System for Thrust-Limiting of Wind Turbines

Abstract

The present invention relates to a control system for thrust-limiting of wind turbines, which wind turbine comprises at least one tower, which tower carries at least one nacelle which nacelle comprises a rotating shaft, which shaft is rotated by one or more blades, which blades at pitch regulated by a pitch control system. It is the object of the present invention to reduce mechanical load and stress of a wind turbine. A further object is to reduce the maximal load on tower of a wind turbine. The thrust-limiting control system performs control of the pitch angle, which thrust-limiting control system performs regulation of the pitch angle based on at least a first input from a wind estimator and a second input from a turbulence estimator. By thrust-limiting control, reduction in the maximum mechanical load on a tower, or maybe also a nacelle, can be achieved by a relatively high percentage of the load in a way where it has only very limited influence on the power production of the wind turbine.

Claims

1. Control system for thrust-limiting of wind turbines, which wind turbine comprises at least one tower, which tower carries at least one nacelle which nacelle comprises a rotating shaft, which shaft is rotated by one or more blades, which blades are configured to be pitch regulated by a pitch control system, whereby the thrust-limiting control system performs control of the pitch angle, which thrust-limiting control system performs regulation of the pitch angle based on at least a first input from a wind estimator and a second input from a turbulence estimator, which first and second inputs are combined for forming an output which output is transmitted to a subtraction function where a signal representing the estimated thrust force is subtracted from the signal, which subtraction function generates a signal, which signal is sent through a post-processing function for generating a signal for performing thrust-limiting of the wind turbine.

2. Control system for thrust-limiting of wind turbines according to claim 1, whereby the wind estimator receives a third input from measurement of inter-blade mean pitch angle, which wind estimator receives a fourth input from a generator torque reference set by a torque controller, which wind estimator receives a fifth input from a measured generator angular velocity.

3. Control system for thrust-limiting of wind turbines according to claim 2, whereby a sixth input to the wind estimator is derived from rotor inertia acceleration.

4. Control system for thrust-limiting of wind turbines according to claim 2, whereby the turbulence estimator receives a seventh input from measured nacelle fore-aft acceleration.

5. Control system for thrust-limiting of wind turbines according to claim 4, whereby the wind estimator generates a first internal signal based on estimated rotor average wind speed, which first internal signal is transmitted to both the turbulence estimator and to the condition-adaptive thrust limiter system.

6. Control system for thrust-limiting of wind turbines according to claim 5, whereby the thrust-limiting system comprises at least the following operational activities such as a condition-adaptive thrust limiter feed forward function, which condition-adaptive thrust limiter feed forward function receives input from the first internal signal based on estimated rotor average wind speed and input from the measured generator angular velocity, which feed forward function has an output.

7. Control system for thrust-limiting of wind turbines according to claim 6, whereby the control system further comprises a safe mode switch function, which safe mode switch function receives input that controls the thrust limit, which safe mode switch function further receives input from a turbulence scaled thrust limiter given by a turbulence detector, which safe mode switch function based on the inputs generates an output, which output is transmitted to the condition-adaptive thrust limiter feed forward function.

8. Control system for thrust-limiting of wind turbines (4) according to claim 4, whereby the signal is transmitted to a PI control function, which PI control function further receives input from the output from the feed forward function, which PI control function further receives input from measurement of inter-blade mean pitch angle, which PI control function generates an output to which output the signal is added for forming a signal representing the minimal pitch angle.

9. Method adapted for thrust-limiting of wind turbines as disclosed in claim 8, whereby at least the following steps of operation: a. receive a first input from a wind estimator, b. receive a second input from a turbulence estimator, c. combine first and second input signals for forming an output, d. perform subtraction of a signal representing the estimated thrust from the output signal, e. generates a signal, which signal is sent through a post-processing function for generating a signal for performing trust-limiting of the wind turbine.

Description

DESCRIPTION OF THE DRAWING

[0042] FIG. 1 shows a curvature concerning static generator power.

[0043] FIG. 2 shows a curvature showing static thrust force.

[0044] FIG. 3 shows a block diagram comprising a thrust-limiting function.

[0045] FIG. 4 shows a block diagram indicating a possible disclosure of a thrust-limiting function.

