A three-stage buck power electronic transformer（PET）topology based on the LLC resonant converter is proposed.In the proposed PET，the high voltage stage on the input side is a diode-clamped three-level rectifier for use in reducing the voltage withstanding grade of switching devices.The isolation stage adopts a three-level LLC series resonant DC-DC converter，the voltage stress of each of whose main switches is half the input voltage.It can realize soft-switching from no-load to full load，which will reduce losses and improve efficiency.The low voltage stage on the output side is a two-level three-phase four-leg inverter to cope with the huge imbalance and nonlinear loads.A control scheme is designed and simulation is performed to verify the performance of the proposed PET.Simulation results show that the proposed PET not only has the basic features of isolation，voltage transformation and energy transmission as does the traditional transformer，but also carries out the power quality regulating function，which can avoid the mutual influence between system and load due to imbalance and distortion.