Stability Boundary Evaluation Method for the Cascaded Bipolar DC Distribution System With CPLs Based on Impedance Analysis

At present, most existing methods for suppressing voltage oscillations induced by constant power loads (CPLs) in bipolar DC distribution systems (Bi-DCDSs) only focus on control frameworks, while the evaluation of the CPL power demand boundary defined by control parameters for ensuring system stability remains unexplored. To help tuning the control parameters, this paper proposes a boundary evaluation method to support control parameter tuning and thereby ensure the stability of Bi-DCDSs. The methods for suppressing voltage oscillations discussed in this paper include: the virtual impedance control (VIC) method based on the positive and negative port components, and the VIC method based on the DC symmetrical components. Firstly, the source impedance models and load admittance models of the Bi-DCDS controlled by the two VIC methods are established. Based on these established models, the input-output gain matrices of the Bi-DCDS are derived, providing foundation for stability analysis. Using the Generalized Nyquist Criterion, the stability of a Bi-DCDS with specified parameters can be assessed. By exploring the stability of the Bi-DCDS under different CPL power demands, the boundary can be identified. The case study demonstrates that, for a ±375 V system, the evaluation results obtained by the proposed method deviate from the simulation-identified boundary by less than 2 kW, verifying the effectiveness of the proposed approach.