The proportion of voltage sag faults in the voltage quality problem is more than 80%，which seriously threatens the safety，economy，and efficiency of electricity consumption.To solve this problem，a voltage quality control system of distribution networks for hybrid transformers is proposed.By hybrid transformers and the integration of photovoltaic units and energy storage units，three operational modes are built，including grid-connected inverter mode，islanding inverter mode，and grid-connected rectifier mode.The grid-connected power supply of photovoltaic units，compensation of voltage sag of energy storage units，and charging function of energy storage units by distribution networks are realized，respectively.Then，the functions of the system are verified by simulations. Finally，a hardware prototype test platform is built，where the rated power of the system is 1 kW.A voltage regulator is used to simulate a unidirectional grid voltage of 220 V，with a load voltage of 110 V，and a tapped transformer with a turn ratio of 10：5：4，for further verification.The research results show that the proposed system is capable of effectively mitigating voltage quality problems in distribution networks across the three operating conditions.