In order to improve the low voltage ride through level of the low voltage auxiliary frequency converter in the thermal power plant and effectively solve problems like the inaccurate time of voltage drop device, which is controlled by the switching structure of the traditional multi-winding transformer, and large inrush current during the closing process, a 30 kW testing device for low voltage auxiliary frequency converter in the thermal power plant is developed and a three-phase four-wire back-to-back power electronic topology is designed, together with adaptive fuzzy PI control algorithm. They can accurately set the voltage dip amplitude and time, achieving automatic cycle test controlled by upper computer. A low voltage ride through scene experiment has been carried out on low-voltage auxiliary frequency inverter both before and after the transformation in a thermal power plant in Jiangxi Province. It proves that the dynamic response speed of power electronic voltage drop device is less than 5 ms, the inverter output voltage harmonic content is less than 0.5%. What’s more, the operation is convenient and the protection strategy is complete and reliable. This work is supported by Science and Technology Project of State Grid Jiangxi Electric Power Company — “Research and Application of LVRT Detection System of First-class LV Auxiliary Frequency Converter in Thermal Power Plant” (No. 52182015000G).