Stability Analysis of Cascaded System With Multiple DC Buses Based on Positive-Mode Damping Criterion

The coupling between various levels of buses in a cascaded system may cause instability of the entire system. The traditional Nyquist criterion is not convenient for predicting the oscillation frequency and some extra steps might be needed for the possible right half plane (RHP) poles. Therefore, new stability criteria are desired to analyze the stability of this type of hybrid system. In this paper, taking a cascaded system containing an AC-DC converter and multiple DC-DC converters as an example, the small signal impedance model of the system is firstly established and analyzed in detail according to the mutual coupling principle between different converters. Secondly, based on positive-mode damping (PMD) criterion, a simple stability assessment method for cascaded systems is proposed. By analyzing the real and imaginary parts of the eigenvalues of the closed-loop transfer function matrix, the stability state and the oscillation frequency of the system can be obtained. Finally, the relationship between the coupling degree of each converter and the oscillation of bus voltage is explored and a case analysis is given. The proposed model and stability analysis results are verified both by simulations and semi-physical hardware experiments.