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As a new generation of high-capacity Voltage Source Converter (VSC), Modular Multilevel Converter (MMC) is a potential topology of VSC based High Voltage Direct Current Transmission System (VSC-HVDC). This paper focuses on the voltage balancing control of floating dc capacitors based on CPS-PWM. The imbalance mechanics of capacitor voltage between and within arms are theoretically revealed. A new balancing control strategy called Capacitor Voltage Balancing Control Loop (CVBC Loop) is proposed, which can directly and accurately regulate the active power injected into the MMC module to guarantee dc voltage balance of capacitors within the same arm. A 10kVA laboratory prototype has been built. The experiment results verify the validity of the proposed control strategy under both steady and dynamic state.