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A method for determining and applying a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine is disclosed. The method includes obtaining a motion quantity indicative of a motion of the rotor and determining the pitch angle offset signal based on the motion quantit

A method for determining and applying a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine is disclosed. The method includes obtaining a motion quantity indicative of a motion of the rotor and determining the pitch angle offset signal based on the motion quantity such that the pitch angle offset signal is adapted to be used for adjusting a blade pitch angle of a rotor blade mounted at the rotor for controlling the rotor frequency in order to reduce a time span during which the rotor is in a critical motion region. A corresponding system and a method for controlling a rotor frequency are also disclosed.

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1. A method for determining a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine, the method comprising: obtaining a motion quantity indicative of a motion of the rotor;determining the pitch angle offset signal based on the motion quantity; determining the pitch

1. A method for determining a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine, the method comprising: obtaining a motion quantity indicative of a motion of the rotor;determining the pitch angle offset signal based on the motion quantity; determining the pitch angle offset signal based on the motion quantity such that the pitch angle offset signal is adapted to be used for adjusting a blade pitch angle; andadjusting, via pitch angle offset signal, the a blade pitch angle of a rotor blade mounted at the rotor for controlling the rotor frequency in order to reduce a time span the rotor spends in a critical motion regionwherein the motion quantity is indicative of rotational frequency of the rotor, andwherein the critical motion region is a critical range of the rotational frequency of the rotor. 2. The method according to claim 1, wherein the critical range is a range around a predetermined or estimated critical rotational frequency of the rotor. 3. The method according to claim 2, wherein the predetermined or estimated critical rotational frequency comprises a frequency equal to a fraction or equal to an integer multiple of a resonance oscillation frequency of an oscillation mode of the turbine, in particular of a tower of the turbine. 4. The method according to claim 2, wherein when the motion quantity is indicative of a rotational frequency the pitch angle offset signal is generated such that the adjusted rotor blade pitch angle is greater when the rotational frequency is lower than the predetermined critical rotational frequency than when the rotational frequency is greater than the predetermined critical rotational frequency. 5. The method according to claim 2, wherein the pitch angle offset signal is generated such that the adjusted rotor blade pitch angle decreases in the critical motion region in at least a portion of the critical motion region, along increasing rotational frequency of the rotor. 6. The method according to claim 2, wherein determining the pitch angle offset signal is based on the motion quantity and on the critical motion region, wherein in particular determining the critical motion region is based on the critical rotational frequency. 7. The method according to claim 1, wherein the determining the pitch angle offset signal occurs while the motion quantity is indicative of a rotation of the rotor below a nominal rotor frequency. 8. The method according to claim 1, further comprising: wherein the determining the pitch angle offset signal occurs while the motion quantity is indicative of a rotation of the rotor below a predetermined threshold rotor frequency lower than the nominal rotor frequency, andwherein below the threshold rotor frequency the turbine supplies zero electric power to a utility grid. 9. The method according claim 1, wherein the determining the pitch angle offset signal is further based on one or more operational parameters selected from the group consisting of a wind speed, an air density, an acceleration signal of the rotor, an acceleration signal of a generator, and a controller state. 10. The method according to claim 1, further comprising: disabling the adjusting the pitch angle of the rotor blade mounted at the rotor based on the pitch angle offset signal while the motion quantity is indicative of a rotation of the rotor at the nominal rotor frequency or above the nominal rotor frequency. 11. The method according to claim 1, further comprising: disabling the adjusting the pitch angle of the rotor blade during start-up of the rotation of the rotor. 12. A system for determining a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine, the system comprising: an input terminal for obtaining a motion quantity indicative of a motion of the rotor;a processing module for determining a pitch angle offset signal based on the obtained motion quantity; andan output terminal at which the pitch angle offset signal is applied to adjust a blade pitch angle of a rotor blade mounted at the rotor for controlling the rotor frequency in order to reduce a time span the rotor spends in a critical motion region,wherein the motion quantity is indicative of rotational frequency of the rotor, andwherein the critical motion region is a critical range of the rotational frequency of the rotor.