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 This paper proposes a fault diagnosis method based on a fuzzy logic for a three-parallel power converter in a wind turbine system. The proposed method utilizes three-phase currents measured from sensors. The method can not only detect open faults but can also identify faulty switch devices without additional voltage sensors or an analysis modeling of the system. Consequently, this particular feature potentially cuts costs to apply the fault diagnosis algorithm. The measured three-phase currents are transformed to two-phase stationary reference frame and these two-phase currents possess specific patterns in accordance to the conditions of the switch devices in the converter. Therefore, the location of a faulty switch can be indicated by pattern recognition of a current vector. The pattern recognition is achieved by a fuzzy system and two diagnosis parameters, a surface of current vector and a current vector angle, are used as inputs for the fuzzy system. A simple fault tolerant for the three-parallel power conversion system is also presented to maintain proper performance under faulty conditions. In the chapter 2, the topology and control strategy of the three-parallel power conversion system are briefly explained. The type of power conversion system fault and its effects are mentioned in the chapter3. The basic principle of the proposed fault diagnosis algorithm, the fuzzy system design, and fault tolerant algorithm are explained in the chapter 4. Finally, simulation and experimental results are illustrated in the chapter 5 and 6, respectively, for verification of the proposed method.