is not required. Summary of the Invention

[0006] This need can be met by the subject matter of the independent claims. The dependent claims describe advantageous embodiments of the invention.

[0007] According to an embodiment of the present invention, a method for controlling an induction generator connected to a public power grid is provided, the method comprising: receiving an actual power grid frequency; and controlling the generator's (e.g., excitation) rotor winding via a rotor control signal having a rotor winding reference frequency (e.g., Ω_ref) set according to the actual power grid frequency (e.g., f).

[0008] This method can be executed, for example, by a controller of an induction generator, which can be, for example, part of a wind turbine controller. The method can be implemented in software and / or hardware.

[0009] An induction generator is an asynchronous generator that produces alternating current (AC) as its rotor shaft rotates relative to the stator windings. To output rotor power at a desired frequency, the rotor needs to rotate faster than synchronous speed. Synchronous speed corresponds to the frequency of the AC power to be supplied to the utility grid. To operate an induction generator, an excitation signal or excitation power (also called a control signal) needs to be supplied to the rotor windings. The relative difference between synchronous speed and operating speed is called slip, and is usually expressed as a percentage of synchronous speed. When used in wind turbines, for example, the secondary shaft of the wind turbine drives the rotor of the induction generator to rotate faster than synchronous speed. This secondary shaft is connected via a gearbox to the main shaft of the turbine, on which multiple rotor blades are connected. Electricity is generated in both the stator and rotor windings.

[0010] Induction generators can be implemented in different ways. An example of an induction generator is a doubly-fed induction generator (DFIG). A DFIG has two sets of windings (e.g., three-phase windings), one set stationary (i.e., stator windings) and one set rotating (i.e., rotor windings). The stator windings can be directly connected to the generator's output terminals. The rotor windings (which are also traditionally called the field windings) can be connected to a converter that provides AC power at a variable frequency, which can be adjusted as needed.

[0011] The AC frequency of the converter connected to the rotor winding can be set specifically according to the rotor winding reference frequency. This rotor winding reference frequency is set based on the actual power grid frequency and varies with the actual power grid frequency.

[0012] A converter can be connected between the output of the stator winding and the rotor winding to convert AC to DC and vice versa. The converter can be bidirectional, allowing power to be transmitted in both directions. Therefore, the converter can be used to actually provide control signals (or excitation signals) to the rotor winding, but simultaneously or concurrently receive rotor power from the rotor winding, convert it to the desired grid frequency, and output that rotor power to the public grid. Since the converter's output can be connected to the output of the stator winding, stator power output from the stator winding, in addition to rotor power, can also be supplied to the public grid.

[0013] A doubly-fed induction generator may include a multiphase wound rotor and a multiphase slip ring assembly with brushes for proximity to the rotor windings. According to one embodiment, the multiphase slip ring assembly can be avoided, but this may present issues with efficiency, cost, and size. According to another embodiment, a brushless wound-rotor doubly-fed motor can be used as an induction generator. Here, the stator windings can be connected to a public power grid, and the rotor windings can be connected to a converter via slip rings and a back-to-back voltage source converter that controls both the rotor and grid currents. This allows the rotor frequency to be freely set to a rotor winding reference frequency that varies with the actual grid frequency. Furthermore, by controlling the rotor current using the converter, the active and reactive power fed from the stator to the public power grid can be regulated independently of the generator's rotational speed. The applied control principles may, for example, involve two-axis vector control or direct torque control.

[0014] Instantaneous grid frequency can be measured, involving sampling and / or averaging and / or filtering to obtain the actual grid frequency. To control the rotor windings, a converter can be used, connected at one end to the rotor winding and at the other end to the stator winding output. When the rotor winding reference frequency for the control signal controlling the rotor windings is set according to the actual grid frequency, the operation of the induction generator can be improved, and in particular, overload or overheating of components can be avoided. The load on the rotor and / or stator windings can, in particular, remain substantially unchanged with variations in the actual grid frequency. Therefore, oversized designs for the rotor and / or stator windings and / or converters can be avoided.

