A Generalized DC Asymmetrical Fault Analysis Method for MMC-HVDC Grids Considering Metallic Return Conductors

Pole-to-ground and pole-to-metallic return conductor (MRC) short circuits are typical dc asymmetrical faults in modular multilevel converter based high voltage direct current (MMC-HVDC) grid. For secure operation, comprehensive analysis of the dc asymmetrical faults is urgently required. However, in MRC contained MMC-HVDC grid, the fault analysis is challenged by two asymmetric factors, viz. the parameter difference between dc transmission lines and the asymmetric distribution of dc limiting reactors. In light of this, a generalized dc asymmetrical fault analysis method is proposed in this paper. Firstly, with symmetrical component decomposition, the MMC- HVDC grid is decomposed into 0-, 1- and 2-modal components. The common-mode network that consists of 0- and 1-modal components is decoupled from the differential-mode network that consists of 2-modal component, which helps to reduce the analysis complexity. Then, combining the fault boundary conditions and the terminal characteristics of common-/ differential-mode networks, analytical expressions of the grid branch current and node voltage are obtained. The proposed analysis method is applicable to different dc asymmetrical faults, and is essentially a generalized form of the exiting dc fault analysis methods. Moreover, the obtained analytical expressions are verified by PSCAD/EMTDC simulation to be accurate enough to estimate the fault characteristics, and can provide insights on the fault mechanism.