Method for operating a wind turbine to control rotational speed of a wind turbine in the event of a grid error

Abstract

A method for operating a wind turbine comprises capturing an actual value of a fed-in power (P.sub.act) and determining a power deviation based on the captured actual value of the fed-in power (P.sub.act). A signal value is set for a correction of a setpoint value for the rotor blade pitch angle when one of the power deviation exceeds a predetermined maximum value and the power deviation exceeds a maximum gradient threshold value over a specified period of time. The signal value is cleared when one of the power deviation falls below a predetermined minimum value and a grid voltage value is located in a predetermined band around a specified target voltage value. A correction value (dβ/dt*) is determined for the setpoint value of the rotor blade pitch angle depending on the power deviation and applied to a rotor blade pitch control when the signal value is set.

Claims

1. A method for operating a wind turbine with at least one rotor blade having a rotor blade pitch angle that is adjustable and a generator configured to feed power to an electrical supply grid, the method comprising: capturing an actual value of a fed-in power (P.sub.act) fed by the generator to the electrical supply grid; determining a power deviation (ΔP) as one of: (1) a difference between the captured actual value of the fed-in power (P.sub.act) and a power setpoint value (P.sub.set); and (2) a time rate of change (dP.sub.act/dt) of the captured actual value of the fed-in power (P.sub.act); determining a correction value (dβ/dt*) for a setpoint value of the rotor blade pitch angle (β.sub.set) depending on the power deviation (ΔP); communicating the correction value (dβ/dt*) for a setpoint value of the rotor blade pitch angle (β) to a rotor blade pitch controller; setting a signal value when one of: (1) the power deviation (ΔP) exceeds a predetermined maximum power deviation value (ΔP.sub.max); and (2) a change of the power deviation over a specified period of time (ΔP/ΔT) exceeds a maximum gradient threshold value during over a specified period of time, (ΔP/ΔT).sub.max; correcting the rotor blade pitch angle (β.sub.set) of the at least one rotor blade when the signal value is set, wherein the rotor blade pitch angle (β.sub.set) is corrected according to the correction value (dβ/dt*) for the setpoint value of the rotor blade pitch angle (β.sub.set) provided by the rotor blade pitch controller; and clearing the signal value when at least one of: (1) the power deviation (ΔP) falls below a predetermined minimum power deviation value (ΔP.sub.min); and (2) a grid voltage value of the electrical supply grid is located in a predetermined voltage band around a predetermined target voltage value for the grid voltage, and wherein the predetermined maximum power deviation value (ΔP.sub.max) is approximately 10%-20% of a nominal power of the wind turbine, and wherein the predetermined minimum power deviation value (ΔP.sub.min) is approximately 5%-10% of the nominal power of the wind turbine.

2. The method according to claim 1, wherein an actual value for a rotational speed of the wind turbine (n.sub.act) is input into the rotor blade pitch controller, and wherein a change speed for the rotor blade pitch angle (dβ/dt) is determined by the rotor blade pitch controller depending on the actual value for the rotational speed of the wind turbine (n.sub.act) and wherein correcting the at least one rotor blade pitch angle is performed at the determined change speed for the rotor blade pitch angle (dβ/dt).

3. The method according to claim 2, wherein the rotor blade pitch controller comprises an integrator configured to integrate the change speed for the rotor blade pitch angle (dβ/dt) to determine the setpoint value of the rotor blade pitch angle (β.sub.set), and wherein the setpoint value of the rotor blade pitch angle (β.sub.set) is communicated to the rotor blade pitch controller.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method according to the invention for operating a wind turbine will be further explained below with reference to an exemplary embodiment. In the figures:

(2) FIG. 1a illustrates an elevational schematic view of an embodiment of a wind turbine;

(3) FIG. 1b illustrates a schematic view of an embodiment of an electrical system of the wind turbine in a wind farm;

(4) FIG. 2 illustrates a partial schematic view of a control of the rotor blade pitch angle; and

(5) FIG. 3 illustrates a partial schematic view of a control of the rotor blade pitch angle of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1a shows a wind turbine 100 with a nacelle 101 and a stand device 102. The stand device is designed as a tower in the example shown. The wind turbine 100 has a rotor 103 with a rotor hub 104, on which three rotor blades 105 are arranged, for a rotation about a mainly horizontal axis. A sensor array 150 for capturing environmental parameters of the wind turbine 100 is provided on the nacelle 101. For example, an anemometer 151 (see FIG. 1b) can be provided for capturing the wind speed at the sensor array 150. The three rotor blades 105 are respectively connected with a rotor blade pitch controller 106 (see FIG. 1b), which permits a rotation of the rotor blades about their longitudinal axis. The rotor blades have a circular blade connection area and are rotatably mounted in the rotor hub for the rotation about their longitudinal axis that is perpendicular to the pitch bearing. The rotational movement can be executed e.g. via an electrical or a hydraulic drive. Alternatively or additionally, an activation of actuators integrated into the rotor blades can also take place, which change the aerodynamic properties of the rotor blades (active flow control). An alternative flow control can support the rotor blade pitch control used according to the invention in a particularly advantageous manner.

