Cascaded Stabilization for ISOP-Type DC Transformers through Impedance Reshaping Regulator

In DC distribution networks, DC transformers (DCTs) are susceptible to cascaded instability due to the equivalent inductance of its input power cables. This issue is often overlooked in existing research owing to the complexity of input impedance modeling of DCTs. Therefore, this paper focuses on deriving the input impedance for the widely used input-series-output-parallel (ISOP) type DCT, of which the submodule is specifically the dual active bridge (DAB) converters, called ISOP-DAB. In addition, this paper evaluates the cascaded stability based on Nyquist Criterion and proposes an active damping method for DCT while considering the input power cables. First, a power-based impedance model of a DAB is adopted to derive the input impedance of ISOP-DAB, offering high simplicity and generality. Subsequently, the impact of the power cable on the cascaded stability is assessed. Then, an active damping method using impedance reshaping regulator (IRR) is proposed for DCTs to achieve cascaded stabilization. The IRR is easy to implement and does not require additional sensors. Finally, experimental results validate the effectiveness and compatibility of the IRR on stabilizing the DCT. Essentially, this work provides an easy and effective way to ensure the cascaded stability of DCTs while maintaining excellent dynamic and steady performance.