Modified Open-Loop Phase Estimation of SOGI-FLL for Frequency Coupling Suppression and Synchronization Stability Enhancement of VSC Systems

The second-order-generalized-integrator-based frequency-locked loop (SOGI-FLL) has widely used in synchronizing voltage source converter (VSC) systems. However, it causes frequency coupling (FC) and stability issues. This article proposes a modified open-loop phase estimation scheme to address these challenges. The root cause of the FLL-induced adverse effects is first examined, revealing that, despite the symmetrical structure of the SOGI-FLL, it introduces asymmetrical dq-frame coupling terms into the VSC control. This results in significant sequence impedance coupling and negative resistance behaviors. To this end, a nonlinear synchronization phase compensation factor is designed to balance the uncovered asymmetrical coupling terms, thereby minimizing the FC. Additionally, to address the negative resistance issue, an impedance phase compensation function with two degrees of freedom is further proposed to regulate the impedance phase and effectively narrow the frequency range of negative resistance. Last, experiments are conducted to confirm the effectiveness of the proposed approach.