The characteristics of two-terminal differential protection on the amplitude-phase flat are discussed. The sensitivity of traditional current differential protection is low and in contradiction with security. By making full use of the relative amplitude and phase relationship of different faults

a new method of dividing the amplitude-phase flat into three regions including the action region

brake region and fuzzy region is proposed

which is capable of meeting both sensitivity and security requirements of current differential protection. Moreover

the proposed method is used to calculate the differential current and restrained current by using the short data window phaselet algorithm to distinguish the external fault causing current transformer saturation and internal high-impedance fault. If it is internal high-impedance fault

protection can act sensitively. The proposed method can solve the contradiction between sensitivity and security

and is capable of acting rapidly when external fault converts to internal fault while ensuring freedom from the influence of the blocking time of current transformer saturation. Simulative results have proved the effectiveness of the proposed method.