Title of Invention	METHOD FOR CONTROLLING A WIND ENERGY PLANT IN THE POWER-LIMITED MODE OF OPERATION
Abstract	A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed, depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value for the rotational speed.

Title of Invention

METHOD FOR CONTROLLING A WIND ENERGY PLANT IN THE POWER-LIMITED MODE OF OPERATION

Abstract

A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed, depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value for the rotational speed.

Full Text	FORM 2 THE PATENT ACT 1970 (39 Of 1970) The Patents Rules, 2003 COMPLETE SPECIFICATION (See Section 10, and rule 13) 1. TITLE OF INVENTION METHOD FOR THE OPERATION OF A WIND ENERGY PLANT IN THE POWER- LIMITED OPERATION 2. APPLICANT(S) a) Name : NORDEX ENERGY GMBH b) Nationality : GERMAN Company O Address : BORNBARCH 2 D-22848 NORDERSTEDT GERMANY 3. PREAMBLE TO THE DESCRIPTION The following specification particularly describes the invention and the manner in which it is to be performed : - The present invention is related to a method for controlling a wind energy plant in a power-limited mode of operation, which is also designated as a power-restricted operation. This application claims priority from German patent application No. 10 2006 054 768.3 filed on November 16, 2006, which is incorporated herein in its entirety by reference. Besides to the standard operation, in which the wind energy plant delivers its power according to the available wind up to a possible maximum power, a wind energy plant can also be operated in a power-limited way. This mode of operation is important in particular in the case that several wind energy plants are embraced in one wind farm and a control of the power from the wind farm has to take place, like in a power station. In this, the controls by the wind farm or the electric utility preset a desired value for the maximum power to be fed by the wind energy plant in the power-limited mode of operation, for the maximum nonreactive power to be fed in particular. In the power-limited operation, there is a series of technical problems. On the one hand, the wind energy plant is operated even in the power-limited operation with rotational speeds which correspond to the standard operation and are therefore too high for the generated power. In this way, a not necessary noise generation takes place. Further, it has proven that in the power-limited operation, wind energy plants with a torque control and a rotor blade control can adopt operating states with respect to rotational speed and torque which lead to unnecessary stress of the wind energy plant and can be controlled only with difficulty. The present invention is based on the objective to provide a method for operating a wind energy plant in the power-limited operation, which creates the possibility to control the wind energy plant reliably with respect to torque and rotational speed even in this mode of operation. 2 According to the present invention, the objective is resolved by a method with the features of patent claim 1. Advantageous embodiments form the subject matters of the subclaims. The method according to the present invention is directed towards the operation of a wind energy plant which has a rotor blade control, a torque control and a control unit. The control unit preset a desired value for the power to be fed. In this, the control unit can generate the desired value itself. Alternatively, it is also possible that the control unit receives signals from a farm control of the wind energy plant and/or a network monitoring system of the electric utility, which trigger a powerlimited mode of operation. In addition to the signal which triggers the switching over into the power-limited mode of operation, the desired value of the power to be fed can also be preset for the control unit from an external side. The external presetting can take place automatically via data communication or manually. In the power-limited mode of operation, a lowering of the torque takes place through the torque control, such that the desired value for the provided power is not exceeded, wherein the control unit determines the threshold rotational speed pertinent to the preset desired value for the power. The value of the threshold rotational speed is smaller than the rated value (nn) for the rotational speed. The method according to the present invention has the advantage that when the demanded power is provided in the power-limited mode of operation, the wind energy plant operates at a specifically matched rotational speed. Further, the method has the advantage that the rotational speed determined depending of the desired value for the moment can be selected with regard to the control of the whole wind energy plant. Thus, conditions of the wind energy plant are avoided where torque and rotational speed are in a ratio to each other which is unfavourable for the operation of the plant. In one preferred embodiment, the threshold rotational speed to be determined is set proportional to the desired value of the power to be fed. In the method according to the present invention, we have: the smaller the difference between the desired values for the power to be fed and the rated power in the normal operation, the smaller will 3 be the difference between the threshold rotational speed and the rated rotational speed. When determining the threshold rotational speed for the power-limited mode of operation, the value of the threshold rotational speed is set such that it corresponds to that rotational speed in the not power-limited mode of operation, which results in the transition to full load operation for that value of the power which is pertinent to the preset power-limited desired value. Thus, departing from the characteristic curve for the standard operation, the limiting value for the rotational speed is selected such that it corresponds to a power value in the standard operation which is matched to the preset desired value for the limited power. Thus, it is made certain that the wind energy plant is operated at values for rotational speed and torque which do not unnecessarily stress the wind energy plant in the power-limited operation. In a preferred embodiment of the method according to the present invention, the rotor blade angle control drives the angle of attack of the rotor blade from on reaching the threshold rotational speed such that the torque decreases linearly up to a maximum value for the rotational speed. In a preferred embodiment, when falling below the threshold rotational speed, the torque is kept approximately constant up to a third rotational speed, and is reduced up to a fourth rotational speed. In a preferred extension, after reaching the fourth rotational speed, the torque is led back again to a characteristic curve of the