Active Power Decoupling for Single-Phase Input–Series–Output–Parallel Solid-State Transformers

Solid-state transformers with input–series–output–parallel structures are being considered for a variety of applications requiring medium-voltage alternating current (MVAC) to low-voltage direct current (LVDC) conversion. Due to the single-phase ac–dc conversion at the input side, all floating cells of the solid-state transformer suffer from the well-known second harmonic pulsating power. Large dc-link capacitors are typically employed to smooth the ripple voltage created by the pulsating power, which leads to low power density. An alternative solution is to adopt active power decoupling technology, which can buffer the pulsating power in separate energy storage components. This article investigates the feasibility and potential benefit of applying active power decoupling to single-phase input–series–output–parallel solid-state transformers. The buck-type active power filter is selected, and the design of a prototype is presented. Simulation and experimental results show that a significantly lower second harmonic ripple voltage is achieved with active power filters and very small dc-link capacitance.