(7) FIG. 1b shows in a schematic view mainly the electrical structure of a wind turbine 100 in a wind farm 180, which is connected to an electrical supply grid 190 and feeds an electrical power to it. The wind flowing against the rotor 103 (left edge) causes the rotor 103 with the rotor hub 104 and the rotor blades 105 to turn. The rotational movement drives a generator 120 via a drive train 110. In the exemplary embodiment shown, the drive train 110 drives the rotor of the generator 120. The electrical configuration of the generator 120 corresponds with that of a double-fed asynchronous machine, its stator winding directly and its rotor winding indirectly, via a main converter 130 with the electrical supply grid 190. The infeed of electrical power to the supply grid 190 takes place via the stator of the generator as well as via the main converter 130. The main converter 130 has a generator-side converter 131 and a grid-side converter 132. Via the generator-side converter 131, a control of the rotor field of the generator 120 and thus of the stator field and of the electrical power fed directly to the supply grid 190 by the stator takes place so that the infeed takes place grid-synchronously. The grid-side converter 132 is set up to feed electrical power from the rotor winding grid-synchronously to the electrical supply grid 190. Both converters 131, 132 are interconnected via a direct current link. For the control, a converter controller 133 is provided, which is connected to a wind turbine controller 140. The wind turbine controller 140 is in turn connected to a wind farm controller 182, which is linked with the other wind turbines in the wind farm 180 for a bidirectional exchange of data. The wind turbine 100 is connected electrically to a medium-voltage power grid 181 covering the area of the wind farm via a transformer 170. The medium-voltage power grid 181 is connected electrically to the electrical supply grid 190 via another transformer 183. The power fed in from the wind turbine is fed in to the electrical supply grid via the medium-voltage power grid 181.

(8) The wind turbine controller 140 is connected to and communicates with the rotor blade pitch controller 106 and is among other things set up to communicate setpoint values for the rotor blade pitch angle and/or its temporal change to the rotor blade pitch controller 106. The wind turbine controller 140 is connected to different sensors of the wind turbine. The drive train 110 of the wind turbine has a rotational speed sensor 160, which is set up to communicate rotational speed measurement values to the wind turbine controller 140. Thus, the actual value of the rotational speed n.sub.t is input to the wind turbine controller 140. Sensors of the sensor array 150, such as e.g. the anemometer 151, are also connected to the wind turbine controller 140. Current and/or voltage values and/or variables calculated from these values, for example the grid frequency, the phase angle and/or the fed-in active power and/or the fed-in reactive power, which are captured via a sensor 161 on the side of the wind turbine 100 in front of the transformer 170 or respectively determined from the measurement variables, are also input to the wind turbine controller 140. The converter controller 133 receives its signals from the wind turbine controller 140 and obtains current and/or voltage measurement values and/or variables calculated from them, which are captured on the rotor or respectively grid side by the main converter 130 via corresponding measurement sensors 134, 135. The wind turbine controller 140 gives the converter controller 133 for example setpoint values for the power, such as for example setpoint values for an active power or setpoint values for a reactive power. The wind turbine controller 140 can also give the main converter controller 130 a priority for reactive and/or effective power. The wind turbine controller 140 is set up in particular to execute the method according to the invention based on FIG. 2 described below or respectively to enable its execution.

(9) FIG. 2 shows the important aspects of the control method for the blade pitch angle, which takes place continuously during the operation of the wind turbine. The control elements described below can be included both in software as well as in hardware and can be part of the wind turbine controller 140, for example. The control has a rotational speed controller 201 as well as a rotor blade pitch controller 202. An actual value for a rotational speed n.sub.act is input to the rotational speed controller 201, from which a change speed is determined for the rotor blade pitch angle dβ/dt. Such a rotational speed controller 201 is known from the prior art and a person skilled in the art is familiar with how the actual value needs to be converted into a change in the rotor blade pitch angle.

(10) The change speed for the rotor blade pitch angle is input to the controller is integrated via the rotor blade pitch controller 202 and a rotor blade angle βset is specified as the setpoint value. The setpoint value for the rotor blade angle βset can then be set for one or several rotor blades. However, the method suggested here can generally also be combined with methods in which an individual pitch control of the individual rotor blades of the rotor takes place.

