Step-up transformer, step-down transformer, single-phase transformer, three-phase transformer and two-winding transformer are some different types of transformers. A transformer is a device used for transforming power from one circuit to another without changing frequency.
The step-up transformer is used to step up the voltage level in the secondary winding. Similarly, the step-down transformer is used to step down the voltage level. The three-phase transformer is commonly used in the three-phase power system, as it is cost effective, but it is difficult to transport due to its large size.
The single-phase transformer is used in the single-phase power system. The two-winding transformer is generally used when the ratio between input voltage and output voltage is more than two.