Stabilization of a Cascaded AC–DC–DC System With Small Bus Capacitance Based on Small Signal Analysis

In this paper, the source converter (voltage source rectifier) of a cascaded system is modeled based on small signal analysis, and the closed-loop transfer functions of the converter are obtained. Then, the output impedance of the source converter is calculated. The analysis of the output impedance indicates that the smaller the bus capacitance, the worse the stability of the cascaded system, and the smaller the power converted by the cascaded system. Three kinds of virtual impedance control methods are proposed to reduce the output impedance of the source converter; all of them improve the stability and increase the maximum power of the cascaded system. The parameters of the three kinds of methods are designed and their effects are compared. Simulation and experimental results show a good agreement with the theoretical analysis, which proves the validity of the virtual impedance control methods.