Method for controlling a wind turbine

Abstract

A method for controlling a wind turbine comprising inputting an actual value of rotational speed and a rotational speed setpoint into a control and outputting a set point value for a generator torque from the control. Inputting the set point value for the generator torque into a limiter with a predefinable upper and lower limit and outputting a limited torque value that is fed into a converter control. Increasing the actual value of the fed-in electrical power by an additional amount of power in response to a boost signal, wherein the fed-in electrical power and the additional amount of power are combined into an aggregated power setpoint value. Determining the set point value for the generator torque from the aggregated power setpoint value and applying the set point value for the generator torque in the boost operation to the limiter both as the upper limit and as the lower limit.

Claims

1. A method for controlling a wind turbine having a generator that is controlled via a converter where electrical power that is fed into an electrical transmission network is increased by a generative deceleration of the generator, the method comprising: inputting an actual value of rotational speed (n.sub.meas) and a rotational speed setpoint (n*) into a control; outputting a set point value (N*) for a generator torque from the control; inputting the set point value (N*) for the generator torque into a limiter with a predefinable upper and lower limit (N.sub.up, N.sub.low); and outputting a limited torque value (N*), wherein the limited torque value (N*) is applied to a control of a converter; increasing the actual value of the electrical power fed into an electrical transmission network (P.sub.freeze) by an additional amount of power P.sub.inc in response to a boost signal (flag.sub.boost), wherein electrical power fed into an electrical transmission network (P.sub.freeze) and the additional amount of power P.sub.inc are combined into an aggregated power setpoint value (P.sub.Total); determining the set point value for a generator torque (N*.sub.boost) from the aggregated power setpoint value (P.sub.Total); and applying the set point value for the generator torque (N*.sub.boost) in a boost operation to the limiter both as the upper limit and as the lower limit, wherein the boost signal (flag.sub.boost) is generated when a frequency in the electrical transmission network falls below a first predetermined frequency value (f.sub.trigger), and wherein the boost signal (flag.sub.boost) is reset when at least one of a predetermined period of time (T.sub.boost) has elapsed since the boost signal (flag.sub.boost) has occurred and the actual value of the frequency (f.sub.meas) in the electrical transmission network is greater than a second predetermined frequency value (f.sub.reset), and wherein the second predetermined frequency value (freset) is greater than the first predetermined frequency value (f.sub.trigger).

2. The method according to claim 1, wherein the boost signal (flag.sub.boost) is generated when a temporal change of a frequency in the electrical transmission network exceeds a first predetermined gradient value.

3. The method according to claim 2, further comprising a plurality of different gradient values that each relate a change in frequency in the electrical transmission network to a predetermined period of time.

4. The method according to claim 1, wherein the set point value for the generator torque (N*) is calculated from the actual value of the electrical power fed into an electrical transmission network(P.sub.freeze) when the boost signal (flag.sub.boost) occurs and an actual value of the rotational speed (n.sub.meas).

5. The method according to claim 1, wherein on of a transition signal trans (flag.sub.trans) and a recovery signal (flag.sub.rec) is generated when the boost signal (flag.sub.boost) is reset.

6. The method according to claim 5, further comprising: setting the upper and the lower limit (N.sub.up, N.sub.low) at the limiter to predetermined values from a normal operation of the wind turbine in response to one of the transition signal (flag.sub.trans) and the recovery signal (flag.sub.rec); and controlling a temporal change of the set point value for the generator torque (dN*/dt) via a ramp function.

7. The method according to claim 6, wherein when the boost signal (flag.sub.boost) is reset, the recovery signal (flag.sub.rec) is generated and a maximum permissible constant value for temporal change of the set point value for the generator torque (dN*/dt) is determined depending on a constant temporal change of the rotational speed (dn*/dt).

8. The method according to claim 6, wherein when the boost signal (flag.sub.boost) is reset, the recovery signal (flag.sub.rec) is generated and a maximum permissible constant value for temporal change of the set point value for the generator torque (dN*/dt) is determined depending on the constant temporal change of the rotational speed via a control with at least one of a P element and a PI element, wherein a constant term (dn*/dt) is added to the temporal change of an actual value (dn.sub.meas/dt) of the rotational speed.