standard operation, with increase of the rotational speed. The method according to the present invention is explained in more detail in the following by means of an example of its execution. The single figure shows a characteristic diagram for the control behaviour of a wind energy plant. In the characteristic diagram, the generator moment 10 is plotted against the generator rotational speed 12. In a first starting sector 14 of the 4 characteristic diagram, the generator moment increases linearly with the rotational speed. Thereafter follows a second characteristic curve sector 16 from on the rotational speed m. The second characteristic curve sector indicates the generator moment according to the ideal characteristic curve for the power received from the wind. The mathematical relation between rotational speed and power results from the following formula: wherein rrot stands for the radius of the rotor blade, ngen for the generator rotational speed, ugetr for the transmission ratio of the gearbox. l(nrot) designates the tip-speed ratio, depending from the rotational speed of the rotor, pluft designates the density of the air, while cp(nrot) describes a power coefficient for the rotor blade, depending from the rotational speed of the rotor. Depending on the technical realisation of the control, the above relation for the power can be calculated continuously or be laid down in a table as a function of the rotational speed. The use of the physical relation between the power received from the wind and the generator power ascertains that a maximum power yield is achieved with the wind energy plant in this range of the rotational speed. The second sector 16 of the characteristic curve is followed by sector 18 of the characteristic curve, in which a rapid increase of the torque with the rotational speed occurs. In the standard operation of the wind energy plant, the torque is increased up to the rated value Mn across the sector 18 of the characteristic curve. In the power limited operation, the control of the wind energy plant takes place according to the sector 18 of the characteristic curve up to a first threshold rotational speed nGr. At the rotational speed nGr -, the desired value for the moment Msoll preset for the wind energy plant is on hand. In the method according to the present invention, the point 5 21 (nGr, Msoll) is on that characteristic curve in the characteristic diagram which is aimed at for rotational speed and moment when changing over into the full load operation in the standard operation. For rotational speeds greater than the first threshold rotational speed nGr, the torque is diminished in a subsequent characteristic curve sector 20, so that the demanded power corresponding to the torque Msoll is met for the reduced power. Along the characteristic curve sector, an increase of the rotational speed up to a maximum rotational speed takes place. Like even in the control of the wind energy plant in the standard operation, a hysteresis can be provided in the control in the power limited operation, too. This means that upon abating wind and with this decreasing rotational speed, even in a region smaller than the first threshold rotational speed, the desired value for the moment is still kept at first. This part of the characteristic curve is indicated through 22. When a lower threshold rotational speed m is reached, the torque is strongly lowered in the characteristic curve sector 24. In this, the lowering runs along the characteristic curve 24, which runs into the second characteristic curve sector 16 in its imaginary continuation. However, when the torque has been sufficiently lowered, a rotational speed increase takes place from on this point before reaching the second 6 characteristic curve sector, in order to lead rotational speed and torque back to the third characteristic curve sector 18. In the represented example of execution, leading back to the third characteristic curve sector 18 occurs when a rotational speed n4 is reached. The point indicated with 26, in which the characteristic curve sector 24 passes into the characteristic curve sector 28, is on the continuation of the characteristic curve sector 24 in this. The characteristic diagram control according to the present invention permits rotational speed optimised control of the wind energy plant even at powers to be generated which are smaller than the rated power of the plant. The advantage of the method is that the wind energy plant can be stably controlled even in the power-limited operation. Just in the rotational speed range around the threshold rotational speed nGr, the wind energy plant is reliably operated for the provided power. Unnecessarily high rotational speeds are also avoided compared to the standard operation. The characteristic curves of the standard operation are drawn in as characteristic curves 30 in broken lines for better overview, wherein control in the form of a hysteresis can take place even in the standard operation. In the standard operation, a control of the rotational speed takes place around the value of the rated rotational speed. 7 WE CLAIM: 1. A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed (nGr) lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed (nGr), depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value (nn) for the rotational speed. 2. A method according to claim 1, characterised in that the threshold rotational speed (nGr) is proportional to the desired value for the power to be fed. 3. A method according to claim 1 or 2, characterised in that in the power-limited mode of operation, the threshold rotational speed (nGr) corresponds to that rotational speed which results in the not power-limited mode of operation of the wind energy plant for a value of the moment which corresponds to the preset desired value for the moment (Msoll). 4. A method according to anyone of claims 1 to 3, characterised in that the rotor blade angle control controls the angle of attack of the rotor blade from on reaching the threshold rotational speed (nGr), in order to linearly reduce the torque up to a maximum value for the rotational speed. 5. A method according to claim 4, characterised in that when falling below the threshold rotational speed (nGr) the moment is kept constant up to a third 8 rotational speed (n3), and is reduced when falling below the third rotational speed (n3). 6. A method according to claim 5, characterised in that the rotational speed decreases up to the fourth rotational speed (n4) and the control unit elevates the rotational speed from on the fourth rotational speed. (14). 9 ABSTRACT A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed, depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value for the rotational speed. To, The Controller of Patents, The Patent Office, Mumbai 10

