Transient Stability Analysis of Hybrid GFL-GFM System Considering Various Damping Effects

As converter-based resources keep expanding in power systems, it is urgent to clarify the transient coupling mechanisms between different control types of converters. It has already been found that both the grid-following (GFL) control and the grid-forming (GFM) control could introduce significant damping effects on transient angle motions. However, the damping effects on the relative motion between different control types of converters remain unclear, which impedes the transient stability evaluation of systems containing various converters. To fill the gap, this article explores the transient damping effects present in a hybrid system containing both the GFL and GFM converters. Two distinct damping effects introduced by the GFL and the GFM algorithms are first identified. It is found that the damping terms can introduce negative damping effects during transients, which would deteriorate the system transient stability. To quantify the damping effects, a critical trajectory approximation (CTA) method for the GFL-type damping and a damping energy approximation (DEA) method for the GFM-type damping are proposed. On this basis, a transient stability criterion for the hybrid systems is developed to directly identify the critical fault-clearing point without any repetitive iteration. Simulations and hardware-in-loop experiments are both implemented for validation. The proposed method effectively evaluates the complex damping effects and quantitatively solves the transient stability boundary for hybrid GFL-GFM systems, which may provide some valuable insights for transient stability analysis of hybrid systems.