World wind energy capacity has been increasing year by year and is expected to continue increasing.The life of a new WT is around 20 years and WT failures are commonly assumed to follow a bathtub curve that indicates high rates of failure will be observed both early on (period of early failures) and towards the end of life (period of wear out) but with lower rates in the middle (period useful life).Wind turbine reliability depends on the reliability of some key components. Maintenance of wind turbines are conducted twice a year at 6 month intervals. A typical maintenance would involve thorough inspection of the entire system, replacement of fluids, lubrication and servicing of mechanical parts.

Dr. Marwa Abd El-Rahman, Researcher of Power & Energy, Mech. Eng. Dept., National Research Centre (NRC), Egypt. General Filed is Mechanical Engineering and specialization in Renewable Energy & SMART Grids for more than 10 yrs.