Self-Excitation Startup Strategy of Cascaded H-Bridge Grid-Connected Converter Based on Dynamic Virtual Impedance

Cascaded H-Bridge (CHB) grid-connected converters face challenges during startup, including issues such as overmodulation and inrush currents due to low dc-link voltage. In addition, the common occurrence of dc-link voltage imbalance among multiple independent submodules (SMs) of CHB exacerbates these risks. In this article, a self-excitation startup strategy based on dynamic virtual impedance is proposed, adjusting the converter's output impedance to a virtual impedance. By establishing a relationship between the modulation index and virtual impedance, the modulation index is controlled within the linear modulation range, effectively suppressing inrush currents. The design of the modulation index reduces power loss during startup and enhances the charging power of the converter, allowing the converter to complete startup within a predetermined time. Furthermore, for typical control-based methods of dc-link voltage balance, a quantitative analysis of the impact of modulation index on the range of SM active power modification is conducted to optimize the modulation index design, thereby improving the dc-link voltage balance capability during the startup. Simulation and experimental results are presented to validate the effectiveness of the proposed method.