Protection of hybrid transformers in the distribution grid

Due to the increasing integration of renewable energy sources and power electronic loads into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low frequency transformer and a fractionally rated power electronic converter, the hybrid transformer can be applied to ensure a high power quality by controlling voltage, current, active and reactive power dynamically. For the application in grids with conventional grid protection infrastructure, hybrid transformers have to withstand considerable overvoltage and -current stresses during voltage surges or grid short circuits. Since the semiconductors are less robust than low frequency transformers with respect to these stresses, the effects of possible fault scenarios and a protection concept are studied in this paper.