In order to ensure fast response of power electronic transformer port fault removal, DC solid-state switches are usually equipped on the DC port side. When the DC port voltage of power electronic transformer has been established, the capacitance of DC port of power electronic transformer will charge the capacitance of solid-state switch at the moment of solid-state switch closing. Considering the line impedance characteristics, the current sensor at the traditional position of DC port will oscillate due to the line impedance and capacitance in solid-state switch, which will cause serious over-current protection rectification. The machine is locked. The DC port of power electronic transformer is modeled and analyzed, which is simplified to the series response characteristics of RLC circuit. The current oscillation waveform on the DC side of power electronic transformer is analyzed, and the principle of optimal selection of measuring points of current transformer is proposed. The correctness of the theory is verified by simulation and experiment.