For the distribution of electrical energy in Germany, application-specific transformers with ratings of 110/20 kV, 110/10kV, 40 MVA, 50 Hz are typically in operation. Due to their weight, these transformers can only be transported with great expense (usually by rail). Therefore a considerable number of reserve transformers has to be held in stock in each distribution area, to guarantee power reliability to utility customers by quick replacement. To reduce assets, a universal, mobile and flexible (adjustable to different medium-voltage levels and truck movable) transformer reserve unit is desired by utility companies. The modular design of an electronic power transformer for this purpose is given in this paper. Based on power electronics, additional features like fast voltage regulation, reactive power compensation, ride-through capability etc. are described.