Comparison of sympathetic inrush influence on differential protection between parallel and series transformers

The equivalent circuits of parallel and series sympathetic inrushes are analyzed and the generation and decay of parallel and series sympathetic inrushes are compared based on the simulative and theoretical analysis. With the consideration of current transformer saturation, current transformer parameters and operating transformer load, the influence of both parallel and series sympathetic inrushes on differential protection is analyzed according to the rules of power system. The analysis shows that, the series sympathetic inrush occurs later and decays more quickly than the parallel sympathetic inrush, and its magnitude is smaller; the current transformer saturation caused by sympathetic inrushes will lead to the disappearance of differential current dead angle with higher second harmonic component; the restraining current of differential protection is bigger than the sympathetic inrush when the operating transformer has load, hardly causing the misoperation of differential protection. This project is supported by the National Natural Science Foundation of China (90610024, 50347026, 50467002), Yunnan Science and Technology Project (2003GG10), the Natural Science Foundation of Yunnan Province (2002E0025M, 2004E0020M, 2005F0005Z), Youth Foundation of Kunming University of Science and Technology (2006 43).