Small-Signal Stability Analysis Based on the Sequence Domain Impedance SISO Models for the VSI-VSC Cascaded System

The increasing “dual-high” grid characteristics make VSI-VSC systems a typical topology, where traditional stability analysis methods prove inadequate. This paper aims at the small-signal stability of VSI-VSC cascaded system. First, the $d q$ impedance model and MSD impedance model of the system are established, and the SISO equivalent model of the sequence subsystem is constructed based on this. This model not only retains the advantage of the clear physical meaning of the sequence impedance method, but also overcomes the shortcomings of the MIMO impedance model in complex calculation. Then, based on this model, the instability mechanism of the system is analyzed, and the influence of some key parameters on the stability of the system is theoretically analyzed. Finally, the correctness of the theory and analysis results are verified through simulation.