Method for determining fatigue damage in a power train of a wind turbine

1. A method for determining the evolution of torque of at least one rotatable shaft and a resulting fatigue damage to different power train components, for design and maintenance operations in a wind turbine, said power train comprising a high speed shaft and a low speed shaft whose speeds of rotation (ωr, ωg) are related by a speed ratio (i), characterized in that said method comprises the steps of:

determining from the generator a torque (Tg) at a high speed shaft;

determining a moment of inertia (Ig) at a high speed shaft;

determining an angular acceleration (αg) at a high speed shaft; and

determining a torque (Tr) at a low speed shaft in the generator through the formula:

Tr =( Tg−Ig·αg )· i.

2. The method of claim 1 , wherein said method further comprises a step of applying a rainflow-counting algorithm to the value of the torque (Tr) at the low speed shaft for determining a number of cycles at ranges of torque for every torque mean value.

3. The method of claim 2 , wherein said step of applying a rainflow-counting algorithm to the value of the torque (Tr) at the low speed shaft is performed at least for one torque mean value.

4. The method of claim 1 , wherein said method further includes a step of determining a relationship between the torque and a stress in a material at every point of every component to be analyzed.

5. The method of claim 4 , wherein said relationship is given by a factor or a non linear equation applied to the torque value (Tr) to obtain cycles at stress ranges for every mean stress value.

6. The method of claim 1 , wherein said method further includes a step of determining the relationship between the torque and a strain in a material at every point of every component to be analyzed.

7. The method of claim 6 , wherein said relationship is given by a factor or a non linear equation applied to the torque value (Tr) to obtain cycles at strain ranges for every mean strain value.

8. The method of claim 4 , wherein said method comprises a further step of determining accumulated fatigue damage during a period of time in any of the parts of the power train by comparing a number of cycles performed at every stress range with a corresponding SN (stress/number of cycles) curve for a mean stress value and a material of the component being calculated, and summing all the obtained damages.

9. The method of claim 6 , wherein said method comprises a further step of determining accumulated fatigue damage during a period of time in any of the parts of the power train by comparing a number of cycles performed at every strain range with a corresponding εN (strain/number of cycles) curve for a mean strain value and a material of a component being calculated, and summing all the obtained damages.

10. The method of claim 8 , wherein said step of determining accumulated fatigue damage during a period of time is carried out by applying the Miner's rule.

11. The method of claim 1 , wherein said method comprises a further step of sending a warning signal when accumulated fatigue damage during a period of time exceeds a predetermined value.

12. The method of claim 1 , wherein said step of determining the angular acceleration at the high speed shaft (αg) is carried out through a time-derivative of the angular velocity (ωg) at the high speed shaft.

13. The method of claim 2 , wherein said data obtained through the rainflow-counting algorithm is in the form of a data matrix.

14. The method of claim 9 , wherein said step of determining accumulated fatigue damage during a period of time is carried out by applying the Miner's rule.