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For grid-connected voltage source converters (VSCs),the transient instability in dc-link voltage control timescale threatens the dc capacitor safety and grid stable operation. However, the transient stability analysis presents great challenges, due to the high-order and nonlinear coupling characteristics of VSC. Existing studies mainly consider single control loop to reduce order and simplify analysis, failing to fully reveal transient instability mechanism. To cope with it, this paper develops a reduced-order model based on singular perturbation, and the fourth-order full model of VSC is decomposed into a second-order nonlinear slow subsystem and a third-order linearized fast subsystem. Subsequently, the transient stability of slow and fast subsystem is analyzed by the Lyapunov's direct method and eigenvalue method, respectively. Meanwhile, a set of sufficient conditions for transient stability are derived, and the influence of several factors (e.g., grid impedance and multiple control loop parameters) on stability are revealed. Further, some guidelines for transient stability enhancement are proposed. Finally, the analysis is validated by the experiments.
Ma et al. (Mon,) studied this question.