For the power transformer tap switching earlier mechanical type of on line tap changer was used. This method has some drawback like arcing, high maintains, slow reaction time and service cost. in order to overcome this limitation power semiconductor device used .In power semiconductor devices mainly used insulated gate bipolar transistor (IGBT) ,Triac ,thyristor .With the help of that devices we can improve the performance of the system using that devices we can improve quick operation of system because of that other problem in the ac main like flicker and sag .Also it have more controllability The experimental results demonstrate that the fast switching is able to correct several disturbances of the ac mains besides, the long duration in variation in time is much lower than the one corresponding to the traditional regulators. https://journalnx.com/journal-article/20150093