The main weaknesses of impedance-based loss-of-excitation (LOE) relay are its maloperation during the power system disturbances (PSDs) and its long detection time during LOE events. In this article, to tackle the mentioned problems, a new LOE protection scheme is proposed based on load angle (δ) variation of parallel synchronous generators (SGs) connected to a point of connection (POC). It will be shown that during a PSD, the variation of δ for all parallel SGs is similar such that their δ increases or decreases simultaneously. However, it will be shown that when the LOE occurs for one of the parallel SGs, the δ of faulty SG increases while other SGs will experience reduction in their δ. Hence, when more than one SG is connected to a POC, the LOE event can be discriminated from PSDs. In the proposed scheme, when one SG is connected to the grid, the impedance-based relay is employed to enable the developed method to detect LOE/PSD. Since in a real-world application, the outage rate of the parallel SGs in a power plant during the emergency situations, periodic services, etc., is very low on a yearly basis, the contribution of the impedance relay in the proposed scheme is quite limited to some special circumstances. Comprehensive simulation studies have been conducted in MATLAB/Simulink environment (R2017b); the results prove high efficiency of the proposed scheme.
