Modular multilevel converter (MMC) has been widely applied in HVDC systems. When MMC is applied in variable-frequency drives, the sub-module (SM) capacitor voltage fluctuation becomes serious at low output frequency. To suppress the fluctuation, a useful method is high-frequency injection. However, the high-value common-mode voltage would cause the insulation problem to the load. To address this issue, a hybrid MMC with a series of full-bridge SMs (FMs) inserted at the dc side is given. The high-value common-mode voltage can be eliminated by the additional arms. Meanwhile, the SM capacitor voltage fluctuation of the upper and lower arms of MMC system can be suppressed by an optimal high-frequency injection method. Simulation and experiment results confirm the validity of the given topology.