A Four-Level Modular Multilevel Converter with Self Voltage Balancing and Extremely Small DC Capacitor

The modular feature of modular multilevel converter (MMC) makes it stand out for medium/high voltage applications. However, as the number of sub-modules increases, the control complexity of voltage balance of each sub-module sharply increases. Conventionally, the MMC sub-module voltage cannot be balanced without voltage monitoring and control. This paper mathematically proofs that four-level MMC has the merit of self voltage balancing by nature. This merit eliminates the voltage monitoring and control for MMC. The four-level MMC simulations are provided for verification purpose. In addition, since the sub-module capacitor voltage tends to converge to nominal value, the low frequency voltage ripple is greatly reduced, which allows the sub-module capacitance to be extremely small. The proposed four-level MMC is 1/8th-1/6th the capacitance compared to conventional MMCs.