Abstract-To detect the DC faults for MMC based DC grids using overhead line transmission, many protection methods in phase-domain have been proposed. These existing protection methods suffer from incomplete function, weak theoretical basis and sensitivity to fault resistance and noise disturbance. To overcome these shortcomings, this paper proposes an improved DC fault protection algorithm using the modal-domain approach for the MMC based overhead DC grids, which decouples interaction between positive and negative poles and mitigates the strong frequency-dependency of the characteristic impedance in phase-domain. The DC fault equivalent circuits are established in modal-domain and the fault characteristics during the initial stage are analysed. Based on the modal-domain analysis, the line-mode reactor voltage which combines fault characteristics of negative and positive reactor voltages, is employed to identify the internal faults. The zero-mode reactor voltage which enlarges the differences between faulty and healthy poles, is employed to select the faulted pole. This method is robust to fault resistance and noise with high detection speed. In addition, it is not affected by power reversal, AC faults and DCCB operation, which are validated and evaluated by simulations in PSCAD/EMTDC.

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