An Enhanced Modular Multilevel Converter With Multiple MVAC Ports Based on Active Fundamental-Frequency Circulating Current Injection to Realize Full-Range Operation

Abstract

The extensive integration of new sources and loads threatens the safe operation of the conventional ac distribution networks. The soft normally open point (SNOP) is proposed as an efficient solution. However, the mainstream back-to-back modular multilevel converter (MMC) scheme requires numerous devices and capacitances, which is not conducive to popularization. Lately, a novel modular multilevel converter with multiple MVac ports (MACP-MMC) has been introduced, offering a significant reduction in the construction cost and volume. Nonetheless, the MACP-MMC has some application limitations: when the MMC's ac output current is minimal, the MACP-MMC's power balanced control submodule has insufficient ability to maintain the internal energy balance, leading to its failed operation. To address this issue, this manuscript presents an enhanced MACP-MMC (eMACP-MMC) that features wide adjustment range. The proposed eMACP-MMC improves the connection structure between the MMC part and the multiport interconnection module part with small cost, thus offering an extra energy exchange path with the circulating current. On the one hand, eMACP-MMC can work as “MACP-MMC mode” under the joint supply or consumption scenario. On the other hand, the internal energy balance can be achieved by actively injecting fundamental-frequency circulating current under the flexible interconnection scenario. Consequently, the eMACP-MMC is suitable for all the application scenario of SNOP. Meanwhile, the circulating current's other degree of freedom is used for eliminate the imbalance between the MMC's arms caused by the former component. Additionally, considering the shortcomings caused by circulating current, the eMACP-MMC's redundant degrees of freedom are used to minimize the injected circulating current. Then, the novel operating principle and the control strategy of eMACP-MMC are researched.