Modular multilevel converters (MMCs) have limited ability to withstand overcurrent. Additionally, the complex characteristics of faults make it extremely difficult to reliably identify the fault area within a short period. In this paper, the waveform of the transient current component is transformed into multiple intersecting curves in an alternative coordinate system, utilizing the principle of coordinate mapping. The internal and external faults are identified by determining the range of angles between the current waveforms and the time axis based on the intersecting regions of the curves. Subsequently, a unit protection scheme based on the transient current coordinate mapping interval is proposed. Finally, a modular multilevel converter-high voltage direct current (MMC-HVDC) system is built using PSCAD/EMTDC to validate the proposed scheme. The simulation results show that the proposed protection scheme is insensitive to factors such as current fluctuations caused by noise and distributed capacitive currents. In addition, it shows high robustness against fault resistance.