Modified Modular Multilevel Converter With Third-Order Harmonic Voltage Injection to Reduce Submodule Capacitor Voltage Ripples

This article presents a modified modular multilevel converter (MMC) equipped with the middle-cells and control algorithm for medium voltage applications. It is intended to reduce the submodule (SM) capacitor voltage ripples by the third-order harmonic voltage injection (THVI) and second-order harmonic current injection (SHCI) without the common-mode voltage injected on the ac-side. On the one hand, using the proposed algorithm, the independent power degrees of freedom can be produced to eliminate the dominant fundamental part in arm powers, which can dramatically reduce the SM capacitor voltage ripples. On the other hand, the smaller amplitude of the SHCI can be obtained, which decreases the power losses. Meanwhile, by properly controlling the proposed middle-cells of the modified MMC, the compensated parts can be produced to eliminate the common-mode voltages caused by the THVI. Additionally, the arm power analysis of the modified MMC under different power factors is conducted, and the obtained results are discussed. The mathematical models of the SM capacitor voltages for the half-bridge based SMs and the proposed middle-cells in the modified MMC are also analyzed. Finally, the effectiveness of the theoretical analysis of the modified MMC is verified by simulation and experimental results.