Method of operating a dfig wind turbine under ssr

A wind turbine controller of a wind turbine with a doubly-fed induction generator (DFIG) the wind turbine controller being arranged to protect the wind turbine against a sub-synchronous resonance (SSR) event acting on the wind turbine, wherein the wind turbine controller is arranged to:

• receive a plurality of values over a given period of time that corresponds to a measurement cycle, the plurality of values being one of power-output values and current-output values;
• determine whether measured plurality of values are indicative of an SSR-event critical for further operation of the wind turbine, wherein the determining comprises monitoring whether at least one oscillation-parameter, obtained using the measured plurality of values and representative of a power-output oscillation or a current-output oscillation, exceeds at least one threshold for at least one measurement cycle, wherein it is a condition for determining the presence of an SSR-event critical for further operation of the wind turbine that the at least one oscillation-parameter exceeds the at least one threshold;
• and
• shut down the wind turbine in response to the measured power-output values or current-output values being determined to be indicative of an SSR-event critical for further operation of the wind turbine.