Determining a frequency of an oscillating movement of a wind turbine tower

A method of operating a wind turbine having a wind turbine tower, the wind turbine tower carrying a nacelle and a generator including a stator and a rotor, the method comprising:

• determining a frequency of an oscillating movement of the wind turbine tower, the oscillating movement causing a rolling movement of the nacelle, wherein the frequency is determined by:
• obtaining a first signal indicative of a speed of the rotor relative to the nacelle;
• obtaining a second signal indicative of the speed of the rotor relative to ground;
• determining a first amplitude spectrum based on the first signal;
• determining a second amplitude spectrum based on the second signal;
• determining a difference function based on the first amplitude spectrum and the second amplitude spectrum, and
• determining the frequency of the oscillating movement of the wind turbine tower as a frequency corresponding to a peak in the difference function;
• and
• controlling the wind turbine according to the frequency of the oscillating movement of the wind turbine tower.