Dynamic Evolution and Stability Analysis of GFM-Based Renewable Energy Resources Considering Repeated & Continuous Current Saturation

The current saturation (CS) strategy of grid-forming renewable energy sources (GFM-RESs) is designed to protect the device from overcurrent damages. However, the potential switching processes of control strategies trigger new stability issues. It is revealed that two emerging stability issues, i.e., repeated current saturation (R-CS) and continuous current saturation (C-CS) governed by the inherent inconsistency of switching conditions, will occur when the system operates into the corresponding virtual power angle (VPA) region. This paper adopts the hybrid system theory to analyze such stability issues by deriving the necessary and sufficient switching conditions of current saturation and desaturation. During R-CS, the GFM-RES externally behaves as an abnormal current source, triggering adverse high-frequency oscillations. When subject to C-CS, the system will suffer from persistent overvoltage issues and cannot return to its initial operating point. To identify the stability risks of these two issues and offer guidance for GFM-RES's controller design, a novel R&C-CS criterion, along with a generic desaturation region-based (DRB) principle, is proposed. Meanwhile, a switching condition and dynamic characteristic decoupling control (S&D-DC) is developed based on the DRB principle to prevent the R&C-CS issues. High-fidelity electromagnetic transient simulations conducted on both single-machine and multi-machine systems validate the theoretical derivations and proposed control strategy.