(11) A captured actual value for the fed-in power Pact is analyzed for a correction value of the rotor blade pitch angle in a different manner. In a difference block 206, a difference value ΔPn between the actual value for the fed-in power Pact is input to the difference block 206 and the power setpoint value Pset input to the difference block 206 is formed in the exemplary embodiment shown. Furthermore, the resulting difference ΔPn is compared with a specified maximum value ΔPmax and a minimum value ΔPmin in the difference block 206. It is hereby such that, when ΔPn exceeds the maximum value ΔPmax, the signal value (delta bit) is set. If the resulting difference value ΔPn is below the minimum value ΔPmin, the signal value is cleared. It can hereby be provided that the maximum value ΔPmax for the setting of the signal value is greater than the minimum value ΔPmin for the clearing of the signal value. An unstable switching back and forth between a switched-on and switched-off signal value can be avoided through the use of such a hysteresis method. The signal value is technically a flag, which, when set or not set, is analyzed by the control. The maximum value ΔPmax and the minimum value ΔPmin can be specified for example as a fraction of the nominal power Pnom of the wind turbine. For example, the maximum value ΔPmax can be specified in the range of approximately 10% to 20% of the nominal power Pnom and the minimum value ΔPmin in the range of approximately 5% to 10% of the nominal power Pnom. Once the signal value is set, it remains set until a condition for clearing the signal value is present. The signal value also remains set when the condition that caused the signal value to be set is no longer applicable.

(12) More complex conditions can also be checked for the setting of the signal value. For example, the comparison of the power difference ΔPn with the maximum value ΔPmax at point in time Tn is linked with another AND condition. As another condition, it can be checked for example whether or not the power difference ΔPn−1 at a previous point in time Tn−1 was greater than that of the maximum value ΔPmax. The setting of the signal value then only takes place if both conditions are met, i.e. if the current power difference ΔPn exceeds the maximum value ΔPmax and the previous power difference ΔPn−1 does not exceed the maximum value ΔPmax. If points in time Tn−1 and Tn are for example only a few milliseconds up to approximately 10 milliseconds apart, a rapid change in the fed-in power can be determined and used as the trigger criterion for the adjustment of the rotor blades. In order to be able to check such an additional condition, the difference block 206 can have a memory in which the input actual values and/or difference values determined from them are saved cyclically. For this, a digital ring buffer can be provided, which continuously saves input values over a certain period of time at a specified cadence and overwrites it again after a specified period of time passes in order to release the storage space for new data again. For example, i determined power differences (ΔPn, ΔPn−1, . . . , ΔPn−1) are saved every 10 milliseconds and cyclically over a period of 50 ms. However, the cadence and the period of time can be reduced or increased as needed. The values ΔPn and ΔPn−1 for the checking of the previously named condition are thus present in the difference block 206.

(13) To set the signal value, in addition to the two previous examples, another condition can be checked, which, as an OR condition, is alone sufficient for the setting of the signal value. For example, the comparison of the power deference ΔPn with the maximum value ΔPmax at a point in time Tn is linked with another AND condition. As another condition, it can be checked for example whether or not the temporal change in the power difference is smaller than a maximum gradient threshold value (ΔP/ΔT)max over a specified period of time. The difference block 206 can be set up for example to determine the difference ΔP between the current power difference ΔP.sub.n at a point in time T.sub.n and the power difference ΔP at an even earlier previous (compared to previous point in time T.sub.n−1) point in time T.sub.n−4. The gradient ΔP/ΔT results from the division of the difference ΔP of the two power differences through the temporal difference ΔT of the difference in two points in time Tn and ΔP.sub.n−4. The signal value is only set when both conditions are met, i.e. when the current power difference ΔP.sub.n exceeds the maximum value ΔP.sub.max and the power gradient (ΔP/ΔT) does not exceed the gradient threshold value (ΔP/ΔT).sub.max. If points in time T.sub.n−4 and T.sub.n are for example 30-50 milliseconds apart, multiple errors can thus be reacted to by overriding the correction variable, even if the error that led to the original triggering of the signal value was already cleared in the meantime by the presence of a condition sufficient for the clearing. The previously described ring buffer should be designed such that the values necessary to check the condition can be read from the memory.

(14) Additionally or alternatively, other conditions can also be checked to clear the signal value. For example, the difference block can be set up to compare further measurement variables with specified threshold values or to analyze error states or error messages present in the control of the wind turbine. For this, measurement variables of the sensors (150, 151, 160, 161) can be accessed, for example. For example, the signal value can be cleared if the grid voltage value returns to a predetermined band around the specified setpoint voltage value.

