The modular multilevel converter (MMC) HVDC grid is a prospective technical route to unravel the high permeability of the renewable energy grid. The protection issues of MMC-HVDC grids are the main obstacle to its further development. In particular

an accurate and fast fault location scheme is essential to speeding up the time of fault clearing and power supply interruption. In this paper

a non-unit fault location scheme

that can be employed as a distance protection criterion for MMC-HVDC grids

is proposed based on the geometric features of the fault voltage. First

the fault transient characteristics are analyzed by solving the built equivalent circuit of MMC-HVDC grids after DC line faults. The multi-refection and refraction of the fault voltages between the line terminal and the fault point are evaluated. A time domain multi-resolution morphological gradient algorithm (MMGA) is utilized to obtain the geometric characteristics of the fault waveform. The change direction and the corresponding time of the initial two heads of the fault voltage can be recorded

and together with the estimated wave velocity

the search region of the target wave head with definite change direction can be obtained. Therefore

the proposed scheme is almost immune to the velocity of fault waves

fault resistances

and fault positions. A four-terminal ±500 kV MMC-HVDC grid built in PSCAD/EMTDC demonstrates the higher performance of the proposed scheme under different fault conditions

size of fault current limiting reactor

sampling frequency

and noise environments.