Adaptive Stator Ground Fault Protection for Variable Speed Operation of Synchronous Machines

Synchronous machines usually operate at constant speed in power systems. However, there are specific applications, such as the generators used in diesel-electric transmissions or Type 4 wind turbines, that operate at variable speed. These machines are provided with various protective relaying functions depending on their rated power and service type, including conventional fixed-setting stator-ground fault (SGF) protection. However, under variable speed conditions, SGF protection with fixed settings often fails to protect the stator winding across the full operating range and may also lead to unintended tripping due to improper harmonic filtering. This paper presents an adaptative SGF protection method for SMs operating at variable speed. The proposed system dynamically adjusts the tripping threshold to the machine's operating frequency, ensuring comprehensive protection. Extensive simulations and experimental tests validate the effectiveness of the proposed protection method, yielding promising results.