Simplified Stability Analysis and Enhancement Methods for Grid-Connected Inverter System in Sequence-Domain

The generalized Nyquist criterion (GNC) can assess the stability of three-phase grid-connected inverters (GCIs) modeled as multi-input multi-output (MIMO) systems, but its computational complexity and poor physical interpretability hinder practical applications. This paper proposes a simplified stability analysis method via loop decomposition: the MIMO system is decomposed into three single-input single-output (SISO) subsystems, which explicitly establishing the relationships between subsystems and global stability. Furthermore, to address stability degradation caused by phase-locked loops (PLLs), an impedance reshaping strategy is developed by integrating PLL phase-angle feedforward control to enhance system stability. Simulations validate that the simplified stability analysis method predicts stability accurately, and the impedance reshaping technique extends the system stability margin effectively.