A reliable and straightforward index to avoid nuisance trip of the conventional Z-based loss-of-field relay in synchronous generators

The conventional impedance (Z)-based relay which is the well-known relay to preserve the power plant's synchronous generator (PPSG) against loss of field (LOF) accident may fail during some stable power swing (SPS); on the other hand, most of the previously introduced schemes may fail during the happening of hybrid events. In this paper to overcome the raised problems, a straightforward index is developed for reliable detection of the hybrid events, and it is combined with an AND-logic cluster with the Z-relay to avoid its spurious trip during SPS events. The reactive power (Q(SG)) of the faulty PPSG is continuously decreased during LOF, while it oscillates under both SPS and hybrid events, leading to non-detection of the hybrid event. Hence, using the integral operator and arithmetic mean value are suggested to eliminate the swinging variations of Q(SG) as a Z-relay blocking function (ZRBF). Thus, Q(SG) decreases during both LOF and hybrid events and does not decrease under SPS events. Therefore, by combination of ZRBF with the Z-relay, the PPSG is properly tripped out during the both LOF and hybrid events, and wrong tripping under SPS is reliably hindered. The reliable performance of developed scheme is completely approved thorough performed simulation studies.