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

Abstract

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.

Keywords

• Stability modeling
• asymmetric condition
• flexible DC power grid
• droop controller
• quasi-PIR suppressor
• parameter analysis