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Loss of excitation (LOE) is one of the most important issues of synchronous alternators, which may be a source of genuine harms and could lead to power system instability. The principle issue to recognize LOE and stable power swing (SPS) is the time delay, which is a primary segment of the recognition time in all ongoing promise strategies. This paper introduces an effective scheme based on the first derivative of equivalent power factor angle (EPFA) of 3-phase voltage and current quantities in the direct-quadrature axis, to detect the LOE. The new algorithm, based on EPFA, is developed to reduce the required time of LOE recognition scheme. Comprehensive simulation studies are accomplished using MATLAB/Simulink as a robust simulation tool for various generator conditions such as full load and sudden load changes in addition to 3-phase faults with different location and duration times. The relay setting is determined and the algorithm performance is evaluated depending on various scenarios in power system. The simulated study results confirm that the proposed method can discriminate SPS from LOE conditions under different modes of operations. [ABSTRACT FROM AUTHOR]