[0015] Changes in grid frequency can alter the synchronization speed of induction generators, thus modifying nominal operation. By adjusting or adapting the rotor winding reference frequency to the changed grid frequency, oversized or out-of-specification electrical components designed to support such events can be avoided or eliminated. This mitigates the excessive costs associated with oversized components, primarily induction generators (e.g., squirrel-cage and doubly-fed) and partial load converters. Furthermore, it reduces wind turbine power derating during grid frequency events due to overheating of components not oversized. This significantly enhances wind turbine availability. In particular, wind turbines can remain connected to the grid as required by grid regulations.

[0016] According to an embodiment of the present invention, the internal control generator speed reference (hereinafter referred to as the rotor winding reference frequency) can be set as a function of the grid frequency deviation, i.e., the deviation between the nominal grid frequency and the actual grid frequency. This method may only be applicable to rated conditions, i.e., when the design limits can be reached.

[0017] According to an embodiment of the invention, the public power grid is designed to operate at a nominal grid frequency, wherein the rotor winding reference frequency (e.g., Ω_ref) is set according to the deviation between the actual grid frequency (e.g., f) and the nominal grid frequency (e.g., f0).

[0018] The nominal grid frequency can reach, for example, 50 Hz in Europe, or 60 Hz in the United States. This method can still be simplified when the rotor winding reference frequency is set based on the deviation between the actual grid frequency and the nominal grid frequency, while ensuring that the actual load or actual power of the rotor winding and / or stator winding and / or converter remains substantially unchanged in the event of a change in grid frequency. In particular, the rotor winding reference frequency can vary linearly with the deviation between the actual grid frequency and the nominal grid frequency. A simple implementation is provided here.

[0019] According to an embodiment of the invention, the rotor winding reference frequency (e.g., Ω_ref) is set such that the slip (e.g., s) follows a predetermined value and remains substantially unchanged with variations in the actual grid frequency, at least within a predetermined frequency range.

[0020] The slip s is given by the following formula:

[0021] s = (Ω(f) - ns(f)) / ns(f),

[0022] Where f is the actual grid frequency, Ω is the actual speed of the generator at the actual grid frequency f, and ns(f) is the synchronous speed at the actual grid frequency f. ns is given in rpm by ns = 60 * f / p, where p is the number of pole pairs of the generator rotor, and f is given in Hz.

[0023] When the slip remains essentially constant with variations in the actual grid frequency, the load on the rotor and / or stator windings can also remain substantially constant for varying grid frequencies. Therefore, oversized designs, as well as overheating or overload, can be avoided.

[0024] According to an embodiment of the invention, the predetermined frequency range is between 0.90 and 1.1 times the nominal grid frequency, particularly between 0.97 and 1.03 times, wherein, for a nominal grid frequency of 50 Hz, the predetermined frequency range is between 45 Hz and 55 Hz, particularly between 47 Hz and 53 Hz, and even more particularly between 48 Hz and 52 Hz.

[0025] When a predefined frequency range is supported, for example, between 0.9 and 1.1 GHz of the nominal grid frequency, the expected frequency deviation can generally be handled. Within the predefined frequency range, or when the actual grid frequency is within the predefined frequency range, the induction generator can adhere to specific grid rules and remain connected to the public grid. In particular, the induction generator can remain connected to the public grid without the risk of overheating or overloading of its components.

[0026] According to an embodiment of the present invention, the rotor winding reference frequency (e.g., Ω_ref) is set such that the rotor power output by the rotor winding follows a predetermined relative rotor power and / or the stator power output by the stator winding follows a predetermined relative stator power, and the rotor power and / or stator power do not change with the actual grid frequency, at least within a predetermined frequency range.

[0027] The relative rotor power can, for example, be between 5% and 20% of the total generator output power, and the relative stator power can, for example, be between 80% and 95% of the total generator output power. Other values ​​are possible. The dimensions of the rotor windings and / or converters can be designed to (substantially) support no more than the predefined relative rotor power (and / or current and / or voltage), where oversized design is not required.

[0028] This can save costs and reduce the weight and space requirements of the rotor windings and / or converters.

