In a symmetrical monopolar connection

there is no obvious DC neutral bus to provide zero potential reference for a VSC-HVDC system based on modular multilevel converters（MMC）.To provide a basis for the selection of grounding methods

we summarized characteristics and applicability of various grounding methods for MMC-based VSC-HVDC systems

followed by their applications in multi-terminal VSC-HVDC systems.Then

we analyzed the zero-sequence electrical quantity of each grounding method

such as current and voltage

propagating from AC power grid to converter stations and from each converter arm to both DC side and AC side.With PSCAD simulation of a real VSC-HVDC project with Y0/Yn/Δ connection transformer

the theoretic studies on zero-sequence electrical quantity propagating mechanism were verified.The results show that the zero-sequence current in the converter arm are effectively shielded by the tertiary winding of the transformer under an asymmetric AC-system grounding fault.The shielding effect of zero-sequence current in the AC side winding of the transformer by the tertiary winding of the transformer under a zero-sequence voltage on the converter arm is decided by the ratio of impedance of the zero-sequence circuit on the AC side and that of the tertiary winding of the transformer.