The main classification of transformers 

1、According to the number of phases：

1）Single-phase transformer: used for single-phase loads and three-phase transformer groups.

2）Three-phase transformer: used to increase and decrease the voltage of a three-phase system.

2、According to the cooling method：

1）Dry-type transformers: rely on air convection for natural cooling or increase fan cooling, which are mostly used for small-capacity transformers such as high-rise buildings, high-speed toll stations, local lighting, and electronic circuits.

2）Oil-immersed transformer: relying on oil as the cooling medium, such as oil-immersed natural cooling, oil-immersed air cooling, oil-immersed water cooling, forced oil circulation, etc.

3、By purpose：

1）Power transformer: used to increase and decrease the voltage of the power transmission and distribution system.

2）Instrument transformers: such as voltage transformers, current transformers, measuring instruments and relay protection devices.

3）Test transformer: It can generate high voltage and conduct high voltage test on electrical equipment.

4）Special transformers: such as electric furnace transformers, rectifier transformers, adjusting transformers, capacitive transformers, phase-shifting transformers, etc.