A Non-Unit Line Protection Scheme for MMC-MTDC Grids Based on Aerial-Mode Voltage Traveling Waves

The line protection of modular multilevel converter (MMC)-based multiterminal direct current (MTDC) grids must detect and locate the faulty line within a few milliseconds after the fault occurs, which is greatly different from the protection of alternative current (AC) systems and traditional line commutated converter (LCC)-HVDC systems. Therefore, a new line protection principle suitable for MMC-MTDC grids must be proposed. The propagation characteristics of voltage traveling waves after DC line faults was first analyzed in this paper. Then, according to the fault transient characteristics of MMC-MTDC grids and the inherent composition of the primary equipment, this paper proposed a novel single-ended faulty line identification principle based on the aerial-mode voltage on the current-limiting inductor at the line terminal and a reliable faulty pole selection method. Besides, the corresponding line protection scheme was designed. Finally, a meshed four-terminal MMC-MTDC grid simulation model was built in PSCAD/EMTDC software to verify the feasibility of the protection scheme. Simulation results show that the proposed protection scheme can identify faulty lines with high speed and sensitivity.