Synchronization Stability Analysis and Fuzzy PI-based Control for Grid-Connected Inverter

Abstract

The grid-connected inverter system employing the phase-locked loop (PLL) as the synchronization unit is prone to lose the synchronization under large disturbances. Therefore, this paper aims to propose an enhanced PLL to improve the system transient stability. At first, the transient synchronous instability mechanism of the system under grid voltage drop is analyzed, and it is learned that the parameters of the PLL have a great influence on the damping and stability of the system through phase portraits. But, it is noted that the derived stability boundary is ambiguous and inaccurate due to some assumptions and nonlinearities. Hence, considering the fuzzy feature, this study tries to apply the existing fuzzy theory to improve the grid-connected inverter behaviors. A fuzzy PI (F-PI) controller is proposed to adaptively adjust the PLL parameters to improve the system damping when the grid voltage drops, so as to improve the transient stability. Combined with the analysis of transient instability mechanism, the design process of F-PI controller is given in detail. Finally, the effectiveness of F-PI control PLL in improving the transient stability is verified by simulation tests and experiments.