Overload Operation of LV-Side Inverter in Smart Transformer

The power electronics-based Smart Transformer (ST) has the ability to provide ancillary services to the ac and dc distribution grids and the potential to reduce the reinforcements in the modern electric grid. However, distribution power transformers, also the ST, often experience overload caused by the peak load and grid faults. Such overloading currents can be withstood by the conventional power transformer but threaten the safety of the power electronics-based ST. Overload operation of the ST is constrained mainly by the thermal limits of the power semiconductors in ST. This paper systematically analyses the vector-sequence-dispatch mode in modulation focusing on the power losses reduction and the impacts of power quality, and then proposes a method to increase the overload capability of the ST. The simulation and experimental results both clearly validate the effectiveness of the improved overload operating capability, and the correctness of theoretical analyses.