Abstract: Modular Multilevel Converters (MMCs) show promise as back-to-back inverters for large capacity wind turbines. But machine-side MMC operation faces the challenge of large capacitance voltage fluctuations in sub-modules under low-frequency conditions. This paper proposes a compound strategy combining doubling circulating current injection superimposed on high frequency. Doubling circulating current injection suppresses double ripple of many capacitor voltages. Superimposed high-frequency injection further increases upper and lower bridge arm energy exchange rates in low frequency operation, significantly reducing sub-module capacitor voltage ripple. A simulation model built on PSCAD/EMTDC verifies effectiveness. Simulations show doubling circulating current injection suppresses 44.19% of capacitor voltage ripple. Superimposed high-frequency voltage and high-frequency circulating current injection further suppresses 51.6% of capacitor voltage ripple, reducing sub-module capacitance requirements for the same voltage ripple. The proposed strategy remains effective under sudden wind speed changes and transient faults at grid connection points, suitable for large-scale offshore wind power grid connection scenarios.