Method of operating a wind turbine

1. A method of operating a wind turbine having a rotor with a plurality of blades, a system for determining one or more loads on the wind turbine, a historical register of data on the wind turbine operation, and a control system for controlling one or more operational parameters of the wind turbine, the method comprising:

determining the loads on the wind turbine;

storing the determined loads on the wind turbine in the historical register:

obtaining, from the historical register, a characteristic indicative of the loads on the wind turbine accumulated over time;

determining one or more wind thrust limits depending on the obtained characteristic indicative of the loads accumulated over time; wherein determining the one or more wind thrust limits comprises determining a minimum wind thrust limit, wherein the determined minimum wind thrust limit depends on a rate of change of the characteristic indicative of the loads accumulated over time, wherein the determined minimum wind thrust limit increases when the rate of change of the characteristic indicative of the loads accumulated over time increases; and wherein the determined minimum wind thrust limit decreases when the rate of change of the characteristic indicative of the loads accumulated over time decreases; and

controlling the one or more operational parameters of the wind turbine to keep a wind thrust on the wind turbine above the determined minimum wind thrust limit.

2. The method according to claim 1 , wherein determining the one or more wind thrust limits comprises determining a maximum wind thrust limit.

3. The method according to claim 2 , wherein the determined maximum wind thrust limit also depends on the rate of change of the characteristic indicative of the loads accumulated over time.

4. The method according to claim 3 , wherein the determined maximum wind thrust limit decreases when the rate of change of the characteristic indicative of the loads accumulated over time increases; and wherein the determined maximum wind thrust limit increases when the rate of change of the characteristic indicative of the loads accumulated over time decreases, and controlling the one or more operational parameters of the wind turbine to keep the wind thrust on the wind turbine within the determined maximum wind thrust limit and the determined minimum wind thrust limit.

5. The method according to claim 4 , wherein determining the one or more wind thrust limits comprises determining a maximum rate of wind thrust variation:

wherein the determined maximum rate of wind thrust variation also depends on the rate of change of the characteristic indicative of the loads accumulated over time;

wherein the determined maximum rate of wind thrust variation increases when the characteristic indicative of the loads accumulated over time decreases; and

wherein the determined maximum rate of wind thrust variation decreases when the characteristic indicative of the loads accumulated over time increases.

6. The method according to claim 5 , wherein the method is implemented as a Model Predictive Control (MPC) based on imposing one or more constraints on the wind thrust on the wind turbine depending on the characteristic indicative of the accumulated loads over time.

7. The method according to claim 2 , wherein determining the one or more wind thrust limits comprises determining a maximum rate of wind thrust variation.

8. The method according to claim 1 , wherein determining the one or more wind thrust limits comprises determining a maximum rate of wind thrust variation.

9. The method according to claim 8 , wherein the determined maximum rate of wind thrust variation depends on the rate of change of the characteristic indicative of the loads accumulated over time.

10. The method according to claim 9 , wherein the determined maximum rate of wind thrust variation increases when the characteristic indicative of the loads accumulated over time decreases; and wherein the determined maximum rate of wind thrust variation decreases when the characteristic indicative of the loads accumulated over time increases.

11. The method according to claim 1 , wherein the method is implemented as a Model Predictive Control (MPC) based on imposing one or more constraints on the wind thrust on the wind turbine depending on the characteristic indicative of the accumulated loads over time.

12. The method according to claim 1 , wherein the system for determining one or more loads on the wind turbine comprises at least one load sensor in the root of at least one of the blades.

13. The method according to claim 1 , wherein the control system for controlling one or more operational parameters of the wind turbine comprises one or more pitch systems for pitching one or more of the blades; and wherein controlling the one or more operational parameters of the wind turbine to keep the wind thrust on the wind turbine within the determined wind thrust limits comprises pitching one or more of the blades to keep the wind thrust on the wind turbine within the determined wind thrust limits.

14. The method according to claim 1 , wherein the wind turbine further comprises a system for determining an instantaneous representative wind speed; wherein the method further comprises determining the instantaneous representative wind speed; and wherein at least one of the wind thrust limits is further dependent on the determined instantaneous representative wind speed.

15. A wind turbine configured to perform the method of operating a wind turbine according to claim 1 .