Medium frequency transformer (MFT) is the key component for solid state transformer (SST) applications. It provides voltage conversion and galvanic isolation between medium/high-voltage side and low-voltage side. The employment of the MFT can significantly reduce volume and weight of power conversion systems. For high-power medium/high-voltage applications, multilevel converters are the most promising topology and applicable for SST applications. Compared to the conventional full-bridge or half-bridge converters producing square excitation voltage waveforms for the MFT, multilevel converters can generate controllable multilevel excitation voltage waveforms, which potentially improves efficiency and controllability of the SST. In this paper, various excitation voltage waveforms for the MFT are investigated and the optimal multilevel excitation voltage waveforms are identified in terms of total losses and system complexity. Meanwhile, factors affecting core loss and winding loss of the MFT are explored. These factors are applicable for transformer loss analysis under arbitrary excitation waveforms.