CONTROL OF AN INDUCTION GENERATOR OF A WIND TURBINE

Abstract

A method of controlling an induction generator is provided connected to a utility grid, the method including: receiving an actual grid frequency; and controlling rotor windings of the generator by a rotor control signal having a rotor winding reference frequency being set in dependence of the actual grid frequency.

Claims

1. A method of controlling an induction generator connected to a utility grid, the method comprising: receiving an actual grid frequency; and controlling rotor windings of the generator by a rotor control signal having a rotor winding reference frequency (Ω_ref) being set in dependence of the actual grid frequency.

2. The method according to claim 1, wherein the utility grid is configured to be operated with a nominal grid frequency, wherein the rotor winding reference frequency is set in dependence of a deviation between the actual grid frequency and the nominal grid frequency.

3. The method according to claim 1, wherein the rotor winding reference frequency (Ω_ref) is set such that a slip adheres to a predefined value and substantially does not change with changing actual grid frequency at least in a predefined frequency range, wherein the slip s is given by:
s=(Ω(f)−ns(f))/ns(f), wherein: f is the actual grid frequency, Ω is the actual speed of generator at the actual grid frequency f, ns(f) is a synchronous speed at the actual grid frequency f, wherein ns in rpm is given by ns=60*f/p, wherein p is the number of pole pairs of the generator rotor and f is given in Hz.

4. The method according to claim 1, wherein the predefined frequency range is between 0.90 and 1.1, or between 0.97 and 1.03 times the nominal grid frequency, wherein for a nominal grid frequency of 50 Hz the predefined frequency range is between 45 Hz and 55 Hz, in particular between 47 Hz and 53 Hz, or between 48 Hz and 52 Hz.

5. The method according to claim 1, wherein the rotor winding reference frequency (Ω_ref) is set such that a rotor power (Protor) output by the rotor windings adheres to a predefined relative rotor power and/or stator power output (Pstator) by stator windings adheres to a predefined relative stator power and the rotor power and/or stator power does not change with changing actual grid frequency, at least in the predefined frequency range.

6. The method according to claim 1, wherein the stator windings are dimensioned according to the predefined relative stator power such that they are required to be operated with a load not higher than 1%, in or 0.1%, above the predefined relative stator power.

7. The method according to claim 1, wherein a converter is connected to the rotor windings for supplying the rotor control signal, wherein the converter in particular comprises a AC-DC converter portion, a DC-link, and a DC-AC converter portion, wherein output terminals of the converter are connected to output terminals of the stator windings.

8. The method according to claim 1, wherein the rotor windings and/or a converter connected to the rotor windings are dimensioned according to the predefined relative rotor power such that they are required to be operated with a load not higher than 1%, or 0.1%, above the predefined relative rotor power.

9. The method according to claim 1, wherein the rotor winding reference frequency (Ω_ref) is set as follows:
Ω_ref(f)=Ω(f0)*(1+(f−f0)/f0), wherein 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, Ω(f0) is the rotor winding reference frequency at the nominal grid frequency, Ω(f0)=f0*s, s is the slip.

10. The method according to claim 1, wherein the actual grid frequency is determined as averaging and/or filtering an instantaneous grid frequency over a predetermined time range, spanning between 1 min and 10 min.

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

12. The method according to claim 1, wherein the induction generator is driven by a rotating shaft of a wind turbine, in or coupled via a gear box to a main shaft at which plural rotor blades are connected.

13. An arrangement for controlling an induction generator connected to a utility grid, the arrangement comprising: an input port configured to receive an actual grid frequency; and a controller configured to control rotor windings of the generator by a rotor control signal having a rotor winding reference frequency (Ω_ref) being set in dependence of the actual grid frequency.

14. A wind turbine, including: an induction generator driven by wind energy, the generator having rotor windings; and an arrangement according to claim 13.

Description

BRIEF DESCRIPTION

[0040] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0041] The FIGURE schematically illustrates a wind turbine according to an embodiment of the present invention.

DETAILED DESCRIPTION

[0042] The wind turbine 1 according to an embodiment of the present invention illustrated in the FIGURE includes a doubly-fed induction generator 3. The induction generator 3 is driven by a secondary shaft 5 which is coupled to a gear box 7. The gear box 7 converts a relatively low rotational speed of a main shaft 9 of the wind turbine to a relatively high rotational speed of the secondary shaft 5. At the main shaft 9 plural rotor blades 11 are connected, which are driven by impacting wind 13. A not illustrated nacelle mounted on top of a wind turbine power harbors the generator 3, the shafts 9, 5, the gear box 7 and a converter 17.

[0043] The generator 3 comprises stator windings 13 as well as rotor windings 15 both of which are only schematically illustrated. The stator windings 13 may for example comprise three-phase windings or coils which are wound around teeth of a ferromagnetic stator yaw. The rotor windings 15 may also comprise for example three-phase rotor windings wound around ferromagnetic cores.

[0044] The three-phase rotor windings 15a, 15b, 15c are connected to a converter 17 comprising 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 both directions. The converter 17 is capable of supplying a control signal (for example three-phase signal) 25a, 25b, 25c to the rotor windings 15.