Full Text

FORM 2
THE PATENT ACT 1970 (39 Of 1970)
The Patents Rules, 2003 COMPLETE SPECIFICATION
(See Section 10, and rule 13)
1. TITLE OF INVENTION
METHOD FOR THE OPERATION OF A WIND ENERGY PLANT IN THE POWER-
LIMITED OPERATION
2. APPLICANT(S)

a) Name : NORDEX ENERGY GMBH
b) Nationality : GERMAN Company
O Address : BORNBARCH 2 D-22848 NORDERSTEDT GERMANY
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed : -

The present invention is related to a method for controlling a wind energy plant in a power-limited mode of operation, which is also designated as a power-restricted operation.
This application claims priority from German patent application No. 10 2006 054 768.3 filed on November 16, 2006, which is incorporated herein in its entirety by reference.
Besides to the standard operation, in which the wind energy plant delivers its power according to the available wind up to a possible maximum power, a wind energy plant can also be operated in a power-limited way. This mode of operation is important in particular in the case that several wind energy plants are embraced in one wind farm and a control of the power from the wind farm has to take place, like in a power station. In this, the controls by the wind farm or the electric utility preset a desired value for the maximum power to be fed by the wind energy plant in the power-limited mode of operation, for the maximum nonreactive power to be fed in particular.
In the power-limited operation, there is a series of technical problems. On the one hand, the wind energy plant is operated even in the power-limited operation with rotational speeds which correspond to the standard operation and are therefore too high for the generated power. In this way, a not necessary noise generation takes place. Further, it has proven that in the power-limited operation, wind energy plants with a torque control and a rotor blade control can adopt operating states with respect to rotational speed and torque which lead to unnecessary stress of the wind energy plant and can be controlled only with difficulty.
The present invention is based on the objective to provide a method for operating a wind energy plant in the power-limited operation, which creates the possibility to control the wind energy plant reliably with respect to torque and rotational speed even in this mode of operation.
2

