Abstract：Aiming at the interconnections requirements of power frequency（PF），low frequency（LF）and

direct current（DC）brought by the offshore wind power，a hybrid multi-frequency converter was designed based

on the nine-bridge-arm modular multilevel converter（9A-MMC），in which the diode-clamp sub-module was

selected for upper and lower bridge-arm，and the cross-connected-double sub-module was selected for intermediate bridge-arm. The converter had fewer components than the existed hybrid 9A-MMC with the ability of blocking DC fault，and its middle arms could output a negative voltage without controlling the current direction. An improved modulation method was designed for the intermediate bridge-arm module to effectively reduce harmonics. The simulation results show that the proposed hybrid 9A-MMC could simultaneously convert PF AC and LF AC into DC，with a low harmonic distortion rate and small DC-side voltage ripple，which is of certain significance for engineering.

      Key words：nine-bridge-arm modular multilevel converter（9A-MMC）；hybrid modular multilevel converter；

DC fault blocking；multi-terminal；offshore wind power