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A novel technique for detecting stator interturn faults in a doubly fed induction generator (DFIG) is proposed by analyzing its rotor current and search-coil voltage. So far, fault-diagnostic techniques proposed for stator-interturn-fault detection in DFIGs are based on analysis of stator current or vibration of generator. Results from these methods are ambiguous because they either fail to account for condition when the DFIG is operating under imbalanced load or these methods are based on experimental results alone without any theoretical basis. Our recent observations suggested that harmonics induced in the rotor circuit are very promising in detecting stator interturn faults in DFIGs. Hence, in this paper, an in-depth investigation is conducted to determine the origin of various harmonic components in rotor currents and their feasibility to detect stator interturn faults unambiguously. Detailed analysis is presented, which explains the mechanism by which the stator-interturn-fault-related harmonics are induced in the rotor circuit. The theory is verified with simulation and extensive experimental results. To confirm the feasibility of the proposed technique for detecting stator interturn faults and obtain results on speed sensitivity of fault detection, a prototype of digital-signal-processor-based fault-diagnostic system has been developed, which is capable of producing very fast trip signal in about 2 s.