Every of the wind turbine (WT) subassemblies and components may fail in course of operation. It was found that 17% of all failures happen in gearbox (12%) and generator (5%), yaw system fails in 8% of cases, and hydraulic system is responsible for 5% of all failures. The “failure rate” is usually expressed by the number of failures per unit of time, which also could be used as a measure of reliability. The estimated failure rate of the electrical system is more than those of the structural and mechanical systems combined. Typical failures in the electrical system are defect components or fuses and burned cables. The damaging consequences are unavoidable if the lubricants used for lubrication of different parts of WT are exposed to high temperatures for extended periods of time. Gearbox of turbine is always lubricated, but high oil temperatures reduce lifetime of oil.