[0029] According to an embodiment of the invention, the dimensions of the stator windings are designed based on a predetermined relative stator power such that they need to operate under a load that is no more than 1%, particularly 0.1%, higher than the predetermined relative stator power. Here, oversized design and cost can be reduced.

[0030] According to an embodiment of the present invention, the converter is connected to the rotor winding to provide rotor control signals, wherein the converter particularly includes an AC-DC converter section, a DC link and a DC-AC converter section, wherein the output terminal of the converter is connected to the output terminal of the stator winding.

[0031] The AC-DC converter section and the DC-AC converter section can be implemented using multiple controllable power switches, such as IGBTs or general-purpose power transistors. Conventionally available components can be used. When the converter's output terminal is connected to the output terminal of the stator winding (e.g., a three-phase stator winding), a combination of rotor and stator power can be supplied to the public power grid. Furthermore, a conventional implementation of a doubly-fed induction generator can be implemented here. The only difference is that controlling the rotor winding by providing a rotor control signal with a rotor winding reference frequency may differ from conventionally known techniques. However, the hardware—the converter and / or rotor winding—can be conventionally available.

[0032] According to embodiments of the invention, the dimensions of the rotor windings and / or the converters connected to the rotor windings are designed based on a predetermined relative rotor power such that they need to operate at a load no more than 1%, particularly 0.1%, higher than the predetermined relative rotor power. Since the rotor windings are controlled using a rotor winding reference frequency based on the actual grid frequency, it can be ensured that the rotor windings and converters will not be overloaded or overheated, because the load may not change substantially with variations in the actual grid frequency due to the adjustment of the rotor winding reference frequency.

[0033] According to an embodiment of the present invention, the rotor winding reference frequency is set as follows:

[0034] Ω_ref(f)=Ω(f0)*(1+(f-f0) / f0), where

[0035] f is the actual grid frequency, f0 is the nominal grid frequency, Ω(f) is the rotor winding reference frequency at the actual grid frequency, and Ω(f0) is the rotor winding reference frequency at the nominal grid frequency.

[0036] Ω(f0) = f0 * s, where s is the slip.

[0037] Here, a very simple formula for implementing this method can be provided. This constraint ensures that the slip remains constant, and that the relative rotor power and relative stator power remain substantially constant with respect to varying grid frequencies.

[0038] According to an embodiment of the invention, the actual grid frequency is determined by averaging and / or filtering the instantaneous grid frequency over a predetermined time range, particularly within a time span between 1 minute and 10 minutes. This allows for smooth operation of the induction generator and avoids other operational problems caused by excessively rapid changes in the rotor winding reference frequency.

[0039] According to an embodiment of the invention, the actual grid frequency deviates from the nominal frequency by less than 5% or less than 3%, and / or wherein the induction generator includes, in particular, a three-phase doubly-fed induction generator and / or a squirrel-cage generator.

[0040] According to an embodiment of the invention, the induction generator is driven by the rotating shaft of a wind turbine, which is connected, in particular, via a gearbox to a main shaft to which multiple rotor blades are connected. The gearbox may have a conversion ratio such that the typical rotational speed of the wind turbine's main shaft is too coarsely converted to a synchronous speed multiplied by a slip corresponding to the nominal frequency of the utility power grid.

[0041] It should be understood that features of a method for controlling an induction generator connected to a public power grid, disclosed, described, explained, or provided individually or in any combination, can also be applied individually or in any combination to an apparatus for controlling an induction generator connected to a public power grid according to embodiments of the present invention, and vice versa.

[0042] According to an embodiment of the present invention, an apparatus for controlling an induction generator connected to a public power grid is provided. The apparatus includes: an input port adapted to receive an actual power grid frequency; and a controller adapted to control the rotor windings of the generator via a rotor control signal having a rotor winding reference frequency (e.g., Ω_ref) set according to the actual power grid frequency (e.g., f).

[0043] Furthermore, according to an embodiment of the present invention, a wind turbine is provided, the wind turbine including an induction generator driven by wind energy, the generator having a rotor winding and the means according to the foregoing embodiment. Attached Figure Description

[0044] Embodiments of the invention will now be described with reference to the accompanying drawings. The invention is not limited to the embodiments illustrated or described.

