Comparative Stability Analysis and Improvement of Grid-Following Converters Using Novel Interpretation of Linear Time-Periodic Theory

This article presents a comparative stability investigation of grid-following (GFL) converters with different advanced phase-locked loops (PLLs) and current control schemes based on linear time-periodic (LTP) theory. First, a time-domain physical interpretation of the LTP theory is proposed, which inspires an iterative LTP eigenvalue computation algorithm. A measure for the influence of the frequency coupling effect on each eigenvalue is defined to demonstrate the necessity of applying the LTP theory. Then, eigenvalue-based stability analysis is carried out to gain insights into dynamic characteristics of two implementations of GFL converters, namely, dual synchronous reference frame PLL (DSRF-PLL) and proportional integral (PI) current control, and dual second-order generalized integrator PLL (DSOGI-PLL) and proportional resonant (PR) current control. The effects of the PLL bandwidth, the current control time constant, the grid strength, and imbalance on the system stability are evaluated. In addition, a damping loop is proposed to improve the stability margin of GFL converters connected to weak grids. Finally, MATLAB/Simulink simulations and experimental measurements validate the correctness of the proposed modeling method and stability assessment results.