9. The method according to claim 6, wherein when the boost signal (flag.sub.boost) is reset, the transition signal (flag.sub.trans) is generated and a maximum permissible constant value for temporal change of the set point value for the generator torque (dN*/dt) is set to a constant value.

10. The method according to claim 9, wherein the transition signal (flag.sub.trans) is reset and the recovery signal (flag.sub.rec) is set when the actual value of the rotational speed (n.sub.meas) increases and a measured electrical power (P.sub.meas) is smaller than an available power (P.sub.aero).

11. The method according to claim 10, wherein the maximum constant value for temporal change of the set point value of the generator torque (dN*/dt) is determined depending on the actual value of the rotational speed (n.sub.meas) and the available power (P.sub.aero) when the recovery signal (flag.sub.rec) is set.

12. The method according to claim 10, wherein the maximum constant value for temporal change of the set point value of the generator torque (dN*/dt) is determined using an actual value of the power (P.sub.meas) that is fed in when the recovery signal occurs and on the actual value of the rotational speed (n.sub.meas) when the recovery signal (flag.sub.rec) is set.

13. The method according to claim 10, wherein the recovery signal (flag.sub.rec) is reset when the set point value of the generator torque (N*) is within a range specified by the upper limit (N.sub.up) and the lower limit (N.sub.low) of the limiter.

14. A method for controlling a wind turbine having a generator that is controlled via a converter where electrical power that is fed into an electrical transmission network is increased by a generative deceleration of the generator, the method comprising: inputting an actual value of rotational speed (n.sub.meas) and a rotational speed setpoint (n*) into a control; outputting a set point value (N*) for a generator torque from the control; inputting the set point value (N*) for the generator torque into a limiter with a predefinable upper and lower limit (N.sub.up, N.sub.low); and outputting a limited torque value (N*), wherein the limited torque value (N*) is applied to a control of a converter; increasing the actual value of the electrical power fed into an electrical transmission network (P.sub.freeze) by an additional amount of power P.sub.inc in response to a boost signal (flag.sub.boost), wherein electrical power fed into an electrical transmission network (P.sub.freeze) and the additional amount of power P.sub.inc are combined into an aggregated power setpoint value (P.sub.Total); determining the set point value for a generator torque (N*.sub.boost) from the aggregated power setpoint value (P.sub.Total); and applying the set point value for the generator torque (N*.sub.boost) in a boost operation to the limiter both as the upper limit and as the lower limit, wherein the set point value for the generator torque (N*) is calculated from the actual value of the electrical power fed into an electrical transmission network(P.sub.freeze) when the boost signal (flag.sub.boost) occurs and an actual value of the rotational speed (n.sub.meas), and wherein a value for boost power is a predetermined fraction of one of a rated power (P.sub.N) and the actual value of the electrical power fed into an electrical transmission network (P.sub.freeze) when the boost signal occurs.

15. The method of claim 14, wherein the boost signal (flag.sub.boost) is generated when a frequency in the electrical transmission network falls below a first predetermined frequency value (f.sub.trigger).

16. The method according to claim 14, wherein the value for the boost power is determined using an actual value of a frequency (f.sub.meas) in the electrical transmission network.

17. A method for controlling a wind turbine having a generator that is controlled via a converter where electrical power that is fed into an electrical transmission network is increased by a generative deceleration of the generator, the method comprising: inputting an actual value of rotational speed (n.sub.meas) and a rotational speed setpoint (n*) into a control; outputting a set point value (N*) for a generator torque from the control; inputting the set point value (N*) for the generator torque into a limiter with a predefinable upper and lower limit (N.sub.up, N.sub.low); and outputting a limited torque value (N*), wherein the limited torque value (N*) is applied to a control of a converter; increasing the actual value of the electrical power fed into an electrical transmission network (P.sub.freeze) by an additional amount of power P.sub.inc in response to a boost signal (flag.sub.boost), wherein electrical power fed into an electrical transmission network (P.sub.freeze) and the additional amount of power P.sub.inc are combined into an aggregated power setpoint value (P.sub.Total); determining the set point value for a generator torque (N*.sub.boost) from the aggregated power setpoint value (P.sub.total); and applying the set point value for the generator torque (N*.sub.boost) in a boost operation to the limiter both as the upper limit and as the lower limit, wherein the set point value for the generator torque (N*) is calculated from the actual value of the electrical power fed into an electrical transmission network(P.sub.freeze) when the boost signal (flag.sub.boost) occurs and an actual value of the rotational speed (n.sub.meas), and wherein the value for the boost power is determined using a measured change in frequency per time in the electrical transmission network.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A preferred exemplary embodiment of the invention is explained in greater detail below based on the figures. In the figures:

(2) FIG. 1a illustrates a side plan view of an embodiment of a wind turbine;

(3) FIG. 1b illustrates a schematic view of an embodiment of a wind turbine feeding into the electrical transmission network;

(4) FIG. 2 illustrates a schematic diagram of an embodiment of a torque control of a converter;

(5) FIG. 3 illustrates a schematic diagram for determining the upper and lower limit for the limiter;

(6) FIG. 4 illustrates a schematic diagram for determining the set point value for the generator torque;

(7) FIG. 5 illustrates a schematic diagram for determining the upper and lower limit during transition operation or recovery operation;

(8) FIG. 6 illustrates a schematic diagram showing the process of switching from transition operation to recovery operation;

(9) FIG. 7a illustrates an embodiment of a method for operating the wind turbine in transition and/or recovery operation;

(10) FIG. 7b illustrates a second embodiment of a method for operating the wind turbine in transition and/or recovery operation;

(11) FIG. 7c illustrates a third embodiment of a method for operating the wind turbine in transition and/or recovery operation;

(12) FIG. 7d illustrates a fourth embodiment of a method for operating the wind turbine in transition and/or recovery operation;

(13) FIG. 8a illustrates the qualitative course of effective power in the electrical transmission network over time;

(14) FIG. 8b illustrates the qualitative course of generator torque in the electrical transmission network over time;

(15) FIG. 8c illustrates the qualitative course of rotational speed in the electrical transmission network over time; and

(16) FIG. 8d illustrates the qualitative course of grid frequency in the electrical transmission network over time.

DETAILED DESCRIPTION OF THE INVENTION

(17) FIG. 1a shows a schematic view of a wind turbine 10 with a tower 11, a nacelle 12, and a rotor 14. The rotor supports multiple rotor blades 16 that take up power from the wind. FIG. 1b shows an embodiment of a wind turbine 10 with a double-fed Asynchronous generator 18 which is connected to an electrical transmission network 25, on the rotor-side via a converter 20 and on the stator-side directly, i.e., without insertion of a converter. Alternatively, the wind turbine can also be configured with a full-scale converter. Feeding into the electrical transmission network 25 takes place, for example, via a transformer 24. The wind turbine 10 can be part of a wind farm consisting of multiple wind turbines 10 that, for example, are connected to the electrical transmission network 25 via a wind farm collector bus and a high-voltage transformer. The wind turbine 10 has a control 26 that is connected for data communication to the converter 20 or respectively to its control. The control 26 is connected for data communication to a wind farm control 27 that is configured for controlling multiple wind turbines 10. The method according to the invention can preferably be implemented in the control 26. In principle, the invention can also be employed on the level of the wind farm in that set point values are determined for each individual wind turbine 10 by the wind farm control 27 and a frequency is measured at the feed-in point or in the wind farm collector bus.

