Rotor Inter-Turn Faults in an Industrial Synchronous Machine Caused by a Stator Ground Fault

Synchronous machines are key pieces in power systems. However, due to their extensive operation periods, they are exposed to a lot of types of faults. Among the possible faults, electrical faults and specially ground faults are very common in large industrial machines. In the case of a ground fault, the stator can be quickly repaired to avoid large loss of profits by bypassing or cutting out the faulty coil. With this repair, the machines can operate at reduced power. There are numerous studies in literature about how to estimate the reduced power operation point once the faulty coil and its homologous coils in the rest of the phases are bypassed. This paper studies a real case of a ground fault repair in a 6-coils 4-branches 3-phases 16MVA synchronous machine. After the repair, the machine led to a rotor inter-turn fault caused by overtemperature when it started up again. The fault occurred after the excitation of the rotor windings and before the grid synchronization during the reduced power tests. The repair process performed in the damaged machine is studied, and lastly, the causes of the rotor fault are detected. Finally, it is concluded that special attention must be given to the repair before its commissioning. Furthermore, a repair process for the stator winding is recommended to avoid this kind of faults.