Excitation transformer includes large harmonic current, which may influence its low-slope percentage restraint characteristic curve and cause the false protection because of over excitation. Therefore, it is necessary to design an over-excitation blocking criterion of percentage differential protection for the excitation transformer. Due to defects in the traditional over-excitation blocking criterion, such as unlock delay and unlock failure, this paper proposes a new type of fifth harmonic blocking criterion which uses the power frequency negative sequence components in the differential current as the auxiliary criterion. The traditional and proposed criteria were compared by MATLAB/Simulink, and the effectiveness of the proposed criterion was proved. The simulation result shows that the new blocking element can not only unlock fast and wide, but also avoid the influence of harmonic current in the system with rectifier load, which is more suitable for the percentage differential protection for devices such as excitation transformer with large harmonic current.
