System and method for controlling blade pitch of wind turbine rotor blades to reduce vibrations and limit loads in a locked condition of the rotor hub

A method for reducing vibrations and loads in one or more rotor blades on a rotor hub of a wind turbine when the wind turbine is in a standstill condition with the rotor hub locked against rotation and the rotor blades are at an initial angle of attack to an incoming wind direction, the method comprising:

• detecting that one or more of the rotor blades are vibrating above a threshold limit;
• determining one or more wind parameters for wind impacting the rotor blades;
• based on the wind parameters and the initial angle of attack, determining or retrieving a first angle of attack for the rotor blades different from the initial angle of attack that will reduce the vibrations in the rotor blades;
• identifying expected loads at one or more wind turbine components that vary with changing wind parameters and varying angle of attacks of the rotor blades;
• determining if the expected loads induced at one or more of the wind turbine components will exceed a threshold limit at the first angle of attack for the rotor blades;
• with a controller, pitching the rotor blades to the first angle of attack when the expected loads do not exceed the threshold limit using an auxiliary drive system in the wind turbine;
• modifying the first angle of attack to a modified angle of attack when the expected loads exceed the threshold limit to reduce the expected loads to below the threshold limit, wherein the modified angle of attack results in an increase in the vibrations in the rotor blades as compared to the first angle of attack;
• and
• with the controller, pitching the rotor blades to the modified angle of attack using the auxiliary drive system.