Medium-frequency transformers are used in high power electronic dc/dc converters. In case of the three-phase dual active bridge topology (DAB3), a six-step voltage is applied to the transformer. In this study, a 200 kVA three phase power transformer prototype is using 0.18 mm thick high permeability Grain Oriented Electrical Steel (GOES) at B = 1.5 T and f = 1000 Hz. The prototype has been designed and built in link with a transformer manufacturer. In this paper, a complete test bench is set up to validate GOES as a suitable material for medium-frequency transformers. The efficiency of the components is evaluated at different operating points.