Second-order Generalized Integrator (SOGI)-based quadrature-signal-generator (QSG), in conjunction with either a phase-locked-loop (PLL) or a frequency-locked-loop (FLL), is widely used as the synchronization unit of grid-tied converters. This paper will reveal and discuss the stability impacts of the frequency-feedback-path (FFP) used in connecting SOGI-QSG and PLL/FLL. Depending on where the FFP is fed into the SOGI-QSG, four types of FFP implementations that are frequently seen in the literature can be obtained. Although these FFP implementations will not affect the steady-state frequency adaptation of SOGI-QSG, their dynamic impacts, particularly in regard to the small-signal stability, remains concealed and a thorough analysis is missing. To bridge this gap, this paper will present a comprehensive stability assessment and comparison of SOGI-PLL/FLL focusing on the FFP issue. To extend the applicability and accuracy of the analysis, a parameter space-oriented stability assessment method is presented, which is formulated in the linear-time periodic (LTP) framework and automatically processed in MATLAB. The obtained results not only provide insights into the stability of SOGI-PLL/FLL but also suggest the appropriate FFP implementations in view of stability. Besides, the presented method can serve as an efficient tool for fast stability tests of SOGI-PLL/FLL when designing their parameters.
- Frequency feedback