(18) FIG. 2 shows a schematic view of a control 26 for the operation of the wind turbine. A torque set point value N* for the converter is determined via a PI control 28 from a control deviation formed using a rotational speed set point value n* and an actual value of the rotational speed n.sub.meas. The torque set point value N* is applied to a limiter 102. The limiter is a limiter with an adjustable lower and upper limit (dynamic limiter). The lower limit N.sub.low and the upper limit N.sub.up are determined by a limiter control 200. The limiter control 200 can also be provided with procedures for braking the wind turbine or have data sets for determining the lower and upper limits for other operating modes. In the present case, the situation of virtual inertia and its procedure, in which rotational energy is captured from the rotating part of the wind turbine when the fed-in electrical power is increased, is discussed. The limiter 102 limits the applied torque set point value N* and provides a limited torque set point value N*. The output value N* of the limiter 102 is used to control the generator torque of the wind turbine. The control of the generator torque takes place depending on the generator used. In the case of the double-fed induction machine 18 mentioned above, the generator torque is adjusted, for example, by actuating the converter. The generator torque is the electrical generated counter-torque to the mechanical torque of the rotor of the wind turbine that must be applied to the generator in order to generate electrical power. In the example represented in FIG. 2, the set point value for the generator torque N* is applied to a control of a converter 30.

(19) Virtual inertia operation uses a frequency f.sub.meas measured at the connecting terminals of the wind turbine. The measured frequency value f.sub.meas represents the actual value and allows a deviation from the rated value of the grid frequency to be detected. In a wind farm with multiple wind turbines that are connected together, a farm control receives the measured frequency and forwards the measured frequency without further processing to all the connected wind turbines or induces a corresponding procedure at the wind turbine. Alternatively, the frequency can also be measured locally for one or multiple turbines, which avoids a delay in operating time and a communication effort in the wind farm.

(20) The frequency measurement is evaluated in the control of the wind turbine in order to trigger the virtual inertia function. When the frequency falls below a triggering frequency f.sub.trigger as the first predetermined frequency value, then the virtual inertia function is activated in the limiter control 200. If the virtual inertia function is activated, a corresponding boost signal is set and a series of operating states is run through: a boost operation and a recovery operation and possibly an interposed transition operation. The triggering of boost operation will be described in the following with reference to FIG. 3 and FIG. 4.

(21) FIG. 4 describes the generation of the boost signal flag.sub.boost as well as the set point value for the generator torque N*.sub.boost. The input variable for the generation of the boost signal flag.sub.boost is the actual value of the frequency f.sub.meas in the electrical transmission network. The chosen representation shows the boost signal represented as a flag bit, which could also be represented by a state machine or in another manner. In step 302, the actual value of the frequency f.sub.meas is compared with a first predetermined frequency value for the triggering frequency f.sub.trigger. If the actual value is smaller than the first predetermined frequency value, the output of the corresponding recognition circuit 304 is set to 1. The output signal of the recognition circuit 304 is applied to the SET input of a flip-flop 306 so that the boost signal flag.sub.boost is output when an underfrequency at its Q output is recognized. If the actual value of the frequency during a comparison 308 is greater than a second predetermined frequency value for a reset frequency f.sub.reset, this is recognized in the recognition circuit 310 and a corresponding 1 value is applied to the input of the OR circuit 312 by the recognition circuit 310. At the other input of the OR circuit 312, the output of a comparator 316 is applied which outputs a 1 value when the boost signal flag.sub.boost is applied to the comparator for a boost period T.sub.boost. The boost period is detected via a time element 314. Further conditions for the OR circuit 312 are described in the following. The output of the OR circuit 312 is applied to the RESET input of the flip-flop 306 which serves to set the Q output to zero so that the boost signal flag.sub.boost is no longer being output.

(22) The boost signal flag.sub.boost is applied to an IF-THEN branch 318 insofar as it is generated by the flip-flop 306. When the boost signal flag.sub.boost is applied to the IF-THEN branch 318, this outputs a frozen electrical power P.sub.freeze at its output. The frozen power P.sub.freeze is fed back via a time element 320. With the frozen power P.sub.freeze, the actual value of the electrical power is frozen before the boost signal occurs and is used as a basis for the control during boost operation. If the boost signal flag.sub.boost is not or is no longer applied to the IF-THEN branch 318, the IF-THEN branch 318 is configured to output an applied actual value of the electrical power P.sub.meas currently delivered by the wind turbine as an ELSE value.

