Voltage balancing of modular smart transformers based on dual active bridges

Modular medium voltage power converters have gained increasing attention in context of Smart Transformers (STs) in the electrical distribution grid. A possible ST architecture can be built by a Cascaded H-Bridge (CHB) converter in the Medium Voltage side and Dual Active Bridges (DABs) in the isolation stage. One of the main issues in the CHB control is represented by the voltage balancing. This work investigates the control for the two converter stages comparing the voltage balancing by the MV stage with the voltage balancing by the isolation stage under unbalanced loading conditions. The proposed balancing technique, applied in the isolation stage is demonstrated to offer higher bandwidth than the commonly used balancing in the MV-stage, which is especially important in case of high number of CHB-cells and different power transfer of the CHB cells. These conclusions are supported by simulation and experimental results.