An Improved Strategy to Detect Stator Inter-Turn Faults in Reluctance Synchronous Machines Using Both Negative Sequence Quantities and Stored Magnetic Energy After Supply Disconnection

The state-of-the-art online condition monitoring tool, based on negative sequence quantities, has been projected to be capable of eliminating the ambiguity arising out of supply unbalance to successfully detect even one turn stator faults in induction motors. Our recent observations raised serious concerns while implementing this scheme to protect a reluctance synchronous motor (RSM) against stator inter-turn faults, particularly under changing supply unbalance. In the earlier practices, the effective negative sequence impedance of the machine has been assumed to remain constant while estimating the negative sequence current caused by supply unbalance. However, in this work, the effective negative sequence impedance has been shown to be sensitive to changing supply unbalance, which can cause serious ambiguity. A fault involving even five shorted turn could not be detected unambiguously. In this work, a major improvement has been suggested to overcome this ambiguity to a great extent. However, the improved method can unambiguously detect faults only down to three shorted turns. Hence, it has been proposed to verify the faulty state of the machine using another robust scheme during the switch-off so that the machine can be brought back into service without loss of much time and labor in the case of a false positive. The back-up scheme can be implemented without any additional measurements.