Transient Modeling and Fault Current Calculation of Hybrid MMC when Riding Through DC-Side Pole-to-Ground Fault

DC fault ride through (FRT) control is one of the typical fault protection methods for modular multilevel converter based high-voltage direct current (MMC-HVDC) system. During dc-side pole-to-ground (PTG) FRT stage, obtain the transient behavior of dc fault current from a theoretical perspective is of vital importance. In this paper, with numerical and Hilbert-Huang Transformation (HHT) based analysis, it is found the path of MMC that is from dc current to dc internal voltage is weak nonlinear Hence, a linear transient model is established for MMC. By performing participation factor analysis, the order of the proposed model is further reduced, so that an analytical expression of dc fault current is approximately calculated. Based on the expression, theoretical guidance can be provided for the selecting for FRT duration. PSCAD/EMTDC simulation verifies the correctness of the transient model and the effectiveness of the analytical expression.