Transient Stability Analysis for Grid-Forming VSCs Based on Nonlinear Decoupling Method

Abstract

With the increasing integration of renewable energy into the power grid, there is a growing demand for converters that not only provide stable power, but also support auxiliary functions such as grid-voltage regulation. Consequently, grid-forming strategies have attracted significant attention. However, due to the complexities of analyzing nonlinear coupling systems, a comprehensive transient stability analysis of grid-forming converters is still being explored. Conventional analysis methods rely on a simplified quasi-steady-state model for grid-forming voltage source converters (VSCs) and focus on analyzing the transient instability phenomenon caused by the outer power loop. However, this oversimplified model may yield incorrect conclusions. To address this limitation, this paper develops a full-order model that includes quadratic nonlinear terms to accurately represent the system’s nonlinear characteristics. The developed model is then decoupled into multiple low-order modes using a nonlinear decoupling method. These low-order modes can be analyzed using the mature inversing trajectory method, indirectly reflecting the transient stability of grid-forming VSCs under large disturbances. Through varying the inner and outer parameters, the transient stability of grid-forming VSCs is analyzed in detail. Furthermore, the analysis results are verified through hardware-in-loop (HIL) experiments.

A Method to Improve Both Frequency Stability and Transient Stability of Virtual Synchronous Generators during Grid Faults

“…However, due to the lack of rotating parts in the renewable energy interface converter, the large-scale substitution of the converters for the traditional synchronous generators (SGs) reduces the inertia of the power systems, resulting in new problems, such as a worse anti-interference ability and weaker frequency stability. A VSG which imitates the inertia and damping characteristics of SGs has become the most promising method to tackle existing problems [1][2][3].…”