The sensitivity and security of transformer differential protection are not the same due to different percentage restraint methods. When IEDs from various suppliers are both applied as main protections for a transformer, since different percentage restraint methods are applied, they will not trip coherently when they are set default percentage restraint coefficients. The paper first compares three percentage restraint methods applied widely at home and abroad, i.e. MAX, AVE and PHA. Besides, via a calculation example, it compares sensitivity and security of the three methods. It is found that when default percentage restraint coefficients are set, differential protections with certain percentage restraint method will not avoid mal-operation when external faults occurs at certain location with CT saturation to some extent. Percentage restraint coefficients should be adjusted to suitable value, or additional fault type criteria should be added to block tripping. This work is supported by National Key Research and Development Program of China (No. 2016YFB0900500) and National High-tech R & D Program of China (863 Program) (No. 2013AA050104).