Stability Modeling and Parameter Analysis for Flexible DC Power Grids Under Asymmetric Conditions

When AC system operates asymmetrically, poor structure and control parameters of the converter station controller will have a negative impact on the stable operation of the flexible DC power grids. In this paper, a small-signal model of flexible DC power grids under asymmetric conditions is proposed, considering the internal characteristics of converter station, positive-sequence current vector controller based on voltage-droop, negative-sequence current suppressor based on quasi-proportional integral resonant (quasi-PIR), and DC power grids. Then, the influence of control parameters in voltage-droop controller and quasi-PIR suppressor on the system stability is analyzed by using Lyapunov stability theory and the root locus method, and their reasonable ranges of values are determined. Finally, the Zhangbei four-terminal flexible DC grid project is simulated in PSCAD/EMTDC to verify the accuracy of the presented small-signal model and the conclusions about the impact of parameters on the system stability, which provides a theoretical basis for the stability analysis and the tuning of control parameters in flexible DC power grids under asymmetric conditions.