Review on Low-Capacitance Techniques for Modular Multilevel Converters

Modular multilevel converter (MMC) has been the most competitive solution to the high voltage dc systems and has been applied to multiple scenarios, such as long-distance electricity transmission, renewable energy integration, and interconnection of non-synchronous ac systems. However, one of the drawbacks of the MMC is the large footprint and high cost because large capacitance is demanded to limit the peak value of the capacitor voltages. Existing studies have conducted in-depth researches on how to reduce the total capacitor usage. This paper firstly illustrates the reason why the MMC typically utilizes the large capacitors. Then, a comprehensive review on the approaches to reducing the capacitor usage in the MMC is conducted. For each technical approach, the principle is firstly illustrated, the development is summarized, and the advantages and challenges are then pointed out. At last, different technical approaches are summarized and compared.