During the transient phase of faults

the observation window is short

constrained by few parameters

and is full of fault details. Fully exploring these transient fault features is crucial for enhancing the protection performance of modern power systems. This paper proposes a novel principle for the differential protection of AC line currents

based on frequency band energy characteristics. Firstly

the frequency composition of transient differential currents in AC lines is analyzed. Then the practical calculation method for the transient dominant frequency is proposed. Secondly

the disparities in high- and low-frequency energy composition of internal and external faults are analyzed. Based on this analysis

a new principle for transient current differential protection is proposed

which effectively accelerates the speed of current differential protection while ensuring the reliability of the protection scheme. Finally

the proposed protection scheme has been tested by simulation results

RTDS hardware experiments

and actual on-site waveform data

respectively.