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In a HVDC transmission system, faults occur in the AC system can result in the variation of impedance-frequency characteristic of the HVDC converter. In this paper, an advanced switching function model of HVDC converter, which considers the effect of the interactions between the AC and DC system and the unbalanced operating conditions of the converter, is presented. Based on this model, a method for calculation of the impedance-frequency characteristic of the HVDC converter under the asymmetric faults conditions is proposed. The method is verified by comparison with the results obtained from calculation and digital simulation using PSCAD/EMTDC.