(23) In an additional block 301, a power P.sub.inc to be additionally provided is determined. To determine the power P.sub.inc, different approaches can be chosen that can be selected with the selection elements 322 and 324. One approach is based on the rated power P.sub.n. In an alternative embodiment, the frozen power P.sub.freeze can be taken as the basis. Both variables are applied to selection element 322, which is configured, depending on which method is selected, to apply one of the variables to a multiplication element. The multiplication element is equipped to scale the applied power value by a percentage in order to determine the power P.sub.inc to be additionally provided. The percentage can be determined by another circuit element (not shown), e.g., depending on the measured grid frequency f.sub.meas. Alternatively, it is also possible for a calculation element 326 to determine a frequency-dependent power value P.sub.boost(f). In the represented exemplary embodiment, a selection element 324 allows the selection of which of the determined power values are output from the block 301 as P.sub.inc. However, it is also possible to provide only one of the variations and to parameterize the solution preferred for a wind farm project in the software. The frozen power P.sub.freeze and the additional power P.sub.inc are added up to an aggregated power set point value P.sub.total by means of an addition element and are divided by the current actual value of the rotational speed n.sub.meas by means of a division element in order to determine the set point value for the generator torque N*.sub.boost.

(24) In summary, it can be noted regarding the procedure in FIG. 4 that, when the frequency below the frequency value f.sub.trigger is detected, a boost signal flag.sub.boost is generated for a predefinable boost period T.sub.boost. By using a second predetermined frequency f.sub.reset, the boost mode can be exited early when the grid frequency returns to its normal range. Further possible conditions for an early exit of boost operation can be, for example, reaching a minimum generator rotational speed below which the wind turbine cannot be operated continuously. By resetting the boost signal flagboost when the minimum generator rotational speed is reached, a switch-off of the wind turbine as the result of boost operation can be avoided. Likewise, a minimum speed or a maximum rotational speed reduction can be defined for the generator in order to avoid stall operation and shutdown of the wind turbine. For this purpose, corresponding input values can additionally be applied to the OR circuit 312 so that boost operation, when corresponding signals are applied, is terminated early and the boost signal flag.sub.boost is reset.

(25) The signals determined according to the exemplary embodiment from FIG. 4 are used, according to the block diagram from FIG. 3, for determining the upper limit N.sub.up and the lower limit N.sub.low for the limiter 102. FIG. 3 represents the limiter control 200. To determine the upper and lower limit for the limiter 102, the actual value of the rotational speed n.sub.meas, the actual value of the frequency f.sub.meas and the actual value of the fed-in electrical power P.sub.meas are applied to the block 300 in FIG. 3. The block 300 possesses the design explained with reference to FIG. 4. The output variables from block 300 are the boost signal flag.sub.boost and the set point value for the torque N*.sub.boost in boost operation. The two signals are each applied to an IF-THEN branch 201 or 202, respectively, and are checked by the branches against the IF condition. In addition, the values for the maximum torque N.sub.max or respectively the minimum torque N.sub.min provided for normal operation is applied to the branch 201 or respectively 202 as ELSE alternatives. If the boost signal flag.sub.boost is set, N*.sub.boost is output as the upper limit N*.sub.up 203 and as the lower limit N*.sub.low 204. If there is no boost signal, the value N.sub.max is set as the upper limit N*.sub.up and N.sub.min is set as the lower limit N*.sub.low for the limiter.

(26) The upper and lower limits determined by the IF-THEN branches are applied to a uniform control block 400. The uniform control block 400 also possesses the boost signal as well as the actual value of the rotational speed n.sub.meas and the actual value of the fed-in electrical power P.sub.meas as further input signals. This is in addition to the value of the power P.sub.aero currently available from the wind flowing over the rotor of the wind turbine. In the uniform control block 400, the upper and lower limits N.sub.low and N.sub.up for the limiter 102 are calculated from the applied set point values for the upper limit N*.sub.up and the lower N*.sub.low. The uniform control block 400 is configured to control the torque of the wind turbine depending on the applied input variables in each operating state that can occur when carrying out a virtual inertia function.

