A three-phase autotransformer is a transformer with only one winding. Used for stepping up or stepping down voltage, compared with an ordinary transformer of the same capacity, the autotransformer is not only smaller in size but also higher in efficiency. It is especially economical when the transformation ratio is close to 1. Therefore, this advantage becomes more prominent in high-power transmission transformers with similar voltage levels. As a result, autotransformers are widely used due to their large capacity, low losses, and low cost, particularly in applications such as power supplies for import/export equipment and starting high-power motors.