According to the present invention, the objective is resolved by a method with the features of patent claim 1. Advantageous embodiments form the subject matters of the subclaims.
The method according to the present invention is directed towards the operation of a wind energy plant which has a rotor blade control, a torque control and a control unit. The control unit preset a desired value for the power to be fed. In this, the control unit can generate the desired value itself. Alternatively, it is also possible that the control unit receives signals from a farm control of the wind energy plant and/or a network monitoring system of the electric utility, which trigger a powerlimited mode of operation. In addition to the signal which triggers the switching over into the power-limited mode of operation, the desired value of the power to be fed can also be preset for the control unit from an external side. The external presetting can take place automatically via data communication or manually. In the power-limited mode of operation, a lowering of the torque takes place through the torque control, such that the desired value for the provided power is not exceeded, wherein the control unit determines the threshold rotational speed pertinent to the preset desired value for the power. The value of the threshold rotational speed is smaller than the rated value (nn) for the rotational speed. The method according to the present invention has the advantage that when the demanded power is provided in the power-limited mode of operation, the wind energy plant operates at a specifically matched rotational speed. Further, the method has the advantage that the rotational speed determined depending of the desired value for the moment can be selected with regard to the control of the whole wind energy plant. Thus, conditions of the wind energy plant are avoided where torque and rotational speed are in a ratio to each other which is unfavourable for the operation of the plant.
In one preferred embodiment, the threshold rotational speed to be determined is set proportional to the desired value of the power to be fed. In the method according to the present invention, we have: the smaller the difference between the desired values for the power to be fed and the rated power in the normal operation, the smaller will
3

be the difference between the threshold rotational speed and the rated rotational speed.
When determining the threshold rotational speed for the power-limited mode of operation, the value of the threshold rotational speed is set such that it corresponds to that rotational speed in the not power-limited mode of operation, which results in the transition to full load operation for that value of the power which is pertinent to the preset power-limited desired value. Thus, departing from the characteristic curve for the standard operation, the limiting value for the rotational speed is selected such that it corresponds to a power value in the standard operation which is matched to the preset desired value for the limited power. Thus, it is made certain that the wind energy plant is operated at values for rotational speed and torque which do not unnecessarily stress the wind energy plant in the power-limited operation.
In a preferred embodiment of the method according to the present invention, the rotor blade angle control drives the angle of attack of the rotor blade from on reaching the threshold rotational speed such that the torque decreases linearly up to a maximum value for the rotational speed.
In a preferred embodiment, when falling below the threshold rotational speed, the torque is kept approximately constant up to a third rotational speed, and is reduced up to a fourth rotational speed. In a preferred extension, after reaching the fourth rotational speed, the torque is led back again to a characteristic curve of the standard operation, with increase of the rotational speed.
The method according to the present invention is explained in more detail in the following by means of an example of its execution.
The single figure shows a characteristic diagram for the control behaviour of a wind energy plant. In the characteristic diagram, the generator moment 10 is plotted against the generator rotational speed 12. In a first starting sector 14 of the
4

characteristic diagram, the generator moment increases linearly with the rotational speed. Thereafter follows a second characteristic curve sector 16 from on the rotational speed m. The second characteristic curve sector indicates the generator moment according to the ideal characteristic curve for the power received from the wind. The mathematical relation between rotational speed and power results from the following formula:

wherein rrot stands for the radius of the rotor blade, ngen for the generator rotational speed, ugetr for the transmission ratio of the gearbox. l(nrot) designates the tip-speed ratio, depending from the rotational speed of the rotor, pluft designates the density of the air, while cp(nrot) describes a power coefficient for the rotor blade, depending from the rotational speed of the rotor.
Depending on the technical realisation of the control, the above relation for the power can be calculated continuously or be laid down in a table as a function of the rotational speed. The use of the physical relation between the power received from the wind and the generator power ascertains that a maximum power yield is achieved with the wind energy plant in this range of the rotational speed.
The second sector 16 of the characteristic curve is followed by sector 18 of the characteristic curve, in which a rapid increase of the torque with the rotational speed occurs. In the standard operation of the wind energy plant, the torque is increased up to the rated value Mn across the sector 18 of the characteristic curve. In the power limited operation, the control of the wind energy plant takes place according to the sector 18 of the characteristic curve up to a first threshold rotational speed nGr. At the rotational speed nGr -, the desired value for the moment Msoll preset for the wind energy plant is on hand. In the method according to the present invention, the point
5