[0045] The accompanying drawings schematically illustrate a wind turbine according to an embodiment of the present invention. Detailed Implementation

[0046] The wind turbine 1 shown in the figure, according to an embodiment of the present invention, includes a doubly-fed induction generator 3. The induction generator 3 is driven by a secondary shaft 5 coupled to a 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 to the main shaft 9, and these rotor blades are driven by impinging wind 13. A nacelle (not shown) mounted on top of the wind turbine power unit contains the generator 3, shafts 9 and 5, gearbox 7, and converter 17.

[0047] The generator 3 includes a stator winding 13 and a rotor winding 15, both of which are shown schematically only. The stator winding 13 may, for example, include a three-phase winding or coil wound around the yaw teeth of a ferromagnetic stator. The rotor winding 15 may also include, for example, a three-phase rotor winding wound around a ferromagnetic core.

[0048] Three-phase rotor windings 15a, 15b, and 15c are connected to converter 17, which includes an AC-DC converter section 19, a DC link 21, and a DC-AC converter section 23. Converter 17 converts AC power into AC power of different frequencies in both directions. Converter 17 is capable of providing control signals (e.g., three-phase signals) 25a, 25b, and 25c to the rotor windings 15.

[0049] Therefore, converter 17 receives drive signal 27 from controller 30, which, in addition to converter 17, can also control other parts of the wind turbine. According to an embodiment of the invention, controller 30 is part of device 35 for controlling an induction generator connected to a public power grid. Controller 30 includes an input port 37 adapted to receive an actual grid frequency signal 39 measured by speed sensor 41, which measures the rotational speed of the induction generator, i.e., the rotational speed of the secondary shaft 5 relative to the stator of generator 3. To obtain the frequency signal 39, the generator speed can be filtered or averaged over a specific time range for smoothing. Controller 30 is adapted to control rotor winding 15 via rotor control signals 25a, 25b, 25c, which have a rotor winding reference frequency Ω_ref set according to the actual grid frequency f.

[0050] The output terminals 43a, 43b, and 43c of converter 17 are connected to the output terminals 45a, 45b, and 45c of stator winding 13. The stator power P_stator and rotor power P_rotor are connected to a step-up transformer, which is connected to a utility grid 47, on which other wind turbines are also connected. Utility grid 47 is intended to operate at the nominal frequency f0. However, due to interference or imbalances between power production and consumption, the frequency of the utility grid may occasionally deviate from the nominal frequency f0, for example, by a value f. The rotor control signals 25a, 25b, and 25c specifically have a rotor winding reference frequency (Ω_ref), which is set based on the deviation (f-f0) between the actual grid frequency f and the nominal grid frequency f0.

[0051] The dimensions of generator 3 and converter 17 are designed such that generator 3 can operate with a specific slip s (e.g., 12% according to an embodiment). This is because, in a doubly-fed induction generator, the total generator power is divided between stator power P_stator and rotor power P_rotor. Furthermore, rotor power P_rotor also affects the size and rating of converter 17. For a larger generator rotor power P_rotor, a larger converter 17 is required.

[0052] Typically, the rotor winding reference frequency may not be set according to the actual grid frequency. However, in this conventional case, the relative power contribution from rotor winding 15 and stator winding 13 changes depending on the grid frequency. For example, at an actual grid frequency of 47Hz, the generator rotor power can increase from 620kW to 950kW. Accordingly, the size of the converter (e.g., the grid-side section) should be designed to operate at this relatively high power, so an oversized design is necessary to avoid damage during operation. The optimal operating point is reached at an actual grid frequency of 50Hz. However, at an actual grid frequency of 53Hz, the generator stator power P_stator increases from 5510kW to 5860kW. The generator rotor power P_rotor decreases from 620kW to 280kW, but as the rotor voltage decreases (from 270V to 130V), the current in the generator rotor eventually increases, and the generator becomes hotter compared to the design operating point.

[0053] Under normal conditions, the rotor winding reference frequency is set to 1120 rpm for the actual grid frequency of 53 Hz, resulting in an increase in stator power P_stator = 5860 kW and a decrease in rotor power P_rotor = 280 kW. Furthermore, the slip is -6% here, which is not the optimal slip value.

