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 under weak boundary conditions. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection scheme 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. The directional overcurrent based pilot protection is adopted as a complementary criterion to detect remote faults. The proposed method can be applied in MMC-HVDC systems with small CLRs that cannot provide strong boundary conditions. Besides, the detection speed is fast (less than 1.5ms). Moreover, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC verifies the effectiveness of the proposed scheme.