模块化多电平变换器（MMC）建模、控制与低频运行技术研究

Compared with diode-clamped, flying-capacitor, and cascaded H-bridge (CHB) multilevel converters, the so-called modular multilevel converter (MMC) realized highest level of modularization, and therefore has been widely considered the most promising multilevel converter schemes in the future. Capacitor voltage balancing of the submodules is crucial for proper and stable operation of the MMC. Present voltage-balancing methods still have room of improvement regarding performance as well as ease of implementation. Circulating current is a unique issue of MMC, and is closely coupled with current stress of the switching devices, system efficiency, voltage fluctuation of submodule capacitors, low frequency operation capability of the system, and even system stability. Control of circulating current is also a challenging issue. Fluctuation of the submodule capacitor voltage is inversely proportional to the output frequency, which constitutes a major obstacle for application of MMCs in AC motor drives, where low frequency starting is often required. Present solutions for this problem focus on the injection of high-frequency circulating currents, which raises current stresses and loss. Control of high-frequency circulating currents is also difficult. While improving present methods, it seems worthwhile to try other possible solutions based on improved submodule topologies, and conducting comparative studies. By carrying out studies on the aforementioned three aspects, this project will help improve the theoretical basis of the MMC, and push forward its application in various engineering fields, especially AC motor drive applications.

与二极管钳位、飞跨电容以及串联H桥（CHB）型多电平变换器相比，模块化多电平变换器（MMC）实现了最高程度的模块化，已被公认为最有发展前景的多电平变换器方案。子模块电容电压平衡控制对于保证MMC正常稳定运行是至关重要的，现有的研究在性能上和实施性方面还有改进的空间。MMC中的环流与器件电流应力、效率、子模块电容电压脉动、MMC低频运行能力乃至系统稳定性之间具有复杂、重要的关系，其自身的控制也具有相当的难度。MMC子模块电容电压的脉动幅度与输出电流频率成反比，导致其低频运行能力受限，这是MMC应用于电机传动领域的主要障碍。现有借助于高频环流注入的解决方案存在器件电流应力大、控制困难等问题。在继续深入研究此类方案的同事，宜探索基于改进型子模块拓扑的其它解决方案。本项目拟主要针对以上三方面的关键技术展开研究，力图进一步完善MMC的理论体系，推动其工程实际应用，尤其是在电机传动领域的突破性应用。

本项目旨在解决模块化多电平变换器在传动应用场合的主要技术障碍——低频运行时子模块电压基频脉动问题，实现模块化多电平变换器在交流传动领域的突破性应用；并将现有有源功率解耦技术引入MMC 变换器,以期大幅提升其工频应用时的功率密度。.主要研究成果为：.（1）对传统的高频注入脉动抑制方案进行了改进，改进其波形后器件电流应力显著下降，并对注入的高频环流提出了精准跟踪策略；基于飞跨电容型MMC提出改进型脉动抑制方法，能在全速范围内减小注入环流的幅值，并显著减小了高频共模电压对电机轴承的影响；.（2）利用全桥子模块的结构特点，将全桥MMC应用于交流传统场合。相比半桥型MMC降低了器件电流应力，并提高了等效开关频率；.（3）利用原有半桥MMC的脉动特征，提出了两种“功率通道”来转移一次子模块脉动功率，适用于高功率密度恒转矩传动负载场合；.（4）将有源功率解耦技术应用于MMC的子模块中，提出了两种功率解耦拓扑，显著减小了子模块电容所需容量，提高了功率密度。