Abstract: The present disclosure relates to methods (100, 200) for controlling and operating wind turbines (10). More particularly, the present disclosure relates to methods and systems for determining a surface condition of one or more wind turbine blades (22) of a wind turbine (10). a method comprises rotating the wind turbine rotor (18) under the influence of a wind in predetermined rotation conditions, wherein the predetermined rotation conditions include at least a predetermined pitch angle (252) of the additional wind turbine blades (222, 223). The method further comprises determining a current value of one or more parameters of the wind turbine (10) when rotating in the predetermined rotation conditions and comparing the current value of the one or more parameters of the wind turbine with one or more reference values (37) to determine the surface condition of the wind turbine blades (22).

Abstract: The present disclosure relates to methods (300) to determine a duration of a period for warming up of a power converter (20) of a wind turbine (1). The method (300) comprises determining (301) a first indicator that is indicative of a time that the power converter (20) has been inactive. Further, the method (300) comprises determining (302) the period for warming up at least partially based on the first indicator. A power converter assembly is also disclosed.

Abstract: The present disclosure relates to a wind turbine comprising a wind turbine rotor with a plurality of blades, a generator operatively coupled to the wind turbine rotor for generating electrical power and a power electronic converter for converting electrical power generated by the generator to a converted AC power of predetermined frequency and voltage. The wind turbine further comprises a wind turbine controller configured to receive values of one or more operational parameters of the wind turbine from one or more sensors and further configured to temporarily increase a speed of the generator to above a nominal generator speed if the values of the operational parameters satisfy a potential trip criterion. The present disclosure also relates to methods for controlling wind turbines.