The present invention relates to a static compensator and a method for controlling thereof, more particularly, to a static compensator including a transformer connected with a bus of a power system; an inverter operating according to a control signal and connected to the transformer in series; a tap changer changing turn ratio of the transformer; and a controller generating the control signal to operate the inverter and generating a tap up or a tap down signal to provide to the tap changer and a method for controlling thereof.