When large-scale new energy，energy sto rage，and large-capacity flexible alternative current （AC） and direct current （DC） transmission equipment are integrated into new electric power systems，steady-state power frequency phase，and sequence component of current and voltage waveforms in transmission line faults are mainly depend on the low voltage ride through strategy of power electronic equipment，emergency control strategy，and current limiting link of electronic components and devices.The performance of traditional relay protection algorithms is significantly based on the power frequency.A new relay protection algorithm based on non-power-frequency transient components can make full use of the transient charging and discharging process caused by transmission line faults to distinguish between internal and external faults.Inspired by information-theoretic methods for characterizing waveform features，a multi-dimensional time-frequency characterization approach is proposed，which is based on the amplitude and variation rate of transient components in both the time-domain waveform and spectral distribution.This method fully exploits the differences in transient waveform features between internal and external faults on transmission lines.The research results show that some time-frequency characterization indexes are less affected by the power electronic equipment in the given time window and frequency band，while exhibiting significant differences between internal and external faults.The proposed characterization method can be applied to the study and testing of new relay protection algorithms，providing strong support for the safe and reliable operation of new electric power systems.