[0054] According to an embodiment of the invention, the rotor winding reference frequency, and in particular the frequencies of the rotor control signals 25a, 25b, and 25c, are adapted to variations in the grid frequency, and especially to the deviation between the actual grid frequency and the nominal grid frequency. Here, even with varying actual grid frequencies, the rotor power and stator power remain substantially constant. Table 1 below shows the influence of different rotor winding reference frequencies on the relative power contributions of the rotor and stator at different grid frequencies of 50Hz and 53Hz.

[0055] Table 1:

[0056]

[0057] For an actual grid frequency of 50Hz, the synchronous speed ns = 1000rpm and the rotor winding reference frequency Ω_ref is set to = 1120rpm. Under this setting, the stator power is 5510kW and the rotor power is 620kW.

[0058] For the actual grid frequency of 53Hz, the synchronization speed is ns = 1060rpm and the rotor winding reference frequency Ω_ref is set to = 1180rpm. As can be seen from Table 1, the rotor power P_rotor(Prot) remains at 620kW and the stator power P_stator(Pst) remains at 5510kW, therefore unchanged compared to the 50Hz case. Furthermore, the slip s remains at -12% in both cases.

[0059] As can be observed from Table 1, the stator current remains at 4610 amps for both 50 Hz and 53 Hz frequencies, and the rotor current remains at 1680 amps for both grid frequencies.

[0060] Embodiments of the present invention may have the advantages of cost optimization; avoid over-specification design of systems and electrical components, ensure and increase the grid connection capability of wind turbines, and avoid potential reduction in the availability of wind turbines, etc.

[0061] According to an embodiment of the present invention, the generator speed can be maintained at an optimal distance from the synchronous speed (ns = 60 x f / p, p = number of pole pairs, f = frequency), and the generator's thermal selection and its associated converters can be in optimal condition. The generator speed can be adjusted according to the grid network frequency level, but can only be adjusted at the rated frequency (50 Hz or 60 Hz).

[0062] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude multiple. Elements described in conjunction with different embodiments may also be combined. It should also be noted that the reference numerals in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method for controlling an induction generator (3) connected to a public power grid (47), the method comprising: Receive the actual power grid frequency (f); as well as The rotor winding (15) of the generator (3) is controlled by rotor control signals (25a, b, c), the rotor control signals having a rotor winding reference frequency (Ω_ref) set according to the actual grid frequency (f). The rotor winding reference frequency (Ω_ref) is set such that the slip (s) follows a predetermined value and remains substantially unchanged with respect to the actual grid frequency (f), at least within a predetermined frequency range. The slip s is given by the following formula: s = (Ω(f) - ns(f)) / ns(f), in: f is the actual power grid frequency. Ω(f) is the actual speed of the generator at the actual grid frequency f. ns(f) is the synchronization speed at the actual power grid frequency f. Where ns is given in rpm by ns = 60 * f / p, where p is the number of pole pairs of the generator rotor, and f is given in Hz. The rotor winding reference frequency (Ω_ref) is set as follows: Ω_ref(f)=Ω(f0)*(1+(f-f0) / f0), where f is the actual power grid frequency. f0 is the nominal grid frequency. Ω_ref(f) is the rotor winding reference frequency at the actual power grid frequency. Ω(f0) is the rotor winding reference frequency at the nominal grid frequency. Ω(f0) = f0 * s, s is the slip.

2. The method according to claim 1, in, The public power grid (47) is designed to operate at the nominal grid frequency (f0). The rotor winding reference frequency (Ω_ref) is set based on the deviation between the actual grid frequency (f) and the nominal grid frequency (f0).

3. The method according to claim 1 or 2, in, The predetermined frequency range is between 0.90 and 1.1 times the nominal power grid frequency. Specifically, for the nominal grid frequency (f0) of 50Hz, the predefined frequency range is between 45Hz and 55Hz.

4. The method according to claim 3, in, The predetermined frequency range is between 0.97 and 1.03 times the nominal grid frequency.

