Abstract: Aiming at the frequent voltage sag problem in power quality problems，a multifunctional voltage sag

treatment equipment was developed for user-side voltage sag treatment. Based on the modeling analysis of the

system，the control algorithm of the system under grid-connected and off-grid compensation states and the current

harmonic control algorithm were studied. By analyzing the characteristics of traditional voltage sag detection

algorithms，a hybrid voltage sag detection algorithm was proposed，which has the advantages of fast detection and stable phase-locking. The fast turn-off method of the triac was designed based on the characteristics of triac and grid voltage. Simulation and experimental results show that the device can achieve rapid detection of voltage sag within 0.2 ms，and it can realize fast and smooth switching of working modes. This can effectively reduce the impact of voltage sags on sensitive loads. At the same time，the equipment also has the functions of reactive power

compensation and load-side harmonic filtering，and it has the advantages of high efficiency and high convenience.

      Key words: voltage sag treatment；three-level inverter；grid-connected and off-grid control；voltage sag hybrid detection algorithm；triac