Stabilization of a cascaded AC/DC system based on small signal analysis

In this paper, the source converter of a cascaded system is modeled by small signal method, and the closed-loop transfer functions of the converter are obtained by reducing the order. Then the output impedance of the source converter is calculated. The analysis of the output impedance indicates that the smaller bus capacitor is, the worse stability of the cascaded system is, and the smaller power can be converted by the cascaded system. A virtual impedance control method is proposed to effectively reduce the output impedance of the source converter, which improves the stability of the cascaded system with a small bus capacitor, and expands the power conversion range. Finally, the validity of the analysis and the effectiveness of the control method are verified by simulation.