Unbypassed Soft-Start of Hybrid Distribution Transformer With Delayed Enablement and Parameters-Varying Control

A soft-start strategy is essential for connecting the hybrid distribution transformers (HDTs) to the medium-voltage power grid. This article proposes a novel soft-start strategy that eliminates the need for bypassing the series converter, thus simplifying the conventional approach. Modeling analysis indicates that, during the uncontrolled rectification stage, the unbypassed soft-start strategy allows both the series and shunt converters of the HDT to charge their dc-link capacitors simultaneously, maintaining approximately fixed current ratio and voltage ratio. In the pulsewidth modulation rectification stage, sequentially enabling the closed-loop control of shunt and series converters allows the dc-link capacitors to be further charged to their rated voltage. The key innovation lies in the slight delay enablement of the series converter and the PI parameters-varying control, which can effectively suppress the inrush current owing to the weak controllability of the shunt converter and the windings voltage change during the switchover period. Simulation and experimental results validate the effectiveness of the proposed unbypassed soft-start strategy, whether using constant or ramped dc-link voltage references.