(27) The manner of operation of the uniform control block 400 is described in more detail with reference to FIG. 5. The conversion of the set point values N*.sub.low and N*.sub.up into control variables N.sub.low and N.sub.up takes place via ramp functions 401, 402. The ramp functions are activated via a ramp signal ramp.sub.active. The activation of the ramp function takes place when either the transition signal flag.sub.trans or the recovery signal flag.sub.rec is set by the switching block 500. In addition, the set point value N* for the generator torque that is obtained from the torque control must be smaller at the comparator 418 than the set point value for the lower torque limit N*.sub.low. Not represented in FIG. 5 is the further alternative of providing a second comparator that additionally checks whether the set point value for the generator torque N* is potentially also greater than the set point value for the upper limit N*.sub.up. In any case, it is a necessary requirement for the activation of the ramp functions 401 and 402 that the set point value N* for the generator torque is outside of the range defined by the set point values for the generator torque N*.sub.low and N*.sub.up.

(28) If the ramp functions are activated, the ramp ramp.sub.low is applied to the ramp function 401 for the lower limit at the limiter and the ramp ramp.sub.up is applied to the ramp function 402 for the upper limit at the limiter. The ramps are maximum permissible temporal changes for the generator torque. For the ramp function 402, constant values for the ramp ramp.sub.up are preset in a memory block 420.

(29) The calculation of the ramp functions takes place in block 600. FIG. 7 shows four exemplary embodiments for the calculations of the ramps in block 600. During the calculation of the ramp functions, it must be distinguished whether transition operation for the wind turbine is provided after the termination of boost operation or whether the control switches directly to recovery operation. FIGS. 7a and 7b relate to an operating mode in which the wind turbine switches directly to recovery mode, while FIGS. 7c and 7d characterize an interposed transition zone. The control signals ramp.sub.rec and ramp.sub.trans correspond to set variables for the generator torque.

(30) FIG. 7a shows the determination of a select signal ramp.sub.rec for the ramp function. The select signal ramp.sub.rec is obtained, for example, from a look-up table in which the temporal change of the generator torque is provided as a function of the change of the rotational speed. The change of the rotational speed is determined from the actual value of the rotational speed n.sub.meas via a differentiating element 606. In the look-up table 604, the ratio of the power P.sub.aero that can be captured from the wind and the electrical power P.sub.meas that is fed in is determined. In the look-up table, it must be considered that the rotational energy in the wind turbine increases as the rotational speed rises, while the electrical power P.sub.el that is fed in also decreases as the generator torque N decreases. The maximum permissible change of the generator torque is limited via the control signal ramp.sub.rec. When applied to the ramp functions, ramp.sub.rec can be applied both as ramp.sub.low to the ramp function 401 and as ramp.sup.up to the ramp function 402.

(31) FIG. 7b shows an alternative embodiment in which a constant change term

(32) ${\frac{{\mathrm{dn}}^{*}}{\mathrm{dt}}}_{/\mathrm{const}}$
is added to the temporal derivative of the actual value of the rotational speed. The change in rotational speed that is excessively increased in this way is corrected via a control 610. The control 610 has a proportional element and can be designed as a P, PI, or PID control. The output variable of the control 610 is the control signal ramp.sub.rec for the actuation of the ramp function 401 and 402.

(33) FIG. 7c shows an example for calculating the control signals ramp.sub.trans and ramp.sub.rec for the ramp functions when there is a differentiation between transition operation and recovery operation after boost operation. During transition operation, a constant value

(34) ${\frac{{\mathrm{dN}}^{*}}{\mathrm{dt}}}_{/\mathrm{const}}$
is set for the maximum permissible change of the generator torque and used as the control signal ramp.sub.trans. This setting is temporally constant for the transition zone. If a switch to recovery operation occurs after transition operation, a quotient variable is subtracted, on the basis of the lower limit N.sub.low that was present before and that is set back by one time increment by means of a time element z.sup.1, by a differentiating element. The quotient variable is composed of the quotient that is formed by the difference between the power P.sub.aero that is available from the wind and a preset power difference P. The denominator of the quotient is formed by the actual value of the rotational speed. The power divided by the rotational speed corresponds to a torque, wherein the quotient can be interpreted as a torque that should be set in order to adjust a constant power differential P below the available power P.sub.aero. The present generator torque is reduced by this variable. The difference is divided by a preset time span T.sub.sample by means of a division element in order to obtain in this manner a maximum permissible change in torque as the control signal ramp.sub.rec for the ramp function. The central idea in recovery operation is that the power differential P is no longer captured from the wind in order to increase in this manner the rotational energy of the wind turbine.

