Loss of Field Protection for Synchronous Generators: Current Practice and Future Trends

The loss of field (LOF) is a common phenomenon that occurs in the excitation system of a synchronous generator (SG), and the LOF relay (ANSI code 40) offers crucial LOF protections for SGs in electrical power plants. Conventional LOF relays employ one or two offset Mho elements and exploit measured impedance in the generator terminal to detect LOF events. Although widely used by industry, these relays have low speed in LOF detection, and inadequate security during power swings. During the last decades, LOF protection schemes in SGs have been improved with a higher detection speed and enhanced security. However, several issues remain to be solved, e.g., effects of the generator modeling and its dynamics on LOF studies, and effects of the flexible alternating current transmission system (FACTS) on LOF protection. To address these issues, this paper reviews SG models for LOF studies, different LOF protection schemes, impacts of FACTS on LOF protections, and the restricted operating period of a SG experiencing LOF to restore its excitation system or a safe planned outage. Future research directions in this area are also recommended.