Modeling and Stability Prediction of the Multi-Port DC Hub for Resilient Interconnection

The multiport DC hub has been evaluated as a cost-effective solution for interconnecting several HVDC grids. To address the stability issues of the isolated multiport DC hub, in this article, a comprehensive stability assessment has been conducted under different working modes, especially after DC fault occurrence. The multiport DC hub is implemented to interconnect several different DC power grids and a coordinated control scheme is designed for the DC hub to achieve power balancing automatically. To establish the mathematical model, the d-q frame impedance-based small-signal model is derived in a uniform rotating frame, providing a global view of system dynamics. And this model has been verified using frequency scanning method in Matlab/Simulink. By selecting dynamic interactions at the internal AC bus, the multiport DC hub is divided into the equivalent models of the source subsystem and load subsystem. Based on the Generalized Nyquist Criterion (GNC), the oscillation frequencies under unstable modes could be predicted by the characteristic loci-plot in normal working mode. Additionally, stability assessment is furtherly conducted on the new steady-state after fault occurrence. The fault tolerant operation, which refers to input power curtailments and control mode switching, have proven to be effective, and their impact on stability has been revealed. Moreover, an active stabilization method based on impedance reshaping is implemented for the unstable mode after fault occurrence. Finally, all theoretical analysis results in frequency-domain (FD) have been validated by the time-domain simulation in Matlab/Simulink. The research work could be beneficial for the future engineering practices of complex DC system.