Transformers are key components for electrical energy transfer in power system. Stability and security of transformer protection are important to system operation; we found that many mal-trip cases of transformer protection are caused by inrush current problems. The phenomenon of transformer inrush current has been discussed in many papers since 1958 [1-5]. Therefore, this paper will only discuss and analyze inrush current problems. Finally, this paper will also present two cases that were analyzed with the use of digital simulation technique to make COMTRADE files, to provide over-current protection and differential protection tests and the analysis of the effect of inrush current on transformer protection.