A single-end protection scheme for hybrid MMC HVDC grids considering the impacts of the active fault current-limiting control

In the hybrid modular multilevel converter (MMC) based high voltage direct current (HVDC) systems, the fault current can be actively suppressed by the converter itself, which endows a smaller requirement for current-limiting reactors (CLR) and a larger time margin for fault detection algorithms, comparing with the half-bridge MMC. But the robustness to fault resistance and noise disturbance of existing boundary protection schemes will be deteriorated with small CLRs. Moreover, the fast response of the fault current-limiting control will change the output DC voltage of hybrid MMC, which affects the fault characteristics and may cause mal-operation of existing protection algorithms. Thus, a single-end protection scheme considering the impacts of the active current-limiting control is proposed for the hybrid MMC based DC grids. The traveling-wave characteristics under different fault stages are analyzed to evaluate the impacts of the fault current-limiting control. In addition, a coordination protection strategy versus different fault conditions is adopted to improve reliability. Various cases in PSCAD/EMTDC are simulated to verify that the proposed method is robust to fault resistance, fault distance, power reversal, AC faults, and immune to noise

An improved DC fault protection algorithm for MMC HVDC grids based on modal domain analysis

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