Protection apparatus for transient voltage drop

The purpose of the present invention is to decrease the repeated charging and discharging frequency of an electrolytic capacitor and achieve long service life and small capacity thereof under the condition that the utilization is carried out with a commercial AC power having larger voltage variation in some countries/regions. Therefore, when an input AC voltage falls within the range from 70% to 90% of the normal input voltage, making an auxiliary inverter perform the DC-AC conversion work to connect the switch of a switching part, generate the compensation power based on the input AC power, and add it to the input AC power through injecting a transformer. If the input AC voltage drops to the level less than 70% of the normal input voltage, disconnect the switch of the switching part and make a main inverter perform DC-AC conversion work, so as to convert the DC power accumulated in the electrolytic capacitor into the substitute AC power to run a load.