21 (nGr, Msoll) is on that characteristic curve in the characteristic diagram which is aimed at for rotational speed and moment when changing over into the full load operation in the standard operation.
For rotational speeds greater than the first threshold rotational speed nGr, the torque is diminished in a subsequent characteristic curve sector 20, so that the demanded power corresponding to the torque Msoll is met for the reduced power. Along the characteristic curve sector, an increase of the rotational speed up to a maximum rotational speed takes place.
Like even in the control of the wind energy plant in the standard operation, a hysteresis can be provided in the control in the power limited operation, too. This means that upon abating wind and with this decreasing rotational speed, even in a region smaller than the first threshold rotational speed, the desired value for the moment is still kept at first. This part of the characteristic curve is indicated through 22. When a lower threshold rotational speed m is reached, the torque is strongly lowered in the characteristic curve sector 24. In this, the lowering runs along the characteristic curve 24, which runs into the second characteristic curve sector 16 in its imaginary continuation. However, when the torque has been sufficiently lowered, a rotational speed increase takes place from on this point before reaching the second
6

characteristic curve sector, in order to lead rotational speed and torque back to the third characteristic curve sector 18. In the represented example of execution, leading back to the third characteristic curve sector 18 occurs when a rotational speed n4 is reached. The point indicated with 26, in which the characteristic curve sector 24 passes into the characteristic curve sector 28, is on the continuation of the characteristic curve sector 24 in this.
The characteristic diagram control according to the present invention permits rotational speed optimised control of the wind energy plant even at powers to be generated which are smaller than the rated power of the plant. The advantage of the method is that the wind energy plant can be stably controlled even in the power-limited operation. Just in the rotational speed range around the threshold rotational speed nGr, the wind energy plant is reliably operated for the provided power. Unnecessarily high rotational speeds are also avoided compared to the standard operation. The characteristic curves of the standard operation are drawn in as characteristic curves 30 in broken lines for better overview, wherein control in the form of a hysteresis can take place even in the standard operation. In the standard operation, a control of the rotational speed takes place around the value of the rated rotational speed.
7

WE CLAIM:
1. A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed (nGr) lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed (nGr), depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value (nn) for the rotational speed.
2. A method according to claim 1, characterised in that the threshold rotational speed (nGr) is proportional to the desired value for the power to be fed.
3. A method according to claim 1 or 2, characterised in that in the power-limited mode of operation, the threshold rotational speed (nGr) corresponds to that rotational speed which results in the not power-limited mode of operation of the wind energy plant for a value of the moment which corresponds to the preset desired value for the moment (Msoll).
4. A method according to anyone of claims 1 to 3, characterised in that the rotor blade angle control controls the angle of attack of the rotor blade from on reaching the threshold rotational speed (nGr), in order to linearly reduce the torque up to a maximum value for the rotational speed.
5. A method according to claim 4, characterised in that when falling below the threshold rotational speed (nGr) the moment is kept constant up to a third
8

rotational speed (n3), and is reduced when falling below the third rotational speed (n3).
6. A method according to claim 5, characterised in that the rotational speed decreases up to the fourth rotational speed (n4) and the control unit elevates the rotational speed from on the fourth rotational speed. (14).

9

ABSTRACT
A method for operating a wind energy plant with a rotor blade control, a torque control and a control unit, which in a power limited mode of operation presets a desired value for the fed power, which is smaller than the rated value for the power, and which from on a threshold rotational speed lowers the torque through the torque control such that the preset desired value for the power is not exceeded, wherein the threshold rotational speed, depending on the preset desired value for the moment (Msoll), is set to a value which is smaller than the rated value for the rotational speed.
To,
The Controller of Patents,
The Patent Office,
Mumbai
10

METHOD FOR CONTROLLING A WIND ENERGY PLANT IN THE POWER-LIMITED MODE OF OPERATION

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