Stability Analysis and Harmonic Filtering Enhancement of Single-Voltage-Loop PI-Controlled Grid-Forming Converters

Grid-forming converters (GFMCs) remain on a strong growth trajectory in the field of power electronics due to ac voltage formation and frequency regulation capabilities. Compared with multiloop control strategies, single-loop voltage control benefits from the simplicity of sensors and controllers, rapid dynamics, and no voltage/current control interaction. However, as revealed by this article, LCL-filtered GFMCs with single-voltage-loop proportional-integral (PI) controllers feature a high resonance frequency for stable operation in grid-tied and islanded scenarios, which deteriorates switching harmonic filtering due to the small filter size. As such, we propose a switching harmonic filtering enhancement strategy for GFMCs by use of the LLCL filter, whose LC-trap forms a serial resonance at the switching frequency and thus allows improved harmonic attenuation. In this way, we achieve strong switching harmonic filtering of LLCL-filtered single-voltage-loop PI-controlled GFMCs. Furthermore, this article derives relevant stability regions. Finally, simulation and experimental results verify the theoretical analysis and switching harmonic attenuation capability.