[0045] Therefore, the converter 17 receives a driving signal 27 from a controller 30 which, beside the converter 17, may also control other portions of the wind turbine. The controller 30 is part of an arrangement 35 for controlling an induction generator connected to a utility grid according to an embodiment of the present invention. The controller 30 comprises an input port 37 adapted to receive an actual grid frequency signal 39 measured by a speed sensor 41 measuring the rotational speed of the induction generator, namely the rotational speed of the secondary shaft 5 rotating relative to a stator of the generator 3. In order to obtain the frequency signal 39, the generator speed may be filtered or average over particular time ranges for smoothing. The controller 30 is adapted to control the rotor windings 15 by the rotor control signal 25a, 25b, 25c which has a rotor winding reference frequency Ω_ref being set in dependence of an actual grid frequency f.

[0046] The output terminals 43a, 43b, 43c of the converter 17 are connected with output terminals 45a, 45b, 45c of the stator windings 13. The stator power P_stator as well as the rotor power P_rotor are connected to a step-up transformer and this is connected to the utility grid 47 where other wind turbines are also connected. The utility grid 47 is intended to be operated at a nominal frequency f0. However, due to disturbances or imbalances between power production and power consumption, the frequency of the utility grid may occasionally deviate from the nominal frequency f0 for example having a value f In particular, the rotor control signal 25a, 25b, 25c has a rotor winding reference frequency (Ω_ref), which is set in dependence of a deviation (f−f0) between the actual grid frequency f and the nominal grid frequency f0.

[0047] The size of the generator 3 as well as that of the converter 17 are designed such that the generator 3 can be operated having a specific slip s (for example 12% according to an embodiment). The reason may be that in a doubly-fed induction generator there is a split of total generator power between the stator power P_stator and rotor power P_rotor. Furthermore, the rotor power P_rotor also effects the size and the rating of the converter 17. For larger generator rotor power P_rotor, the larger the converter 17.

[0048] Conventionally, the rotor winding reference frequency may not have been set in dependence of the actual grid frequency. In this conventional case, the relative power contributions from the rotor windings 15 and the stator windings 13 however change depending on the grid frequency. For example, at an actual grid frequency of 47 Hz, the generator rotor power may increase from 620 kW to 950 kW. Accordingly, the converter (e.g., the grid side portion) should be sized to work with this relatively high power so should be over-dimensioned in order to avoid damage during operation. At 50 Hz actual grid frequency, the optimal operating point is reached. However, at 53 Hz actual grid frequency, the generator stator power P_stator increases from 5510 kW to 5860 kW. The generator rotor power P_rotor is reduced from 620 kW to 280 kW, but as the rotor voltage is more reduced (from 270 V to 130 V), finally the currents in the generator rotor are higher and the generators become hotter compared to the design operation point.

[0049] For the conventional case, the rotor winding reference frequency was set for 53 Hz actual grid frequency at 1120 rpm resulting in an increase of the stator power P_stator=5860 kW and to a decrease of the rotor power P_rotor=280 kW. Furthermore, the slip was thereby −6%, i.e., not the optimal value for the slip.

[0050] According to an embodiment of the present invention, the rotor winding reference frequency, in particular the frequency of the rotor control signal 25a, 25b, 25c is adapted in dependence of the grid frequency variation, in particular in dependence of the deviation of the actual grid frequency from the nominal grid frequency. Thereby, the rotor power as well as the stator power substantially remains constant even for changing actual grid frequencies. In the following Table 1, the effects of different rotor winding reference frequencies for different grid frequencies on the relative power contributions of the rotor and the stator are indicated for grid frequencies of 50 Hz and 53 Hz.

TABLE-US-00001 TABLE 1 f 50 Hz f 53 Hz ns 1000 rpm ns 1060 rpm Ω_ref 1120 rpm Ω_ref 1180 rpm s −12% s −12% P_stator 5510 kW P_stator 5510 kW P_rotor 620 kW P_rotor 620 kW V_rotor 270 V V_rotor 270 V I_stator 4610 A I_stator 4610 A I_rotor 1680 A I_rotor 1680 A

[0051] For an actual grid frequency of 50 Hz the synchronous speed ns=1000 rpm and the rotor winding reference frequency Ω_ref is set to=1120 rpm. With this setting the stator power is 5510 kW and the rotor power is 620 kW.

[0052] For an actual grid frequency of 53 Hz, the synchronous speed is ns=1060 rpm and the rotor winding reference frequency Ω_ref is set=1180 rpm. As can be seen from Table 1, the rotor power P_rotor (Prot) stays at 620 kW and the stator power P_stator (Pst) stays at 5510 kW, thus unchanged relative to the situation at 50 Hz. Furthermore, also the slip s remains for both cases at −12%.

[0053] As can also be observed from Table 1, the stator current stays for both frequencies of 50 Hz and 53 Hz at 4610 Ampere and the rotor current stays also for both grid frequencies at 1680 ampere.

[0054] Embodiments of the present invention may have the advantage of cost-optimization; avoidance of over-specifications of systems and electrical components secured and increased grid connection capabilities of a wind turbine, avoidance of possible reduction of wind turbine availabilities and others.

[0055] According to an embodiment of the present invention, the generator speed may be maintained at the optimum distance from the synchronous speed (ns=60×f/p, with p=number of pole pairs, f=frequency), the thermal sizing of generator and its related converter may be at the optimum. The speed of the generator may be adjusted in dependence of the grid network frequency level but may only be focussed in the rated frequency (50 Hz or 60 Hz).

[0056] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0057] For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.