Research on a current calculation method and characteristics of pole-to-ground faults in true bipolar MMC-HVDC grids considering line coupling✰

Abstract The pole-to-ground fault is the most common fault in the true bipolar modular multilevel converter based high voltage direct current (MMC HVDC) grid. The study of a fault current calculation method and characteristics is the theoretical basis for relay protection and fast fault isolation technology of MMC HVDC grid. Considering the mutual inductance coupling between lines, this paper proposes a general fault current calculation method for pole-to-ground faults in MMC HVDC grids. Firstly, considering the influence of AC system on MMC and the mutual inductance coupling between lines, the complex frequency domain equivalent model of the MMC HVDC grid and the simplified model of the remote converter station are studied. Based on the characteristics of the true bipolar MMC HVDC grids, the circuit is analyzed in the complex frequency domain, and an improved node voltage method combined with voltage current relation (VCR) is proposed to calculate the MMC HVDC fault current and obtain the corresponding expression. Finally, the proposed method is validated by comparing its calculation results with the simulation results performed on PSCAD/EMTDC, and the characteristics of the pole-to-ground fault in the MMC HVDC grid are further summarized.