Analysis and Suppression of the Capacitor Voltage Ripple for Hexverter-Based Modular Multilevel Converters

Hexverter is a direct AC-AC modular multilevel converter based on cascaded full-bridge sub-modules, which is well suited to connect two systems operating at different frequencies. However, its operating range is limited by the capacitor voltage of sub-modules. A strategy based on the optimal neutral voltage and circulating current is proposed in this paper to limit the capacitor voltage within a specific range while minimizing the switching loss. First, the capacitor voltage's general fluctuation law is studied, and the detailed mathematical expression of the capacitor voltage is given. After that, constant neutral voltage and circulating current are introduced to limit its magnitude. In addition, the optimal values of neutral voltage and circulating current are calculated by minimizing switching loss, and the complete suppression strategy is put forward. Finally, it is verified by simulation that the proposed strategy suppresses the magnitude of the capacitor ripple voltage and reduces the switching loss.