Power transformers are classified by application: step-up (6.3kV/10.5kV or 10.5kV/110kV, etc. for power plants), interconnection (220kV/110kV or 110kV/10.5kV between substations), and step-down (35kV/0.4kV or 10.5kV/0.4kV for distribution).
Power transformers are also classified by the number of phases: single-phase and three-phase.
Power transformers are classified by winding type: two-winding (each phase is mounted on the same core, primary and secondary windings are wound separately and insulated from each other), three-winding (each phase has three windings, primary and secondary windings are wound separately and insulated from each other), and autotransformer (one set of windings with a tap in the middle serves as the primary or secondary output). Three-winding transformers require the capacity of the primary winding to be greater than or equal to the capacity of the secondary and tertiary windings. The percentage of capacity for three-winding transformers, in the order of high voltage, medium voltage, and low voltage, is: 100/100/100, 100/50/100, 100/100/50. It is required that neither the second nor third windings operate at full load. Generally, the third winding voltage is lower and is mostly used for near-field power supply or connecting compensation equipment, connecting three voltage levels. Autotransformers:
Available in step-up or step-down types, they are widely used in ultra-high voltage power grids due to their low losses, light weight, and economical operation. Commonly used small autotransformers are 400V/36V (24V) models, used for powering safety lighting and other equipment.
Power transformers are classified by insulation medium: oil-immersed transformers (flame-retardant and non-flame-retardant types), dry-type transformers, and 110kV SF6 gas-insulated transformers.
Power transformer cores are all core-type structures.
Three-phase power transformers configured in general communication engineering are two-winding transformers.