Reaction to an overspeed event

1. A method comprising:

determining a rotational speed of a main rotor of a wind turbine;

identifying a controller overspeed situation when the rotational speed of the main rotor of the wind turbine exceeds a first predefined rotational speed limit but is lower than a second predetermined rotational speed limit; and

continuing to operate the wind turbine and controlling the wind turbine during the controller overspeed situation, wherein controlling includes:

determining a current state related to the wind turbine;

providing data related to the current state as input to a turbine model;

predicting a load of at least one wind turbine component and power output of the wind turbine using the turbine model provided with the input for plural control strategies, wherein the plural control strategies include curtailing the rotational speed of the wind turbine rotor and/or the power output by adjusting at least one blade pitch angle and/or adjusting a generator torque;

comparing the predicted load and power output for the plural control strategies; and

selecting that control strategy among the plural control strategies that satisfies a target criterion comprising the load and the power output;

further wherein when the rotational speed of the main rotor of the wind turbine exceeds the second predetermined rotational speed limit a safety overspeed situation is identified and the wind turbine is shut down.

2. The method according to claim 1 , wherein the target criterion includes the predicted load of at least one or plural or all wind turbine components is in an allowed range; and/or the predicted power output is maximized wherein the target criterion is in particular configurable.

3. The method according to claim 1 , wherein predicting the load and power output includes modelling the behavior of the wind turbine using the turbine model provided with the input for each of the plural control strategies for a predetermined time interval and/or wherein the turbine model models each rotor blade as one single component.

4. The method according to claim 1 , wherein the state comprises at least one measured and/or estimated operational parameter relating to an operation of the wind turbine, including:

a measured load of at least one blade;

a rotational speed;

a power output;

a temperature of a wind turbine component; and/or

a pitch position of at least one blade,

wherein the state comprises at least one measured and/or estimated environmental parameter relating an environmental condition of the wind turbine, including:

a wind speed;

a wind direction;

a thrust;

an air density;

an air humidity; and/or

an air temperature.

5. The method according to claim 1 , wherein information from a lidar and/or a radar and/or at least one wind turbine upstream is acquired for determining evolvement and/or state of environmental condition.

6. The method according to claim 1 , wherein at least one is assumed during the predetermined time interval:

at least one environmental parameter is constant;

at least one environmental parameter is extrapolated according to a forecast, and/or wherein an evolvement of at least one operational parameter is predicted using the wind turbine model based on the current state and/or a change of the current state.

7. The method according to claim 1 , wherein the plural control strategies include

maintaining residual power output.

8. The method according to claim 1 , wherein the wind turbine component includes:

a wind turbine tower;

a wind turbine rotor blade;

a wind turbine drive train; and/or

at least one bearing of a rotor blade and/or of a wind turbine drive train.

9. The method according to claim 1 , wherein the turbine model is an online turbine model predicting the load and power output during performing the method.

10. The method according to claim 1 , wherein the turbine model is an offline turbine model that has, for plural discrete states and for the different plural control strategies, loads and power outputs pre-calculated, the method comprising:

discretizing the data related to the state and feeding the discretized data to the offline turbine model; and

retrieving the pre-calculated loads and power outputs based on the discretized data.

11. The method according to claim 1 , further comprising:

controlling the wind turbine according to the selected control strategy.

12. An arrangement for controlling at least one wind turbine, the arrangement comprising:

a determining portion adapted to determine a current (operational) state related to the wind turbine;

a turbine model connected to receive data related to the current (operational) state as input, wherein the model is adapted to, provided with the input, predict a load of at least one wind turbine component and power output of the wind turbine for plural predetermined control strategies; and

a processor adapted to compare the load and the power output for the plural control strategies and select that control strategy among the plural control strategies that satisfies a target criterion comprising the load and the power output;

wherein when a rotational speed of a main rotor of the at least one wind turbine exceeds a first predefined rotational speed limit but is lower than a second predetermined rotational speed limit a controller overspeed situation is identified and the selected control strategy includes continuing to operate the wind turbine but curtailing the rotational speed of the at least one wind turbine rotor and/or the power output by adjusting at least one blade pitch angle and/or adjusting a generator torque; and

wherein when the rotational speed of the main rotor of the at least one wind turbine exceeds the second predetermined rotational speed limit a safety overspeed situation is identified and the selected control strategy includes shutting down the at least one wind turbine.

13. A wind turbine including the arrangement according to claim 12 .