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

摘要

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 poleto-ground fault in the MMC-HVDC grid are further summarized.