(35) Another embodiment is explained in FIG. 7d. Here, too, the control signal 602 is set for the ramp function during transition operation to a constant value

(36) ${\frac{{\mathrm{dN}}^{*}}{\mathrm{dt}}}_{/\mathrm{const}}.$
In recovery operation, the control signal ramp.sub.rec is calculated with a similar approach as in FIG. 7c. However, the torque to be subtracted that is applied to the differentiating element is not determined based on a preset power difference, but is determined on the basis of the actual power value P.sub.meas and the actual value for the rotational speed. As long as the recovery signal flag.sub.rec is an applied to IF-THEN, branch, the actual power value is held constant via a time element z.sup.1, i.e., it is held at the value before the switch to recovery operation occurs. For the rotational speed n.sub.meas, the current actual value is chosen that is applied to a division element together with the power value. The central idea in this embodiment is that, by holding the power value, a proportional correction of the generator torque takes place corresponding to the actual value of the power when recovery operation begins.

(37) With reference to the uniform control block 400, this means that, when the transition signal flag.sub.trans is applied to the IF-THEN branch 403, the control signal ramp.sub.low for the ramp function 401 corresponds to the calculated control signal ramp.sub.trans. If, however, the recovery signal flag.sub.rec is applied to the IF-THEN branch 403, the control signal ramp.sub.low for the ramp function 401 is set by the control signal 616 ramp.sub.rec. Switching from the boost signal (flag.sub.boost) to the transition signal (flag.sub.trans) or the recovery signal (flag.sub.rec) is represented in the switching block 500 in FIG. 5. A possible more detailed design for the switching block 500 can be found in FIG. 6.

(38) Central elements for switching are two flip-flop circuits 502, 504. The boost signal flag.sub.boost, which applies an input signal to the SET input of the flip-flop from the ON state to the OFF state via a recognition circuit 506 in the case of a reduction of the boost signal and thereby generates a transition signal flag.sub.trans at the Q output of the flip-flop circuit 502, is applied to the SET input of the flip-flop circuit 502. The end condition for the transition zone is triggered by setting the RESET input of the flip-flop 502. It can either be taken into account for a termination of the transition zone that the change in rotational speed is positive. This means the rotational speed increases again so that the transition signal flag.sub.trans can be reset. Alternatively to an increase in rotational speed, a signal can also be applied to the RESET input of the flip-flop circuit when the measured power P.sub.meas is smaller than the power P.sub.aero available at the time from the wind.

(39) In both cases, the transition zone is terminated and the transition signal flag.sub.trans is reduced. In response to the reduction of the transition signal flag.sub.trans, the detection unit 508 applies a signal to the SET input of the flip-flop circuit 504. Correspondingly, the recovery signal flag.sub.rec is set at the Q output. Recovery operation then continues until the output signal of the comparator 510 is applied to the RESET input of the flip-flop circuit 504, which indicates when the set point value for the generator torque N* is greater than the lower limit N.sub.low at the limiter. In addition, another comparator (not shown) can be provided that compares whether the set point value N* is smaller than the upper limit value N.sub.up at the limiter.

(40) The functionality of transition operation and recovery operation can be summarized as follows. A signal for transition operation is set once it is detected that boost operation has been deactivated and the signal for virtual inertia operation is set. Transition operation can be deactivated when one of the two conditions from FIG. 6 that are applied to the RESET input of the flip-flop circuit 502 is fulfilled. When the rotational speed is greater than a predefinable small positive value and is large enough to trigger speed oscillations in the generator. The second condition relates to the actual value of the electrical power that is fed in, for example in the form of effective power, in relation to the estimated aerodynamic power P.sub.aero made available by the wind. A predefinable difference in the power is based on the losses between aerodynamic power and fed-in power and should be large enough to prevent a triggering of oscillations in the estimated aerodynamic power or in the effective power output.

