However, wind turbines (WT) are complex electromechanical systems which extract kinetic power of the wind and convert it into electrical power. The wind turbines oftenwork under very harsh environment, with rapidly changing environment temperature, air pressure and alternating load operation conditions. Thus wind turbines are subjected to different sort of failures, and the wind energy industry has main bearings, gearboxes, and generators. Moreover, wind turbines are hard-to-access structures, and often located in rural areas, such as a mountainous area or at sea with traffic inconvenience.