Consideration of Control-Loop Interaction in Transient Stability of Grid-Following Inverters using Bandwidth Separation Method

Grid-following inverters have been widely adopted as a grid interface for renewable energy resources, but ensuring their transient stability has been a growing challenge in part because of a lack of clarity on how interactions between the inverter's various outer control loops impact transient response. In this paper, the nonlinear dynamics and transient stability of grid-following inverters are investigated, where not only the phase-locked loop (PLL) but also the DC-link voltage control (DVC) and the AC terminal voltage control (TVC) are taken into account. The use of bandwidth separation method is proposed, though which the original differential equations can be simplified, and their order can be reduced, providing that the separation between their bandwidths is sufficiently large. Analysis of the interaction between the DVC and the PLL is developed, proving that this interaction negatively impacts stability. Furthermore, optimal combinations of bandwidth of PLL and DVC are identified under various types of grid faults. The impact of TVC on transient stability is analyzed. All of the results and the conclusions drawn from them are verified by simulation.