Wind energy promises to become an important source of energy in the near future. Penetrating large-scale wind power into power systems presents a lot of challenges to power system operators, generation companies and wind turbine manufactures. In order to design less expensive and more efficient wind turbines, manufacturers have tried a lot of possibilities. However reliability is also an important issue. For this reason much attention needs to be paid to the reliability improvement. In the beginning, this thesis introduces and discusses modern wind turbines, gives failure rates to express the reliability, analyzes problems and the most critical subassemblies of wind turbines, and gives the possible methods to make wind turbines more reliable. For electrical part of wind turbine, generator is a crucial component. In this thesis a fault tolerant energy conversion system was designed to meet the specification. With certain faults the system can continue operating and output power. Therefore the reliability of wind turbines is increased. Finally the design procedure, and comparisons of this fault tolerant generator system and conventional generator system are given.