5. The method according to claim 3, in, For a nominal grid frequency (f0) of 50 Hz, the predefined frequency range is between 47 Hz and 53 Hz.

6. The method according to claim 5, in, For a nominal grid frequency (f0) of 50 Hz, the predefined frequency range is between 48 Hz and 52 Hz.

7. The method according to claim 1 or 2, in, The rotor winding reference frequency (Ω_ref) is set such that the rotor power (Protor) output by the rotor winding follows a predefined relative rotor power and / or the stator power (Pstator) output by the stator winding follows a predefined relative stator power, and the rotor power and / or stator power do not change with the actual grid frequency (f) at least within the predefined frequency range.

8. The method according to claim 7, in, The dimensions of the stator windings (13) are designed according to the predetermined relative stator power such that they need to operate under a load that is no more than 1% higher than the predetermined relative stator power.

9. The method according to claim 8, in, The dimensions of the stator windings (13) are designed according to the predetermined relative stator power such that they need to operate under a load that is no more than 0.1% higher than the predetermined relative stator power.

10. The method according to claim 7, in, A converter (17) is connected to the rotor winding (15) to provide the rotor control signals (25a, b, c), wherein the converter includes an AC-DC converter section (19), a DC link (21), and a DC-AC converter section (23). The output terminals (43a, b, c) of the converter are connected to the output terminals (45a, b, c) of the stator winding (13).

11. The method according to claim 7, in, The dimensions of the rotor winding (15) and / or the converter (17) connected to the rotor winding are designed according to the predetermined relative rotor power such that they need to operate at a load no more than 1% higher than the predetermined relative stator power.

12. The method according to claim 11, in, The dimensions of the rotor winding (15) and / or the converter (17) connected to the rotor winding are designed according to the predetermined relative rotor power such that they need to operate at a load no more than 0.1% higher than the predetermined relative stator power.

13. The method according to claim 1 or 2, in, The actual grid frequency (f) is determined by averaging and / or filtering the instantaneous grid frequency over a predetermined time range.

14. The method according to claim 13, in, The actual grid frequency (f) is determined by averaging and / or filtering the instantaneous grid frequency over a time span between 1 minute and 10 minutes.

15. The method according to claim 1 or 2, in, The actual grid frequency (f) deviates from the nominal grid frequency (f0) by less than 5% or less than 3%, and / or The induction generator (3) includes a three-phase doubly fed induction generator and / or a squirrel-cage generator.

16. The method according to claim 1 or 2, in, The induction generator is driven by the rotating shaft (5) of the wind turbine (1), which is connected to the main shaft (9) via a gearbox (7), on which a plurality of rotor blades (11) are connected.

17. A device (35) for controlling an induction generator connected to a public power grid, said device comprising: Input port (37), which is adapted to receive the actual power grid frequency; as well as A controller (30) is adapted to control the rotor winding (15) of the generator (3) via rotor control signals (25a, b, c), the rotor control signals having a rotor winding reference frequency (Ω_ref) set according to the actual grid frequency (f). The rotor winding reference frequency (Ω_ref) is set such that the slip (s) follows a predetermined value and remains substantially unchanged with respect to the actual grid frequency (f), at least within a predetermined frequency range. The slip s is given by the following formula: s = (Ω(f) - ns(f)) / ns(f), in: f is the actual power grid frequency. Ω(f) is the actual speed of the generator at the actual grid frequency f. ns(f) is the synchronization speed at the actual power grid frequency f. Where ns is given in rpm by ns = 60 * f / p, where p is the number of pole pairs of the generator rotor, and f is given in Hz. The rotor winding reference frequency (Ω_ref) is set as follows: Ω_ref(f)=Ω(f0)*(1+(f-f0) / f0), where f is the actual power grid frequency. f0 is the nominal grid frequency. Ω_ref(f) is the rotor winding reference frequency at the actual power grid frequency. Ω(f0) is the rotor winding reference frequency at the nominal grid frequency. Ω(f0) = f0 * s, s is the slip.

18. A wind turbine (1), comprising: A wind-powered induction generator (3) having a rotor winding (15). as well as The apparatus (35) according to claim 17.

Images