Interactions between individually designed power subsystems in a cascaded system may cause instability. This paper proposes an approach, which connects a virtual impedance in parallel or series with the input impedance of the load converter so that the magnitude or phase of the load converter's input impedance is modified in a small range of frequency, to solve the instability problem of a cascaded system. The requirements on the parallel virtual impedance (PVI) and series virtual impedance (SVI) are derived, and the control strategies to implement the PVI and SVI are proposed. The comparison and general design procedure of the PVI and SVI control strategies are also discussed. Finally, considering the worst stability problem that often occurs at the system whose source converter is an <tex-math notation="LaTeX">$LC$</tex-math> filter, two cascaded systems consisting of a source converter with an <tex-math notation="LaTeX">$LC$</tex-math> input filter and a load converter, which is either a buck converter or a boost converter, are fabricated and tested to validate the effectiveness of the proposed control methods.
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Zhang et al. (2015) studied this question.
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