Stability assessment and coordinated impedance shaping strategy for DC bidirectional cascaded system with LC input filter

In the ac/dc hybrid power system, bidirectional dc/ac converters, LC input filters, and energy storage bidirectional dc/dc converters form the dc bidirectional cascaded system. However, the system suffers instability issue in the different power flows, especially when one of the converters is constant power-controlled. Furthermore, the LC filter circuit will increase the output impedance of the cascaded system, which degrade the stability margin and might lead to the dc bus voltage oscillation. In order to analyze and solve such instability issue of the bidirectional cascaded system, this paper presents an impedance-based stability assessment; then, a coordinated impedance shaping strategy based on the voltage proportional controller is proposed. The proposed strategy can reduce the output impedance of the dc bus voltage-controlled converter and increase the phase of input impedance for the constant power load simultaneously, thus increasing the stability margin. Moreover, the proposed strategy eliminates the steady-state error of dc voltage by introducing power feed-forward control. Finally, the efficiency of this proposed control strategy and theoretical analysis are validated by simulation and experimental results.