Systematic Approach for Transient Stability Evaluation of Grid-Tied Converters during Power System Faults

Grid-tied converters subject to severe grid faults might experience transient instability and loss of synchronization. As studying this phenomenon deals with large-signal disturbances, a linearized equivalent will no longer be an accurate approximation of the system. Consequently, a highly used approach is to perform time-domain simulation studies of a detailed model of the system to assess the transient stability and overall performance. However, such an approach might result in a large computational burden and limited physical insight to the system. To address this issue, this paper presents a systematic approach to assess the transient stability of the inherently nonlinear problem alongside an methodology for setting the control parameters to avert transient instability. Using a simplified model for the converter, it is shown that phase-plane analysis is accurate for analyzing the transient synchronization stability. To that end, a critical damping ratio of the phase-locked loop can be found to identify the domain of attraction of the state trajectories, which has been experimentally verified. Using this together with an engineering insight to the system, one can set the controller parameters of the phase-locked loop such to guarantee stability during severe grid faults and perform worst-case planning settings for the controller and protection devices.