Autotransformers are applied in the transmission grid to transfer the transmission voltage to another voltage level. They have auto-connected windings, either 1 winding (series + common winding) or 2 windings (series + common + tertiary winding). The voltage regulation takes place through off-circuit tap changers and/or on-load tap changers. These transformers are either single-phase or three-phase. Autotransformers have a smaller frame size as compared to power transformers.