A Modular Multilevel Converter with Self Voltage Balancing

Abstract

The modular feature of modular multilevel converter (MMC) distinguishes itself from other topologies for medium-/high-voltage applications. However, as the count of sub-modules increases, the control complexity of voltage balancing for each sub-module sharply increases. Conventionally, the MMC is deemed to have no inherit voltage balancing property without voltage monitoring and control. This paper mathematically proves that MMC capacitor voltage is self-balanced by nature. This implies that MMC could achieve the sub-module capacitor voltage balancing without any monitoring or control. Based on the mathematical proof, a novel modulation, namely Y-Matrix Modulation (YMM), is proposed to transform the math analysis into modulation practice. With the proposed YMM, MMCs are secured self voltage balancing. Conventionally, either a complicated voltage balancing control, or extra components must be embedded to MMC to balance the capacitor voltage. Compared to conventional MMC capacitor voltage balancing strategies, YMM features extremely simple algorithms and good reachability to high-level MMCs while maintaining the original half-bridge sub-module topology.