[0046] FIG. 5 shows a wind turbine.

DETAILED DESCRIPTION OF THE INVENTION

[0047] FIG. 1 shows one possible curvature indicating wind speed versus power. At the curvature, it can be seen that invention as disclosed hereafter has very limited influence on the power generated. Only in a very small window of wind speed are there any changes. It is indicated that the thrust limitation system will only reduce the power production in this very limited area, and here the influence is less than 1 per cent of the power production.

[0048] FIG. 2 indicates instead wind speed vs. thrust force. The two curves indicate that the maximum thrust force is achieved by a wind speed of just above 10 metres/second. The upper curve indicates a peak which is reduced by the lower curve. The lower curve indicates active thrust limitation because the pitch has additional advantages, such as reaction speed and turning options with only few secondary load changes. The reaction time is a motivation for thrust-limiting control features so as to reduce the thrust peak, and to reduce the load associated with extreme external events. Because of the reaction time, the slight change in the pitch to reduce the thrust is having only very limited influence on the power production of the system as such. Therefore, the thrust limitation is performed with only very small changes in the power production, but because the thrust limitation is performed in an area where the wind turbine typically operates most of the time, viz. in the range of app. 10 metres/second, the thrust limitation can have major influence on the total load on for example a tower or maybe also the whole rotor system. In this way, mechanical stress on the tower and maybe also the blades is reduced.

[0049] FIG. 3 indicates a system comprising a thrust-limiting function 2. This limiting function receives a first input 18 from a wind estimator 20. A second input 21 is generated by a turbulence estimator 22. The signal 21 controls the level for power reduction and thrust reduction. This wind estimator 20 has a first input 26 based on measurement of inter-blade mean pitch angle. Further, the wind estimator 20 receives input 30 from a generator torque reference. Further the wind estimator 20 receives input 34 from measured generator angular velocity. The wind estimator 20 generates an output 40 sent to the turbulence estimator 22 and directly to the condition-adaptive thrust limiter 2. The turbulence estimator 22 receives an input 36 based on measured nacelle fore-aft acceleration.

[0050] As can be seen from FIG. 3, there is a plurality of signal inputs that are used as inputs for wind estimator 20 and turbulence estimator 22 which are used for the condition-adaptive thrust limiter which condition-adaptive thrust limiter has an output 27 which is the signal that controls the actual pitch position.

[0051] FIG. 4 shows a more detailed disclosure of the condition-adaptive thrust limiter 2. The condition-adaptive thrust limiter 2 comprises at least a condition-adaptive thrust limiter feed forward function 42, a soft safety switch 48, a post-processing procedure 60 and a PI control 64. The condition-adaptive thrust limiter receives an input 40 generated by the wind estimator 20 which can be seen in FIG. 3. Further, the condition-adaptive thrust limiter feed forward function 42 receives another input 34 representing measured generator angular velocity. Further the safe mode switch 48 can have a first input 50 which input can control the thrust limitation function. In this way it is possible to activate or de-activate the thrust-limiting system from an external control. Further the safe mode switch receives an input 52 from a turbulence-scaled condition-adaptive thrust limiter given by a turbulence detector. The safe mode switch 48 has an output 54 which at first is transmitted to the condition-adaptive thrust limiter feed forward function 42, and further to subtraction function 56. Here a signal 59 represents the estimated thrust force subtracted from the signal 54 coming from the safe mode switch 48. Hereby a new signal 58 is generated which is transmitted to a post-processing module 60. This module generates a signal 62. This signal is sent to a PI control 64. This PI control further receives an input 46 which is generated by a condition-adaptive thrust limiter feed forward module 42. Further, a signal 26 is representing the measurement of interblade mean pitch angle also sent to the PI control. The PI control has an output 68 which is sent to an adder 70 where the signal 46 generated by the condition-adaptive thrust limiter feed forward module 42 is added to the signal 46 generating the output signal for controlling the pitch position 72.

[0052] FIG. 5 shows a wind turbine 4 comprising a tower 6, a nacelle 8, and rotor with blades 12. Further, a power and pitch control system 14 is indicated.

[0053] By the present invention it is possible to reduce the maximum thrust, both at the tower 6 and at the rotating system 12. The advantage of the present invention is that this reduction of thrust can be performed.