Power electronic transformer is a new type of power equipment for building smart grids. However, when the grid voltage drops deeply, it will cause its output voltage to be distorted and affect the stable operation of the load. The hybrid energy storage system composed of lithium battery and super-capacitor through bidirectional half-bridge DC/DC converter and dual active bridge DC/DC converter is proposed to be connected to the low-voltage DC side of power electronic transformer, so as to stabilize the output voltage of the power electronic transformer. The dual active bridge DC/DC converter and the bidirectional half-bridge DC/DC converter use phase-shift control and droop control, respectively. The result of the simulation shows that the addition of the hybrid energy storage system can enable the power electronic transformer to have the ability to compensate for the deep voltage drops, thereby improving the reliability of the system.