(41) If transition operation is terminated, recovery operation is initiated. Recovery operation is switched on as long as the torque set point value for normal operation is limited by the lower limit of the limiter (cf. comparator 510). Deactivation of recovery operation indicates that the wind turbine returns once again to its normal operation.

(42) FIG. 8 shows the qualitative course of effective power P, generator torque N, rotational speed , and frequency f in the electrical transmission network over time. Boost operation is triggered at the point in time t.sub.1, in which the grid frequency f falls below a value f.sub.trigger. At the point in time t.sub.1, boost operation is triggered. After a short rise in the effective power that is fed in and the generator torque N, the power that is fed in reaches a constant value that is higher by P.sub.inc than the power value P.sub.pre that was previously fed in before the frequency drop occurred. In boost operation, the generator torque N rises and the rotational speed w decreases.

(43) After the predetermined period of time for the increased feeding in of power T.sub.boost, the point in time t.sub.2 is reached and the transition zone takes place. The transition zone is characterized in that the generator torque decreases with a constant gradient between the time span t.sub.2 and t.sub.3. In this time span, the rotational speed stabilizes. The transition zone is terminated when a lower power limit P.sub.dip is reached. In this manner, it is ensured that the power at the wind turbine does not fall too strongly and the wind turbine does not shut down. After the termination of transition operation, recovery operation begins at the point in time t3. In recovery operation, both the torque and the rotational speed increase until recovery operation is terminated. At the conclusion of recovery operation, the electrical power that is fed in corresponds to the electrical power fed in before the drop in frequency.

REFERENCE SIGN LIST

(44) 10 Wind turbine 11 Tower 12 Nacelle 14 Rotor 16 Rotor blades 18 Asynchronous generator 20 Converter 24 Transformer 25 Electrical transmission network 26 Control 27 Wind farm control 28 PI element 102 Limiter 200 Limiter control 201 IF-THEN branch 202 IF-THEN branch 300 Block 301 Additional block 302 Comparator 304 Recognition circuit 306 Flip-flop 308 Comparator 310 Recognition circuit 312 OR circuit 314 Time element 316 Comparator 318 IF-THEN branch 320 Time element 322 Selection element 324 Selection element 326 Calculation element 400 Uniform control block 401 Ramp function 402 Ramp function 403 IF-THEN branch 418 Comparator 420 Memory block 500 Switching block 502 Flip-flop circuit 504 Flip-flop circuit 506 Recognition circuit 508 Recognition circuit 510 Comparator 600 Control block 604 Look-up table 606 Differentiating element 610 Control n* Rotational speed set point value n.sub.meas Rotational speed actual value N* Torque set point value N* Limited torque set point value N*.sub.boost Set point value for the generator torque N.sub.low Applied lower limit N.sub.up Applied upper limit N*.sub.low Set point value for the lower limit N*.sub.up Set point value for the upper limit N.sub.min Minimum torque N.sub.max Maximum torque f.sub.meas Measured grid frequency f.sub.trigger Triggering frequency f.sub.reset Reset frequency T.sub.boost Boost period P.sub.aero Available power P.sub.boost(f) Power value determined depending on the frequency P.sub.freeze Frozen electrical power P.sub.inc Power to be additionally provided P.sub.el Electrical power fed in before the frequency dip occurs P.sub.meas Measured electrical power P.sub.n Rated power P.sub.total Aggregated power set point value N*.sub.boost Set point value for generator torque in boost operation N.sub.max Maximum torque in normal operation N.sub.min Minimum torque in normal operation flag.sub.boost Boost signal flag.sub.trans Transition signal flag.sub.rec Recovery signal ramp.sub.low Ramp for the lower limit ramp.sub.up Ramp for the upper limit ramp.sub.rec Control signal for ramp function in recovery operation ramp.sub.trans Control signal for ramp function in transition operation ramp.sub.active Ramp signal

(45) ${\frac{{\mathrm{dn}}^{*}}{\mathrm{dt}}}_{/\mathrm{const}}$
Constant temporal change of the rotational speed

(46) ${\frac{{\mathrm{dN}}^{*}}{\mathrm{dt}}}_{/\mathrm{const}}$
Constant value for temporal change in torque P Constant power difference P.sub.dip Lower power limit