This paper analyzes the causes of mal-operation in transformer differential protection. By referring to the analysis of the physical characteristics of electric quantities

the integrated analytical method for electric quantities on all sides and graphics analytical method

key problems about sampling abnormality and current transformer saturation are thoroughly analyzed and measures are presented. The definite relationship between the differential current and zero sequence current during internal fault is proved in theory

by which the mal-operation of transformer differential protection can be reliably prevented.