The disclosure relates to checking a rotational speed relay of a wind turbine. The wind turbine comprises a rotational speed sensor for the rotational speed of a shaft. The rotational speed sensor outputs a rotational speed signal, which is fed to a signal input of the rotational speed relay. According to disclosure, the rotational speed signal fed to the rotational speed relay is first inactivated. Then a signal generator is activated, which produces a check signal equivalent to the rotational speed signal. The check signal is fed to the signal input of the rotational speed relay. The signal generator is operated with a check signal that is beyond a rotational speed limit, and a check is performed to determine if the rotational speed relay generates a switch-off command. This allows the functional capability of the rotational speed relay to be checked reliably and at low cost.

The present disclosure relates to methods (100, 200) for estimating an actual value of an operational parameter (67) of a wind turbine (10). A method (100) comprises determining (120) which sensors of a plurality of sensors (61) of the wind turbine (10) are reliable and which sensors of the plurality of sensors (61) are unreliable. The method (100) further comprises estimating (130) the actual value of the operational parameter (67) based on a mathematical model and on data related to the operational parameter (63) measured by the reliable sensors.

The present disclosure is related to a torque transmission system (203) for a slip ring unit (200) The slip ring unit (200) is configured for being mounted along a rotational axis (30) of the wind turbine rotor (18). The slip ring unit (200) comprises an encoder, a rotating part (201) configured for connection to a rotating component of the wind turbine and a static part (202) configured for connection to a static component of the wind turbine. The torque transmission system (203) is configured for connecting the rotating part (201) of the slip ring unit (200) to the rotating component of the wind turbine while having a degree of freedom in an axial and/or in a radial direction. Furthermore, the torque transmission system (203) is configured to prevent relative displacement between the rotating part (201) of the slip ring unit (200) and the rotating component of the wind turbine in a tangential direction. The present disclosure also relates to methods (100) of assembly of a slip ring unit (200) in a wind turbine.