Impedance Modeling and Stability Analysis of Two-Terminal MMC-HVDC Systems Based on Multi-Port Generalized Transmission Parameter Models

Two-terminal MMC-based HVDC systems exhibit intricate multi-port coupling and high-order dynamics, presenting significant challenges for stability assessment. Conventional methods based on the source-load impedance ratio and the generalized Nyquist criterion (GNC) encounter difficulties in high-order matrix-form systems due to the complexity of identifying right half plane (RHP) poles. To address this, this paper proposes a stability analysis framework based on the multi-port generalized transmission parameter (GTP) model. By partitioning the system into subsystems, the proposed method facilitates indirect determination of RHP poles, circumventing numerical challenges associated with direct calculation. Specifically, the MMC-HVDC system is decomposed into a wind farm (WF) module, an AC grid module, and an MMC back-to-back (MMC-BTB) module. Utilizing the harmonic state space (HSS) method and power conservation principle, a hybrid admittance model is developed. Subsequently, the multi-port GTP model for the MMC-BTB is derived via Schur complement transformation. Furthermore, a stability margin quantification approach is established based on the stable winding distance index and the GNC. Finally, time-domain simulations validate the accuracy and effectiveness of the proposed GTP-based method in assessing the stability of two-terminal MMC-HVDC systems.