Stability Assessment Study for a Triple-Stage Three-Phase Solid-State Transformer

This paper proposes an accurate stability and performance analysis for a triple-stage three-phase modular solid-state transformer (SST), based on the cascaded H-bridge (CHB) and on the dual active bridge (DAB) topologies. The control strategy of this system consists of a complex multi-layer structure, mainly because of the need to balance the various dc-links of the converter. Such a complex system is highly prone to instability, both for the cascaded topology and for the control structure. In this paper, the small-signal modelling approach and the Middlebrook's criterion are used to address the stability issues of the converter. For this purpose, the small signal model for each converter stage is developed and through those the SST impedance transfer functions are derived. To validate the theoretical impedance equations, a numerical simulation has been carried out in order to map the frequency response of the converter stages. Then, the stability of the whole system is discussed, pointing out what parameters may cause instability in the analyzed SST system. Four relevant case studies corresponding to four different operating modes are assessed and verified via Simulink simulations.