1. According to the purpose: there are step-up transformers (power plant 6.3V10.5kV or 10.5KV/110KV, etc.), contact transformers (220KV/110KV or 110KV10.5KV between substations), step-down transformers (35kV, 10.4kV or 10.5kV, 10.4kV).

2. According to the number of phases: there are single-phase and three-phase power transformers.

3. According to the winding: there are two-winding power transformers (each phase is installed on the same iron core, the primary and secondary windings are wound separately and insulated from each other), three-winding power transformers (each phase has three windings, primary and secondary windings) are individually wound and insulated from each other), self-transformer (the tap in the middle of a set of windings serves as the primary or secondary output).

A three-winding transformer requires the capacity of the primary winding to be greater than or equal to the capacity of the secondary and tertiary windings. The percentages of the three winding capacities in the order of high voltage, medium voltage, and low voltage are: 100/100/100, 100150/100, and 100/100/50, the second and third windings cannot operate at full load.

Generally, the voltage of the tertiary winding is low, which is mostly used for power supply or connecting supplementary equipment in nearby areas. It is used to connect autotransformers with three voltage levels: there are two types: step-up or step-down. Due to its small loss, lightweight, and economical use, it is widely used in ultra-high voltage power grids. The commonly used model of small autotransformers is 400V136V (24V) and is used to power security lighting and other equipment.

4. According to the insulation medium: oil-immersed transformer (flame-retardant, non-flame-retardant), dry-type transformer, gas-insulated transformer.