Load Angle Based Loss of Excitation Protection Scheme for Parallel Connected Synchronous Generators

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 (delta) variation of parallel synchronous generators (SGs) connected to a point of connection (POC). It will be shown that during a PSD, the variation of delta for all parallel SGs is similar such that their delta increases or decreases simultaneously. However, it will be shown that when the LOE occurs for one of the parallel SGs, the delta of faulty SG increases while other SGs will experience reduction in their delta. 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.