A differential current threshold value is usually needed to avoid the effect of unbalancing current when the current is passing though the transformer. For ratio differential protection when a winding fault or a high impedance ground fault occurs， often it requires the value to be as small as possible. The ratio of differential protection can affect the action zone of differential current and restraint current. A high ratio is advantageous for an external fault but the small ratio is better to an internal fault. Selection of the fixed value and the ratio directly affects the protection's sensitivity and reliability. Hence this paper investigates the fault increment ratio differential protection and the traditional ratio current differential protection. It proves that the fault increment ratio differential protection precedes the traditional ratio current differential protection. It is also demonstrated in the paper that the fault increment floating ratio differential protection tested by examination and simulation has much obvious superiority and practicability for some faults.