(15) The captured actual value for the fed-in power Pact is also input to a power controller 207. The power controller 207 is set up to control the power infeed of the wind turbine. For this, among other things, power setpoint values of the wind farm controller 182 and additional measurement variables are input to it. The power controller 207 determines the power setpoint value Pset, which is input to a converter or other electrical control for the generator (not shown here) in order to generate and feed in the corresponding power. Moreover, the power setpoint value Pset determined by the power controller 207 is input to the difference block 206, where the delta bit is determined as the signal value together with the actual value for the fed-in power Pact. The power controller 207 is set up to cyclically save certain power setpoint values identified to control the wind turbine. The memory is thereby configured such that temporally previous values remain saved at least for a specified short period of time. A digital ring buffer can be provided for this. The delta bit is input to the power controller 207 as a signal value that is output by the difference block 206. The power controller 207 is set up to cyclically check whether the signal value is set and, if it is set, to retrieve and save a temporally previous power setpoint value Pset,freeze from the memory until the signal value is cleared again. An additional memory can be provided for this. The power controller 207 is furthermore set up so that it, as long as the signal value is set, outputs to the difference block 206 the saved previous power setpoint value Pset,freeze instead of the current power setpoint value Pset. The determination of the power deviation can thereby take place in the difference block 206 depending on the power setpoint value set before the error occurred.

(16) Additionally, the saved previous power setpoint value Pset,freeze can also be used to control the wind turbine and can be specified for example as the setpoint value for a transition period after the signal value has been cleared. The retrieval takes place such that a temporally previous power setpoint value is thereby selected, which was determined sufficiently prior to the occurrence of the error. Since the suggested method is suggested for errors, which necessitate quick intervention in the rotor blade control, the prior value can lie 1 to a few 10 milliseconds back with respect to the current value. The cadence can be selected to be the same for all method steps, e.g. 5 or 10 milliseconds. Otherwise, the power controller 207 can be set up to continue to determine and to save power setpoint values even if the signal value is set. After clearing the signal value or respectively after a transition period has passed, the power controller 207 can continue the power setpoint value specification for normal operation.

(17) FIG. 2 also shows a rotor blade pitch angle pilot block 205, in which a correction value dβ/dt* is determined depending on the captured actual value for the fed-in power Pact. The temporal change in the power actual value dPact/dt is accessed here. The functional correlation between the temporal change of the power actual value and the correction value is thereby such that at least in areas a strict monotony is present, i.e. an increasing change in the power deviation with dPact/dt leads to a larger correction value dp/dt*. The rotor blade pitch angle pilot block 205 is set up to cyclically save input actual values Pact and to determine the temporal change in the actual power dPact/dt over at least one period of time from the most current and at least one previous actual value. A digital ring buffer can also be provided for this. The rotor blade pitch angle pilot block 205 can be set up to determine power changes ΔP over infinitesimal periods of time Δt of 1 ms or ΔP/Δt over larger periods of time Δt, e.g. a quarter or half of a grid period (5 or respectively 10 ms in 50 Hz grids). The pilot block can also be set up to perform a smoothing over several measurement values.

(18) It is provided in the method according to the invention that a switch 204 is opened or closed depending on the signal value. In the case of a closed switch 204, the correction value dβ/dt* is input to an addition member 203 and is added to the change speed for the rotor blade pitch angle dp/dt output by the rotational speed controller 201. The sum of the setpoint value dβ/dt determined by the rotational speed controller 201 and the correction value dβ/dt* determined by the rotor blade pitch angle pilot block 205 is then input to the rotor blade pitch controller 202. If the correction term is positive, then the setpoint value for the change speed for the rotor blade pitch angle is increased in the case of the addition. The value of the integral of the rotor blade pitch controller 202 is increased accordingly. It is thus pitched faster, i.e. the rotor blade pitch angle is changed with a higher speed.

LIST OF REFERENCE NUMBERS

(19) 100 Wind turbine 101 Nacelle 102 Stand device 103 Rotor 104 Rotor hub 105 Rotor blade 106 Blade angle pitch device 110 Drive train 120 Generator 130 Main converter 131 Generator-side converter 132 Grid-side converter 133 Converter controller 134 Current/voltage sensor 135 Current/voltage sensor 140 Wind turbine controller 150 Sensor array 151 Anemometer 160 Rotational speed sensor 161 Current/voltage sensor 170 Transformer 180 Wind farm 181 Medium-voltage grid 182 Wind farm controller 183 Transformer 190 Electrical transmission grid 201 Rotational speed controller 202 Rotor blade pitch controller 203 Addition member 204 Switch 205 Rotor blade pitch angle pilot block 206 Difference block 207 Power controller 208 Memory block