A Coupling-Factor-Based Nonlinear Decoupling Method for Transient Operation Analysis of LCL-Filtered Grid-Connected VSC

In light of the exponential growth in renewable energy generation, voltage source converters (VSCs) have garnered extensive attention, particularly concerning control methodologies and steady-state operation analysis. Nevertheless, the comprehensive transient operation analysis under large disturbances remains a critical issue, owing to the challenges in the analysis and characterisation of high-order nonlinear VSC systems. In this regard, a coupling-factor-based nonlinear decoupling (CFND) method is proposed in this paper. Firstly, by evaluating the correlation degree between state variables with coupling factors, the original nonlinear high-order system can be divided into multiple decoupled first- or second-order cubic modes, which can facilitate the transient stability analysis. Secondly, with the proposed CFND method, the full-order large-signal model of LCL-filtered grid-connected VSCs is created to analyse the transient stability under grid fault conditions. It is revealed that increasing phase-locked loop (PLL) bandwidth and decreasing current loop bandwidth are unfavourable to the transient stability. Besides, decreasing the active current reference or increasing the reactive current reference is beneficial for transient stability during the low voltage ride-through (LVRT) process. Finally, the effectiveness of the stability analysis results is verified through hardware-in-loop (HIL) experiments.