Transient Stability Analysis for Hybrid Parallel-Connected Converters by Two-Dimensional Phase Portrait

A hybrid parallel-connected system with grid-following (GFL) and grid-forming (GFM) converters can be characterized as a high-order nonlinear dynamic system when performing transient synchronous stability analysis, which cannot be analyzed clearly by the existing method. The existing methods analyze the power angle for each converter individually, which fails to elucidate the transient coupling dynamic process of the power angles among different converters. To fill this gap, a 2-D phase portrait method is proposed by structure-preserving dimensionality reduction mapping, which can effectively reveal the interactive dynamics between GFM converter and GFL converter. The four types of factors, which impact the transient stabilization are elaborated by the proposed method. Furthermore, the influence of current limiting strategies on the transient stability is revealed by applying the 2-D phase portrait. Finally, experimental results are provided to verify the theoretical analysis.