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The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for medium and high voltage applications. In order to enhance power handling capacity and power ratings, this paper proposes a novel parallel-connected modular multilevel converters topology, which assumes two modular multilevel converters directly connected at both AC and DC sides. However, two key problems should be solved in the parallel-connected MMCs: voltage balancing of all sub-modules and the circulating currents between two MMCs. Firstly, this paper focuses on circuit configurations and voltage-balancing control of parallel-connected modular multilevel converters (PMMCs), and the combination of average and individual voltage control methods enables the parallel-connected MMCs to achieve the voltage balancing of all sub-modules without using any external circuit. Next, a zero-sequence model is developed to predict the dynamics of the zero-sequence current, and then an effective zero-sequence current control loop is designed to attenuate the circulating current. Finally, Matlab simulations verified the performance of the proposed control strategy.

This paper investigates the novel modulation method of six-switch back-to-back converter for the application of an online uninterruptible power supply. With an additional current direction judgment logic and related modulation adjustment, the total modulation index can be increased and consequently the overall losses and the voltage stress across semiconductors will decrease significantly, being much different from other modulation methods of the six-switch converter. Matlab simulation and experimental prototype are constructed to verify the performance of the proposed modulation method.