An improved DC fault protection scheme independent of boundary components for MMC based HVDC grids

For Modular Multilevel Converter (MMC) based DC grids, current-limiting reactors (CLRs) are mainly employed to suppress the fault current and provide boundary effects to detect internal faults. Thus, most existing protection schemes are highly dependent on the larger CLRs to guarantee high selectivity. However, in existing MMC based HVDC projects, the size of CLRs is restrained by the cost, weight and system stability under normal state. Thus, boundary protections may fail to detect high-resistance faults and pole-to-ground faults. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection algorithm independent of boundary components. The propagation characteristics of line-mode backward traveling-waves (TW) are analyzed to identify external and internal faults. The polarities of zero-mode backward TWs are employed to select faulted poles. To detect remote faults, a pilot protection scheme based on the directional overcurrent is adopted as the complementary criterion. The detection speed of the proposed protection is fast, with a delay less than 1.1ms. Besides, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC demonstrate that the proposed method is not affected by AC faults, fault distances and fault types