"Transformer" covers very different machines. They are best classified along three axes: insulation technology, function in the grid, and integration into a substation. Here is the map.
The most structuring distinction. In an oil-immersed transformer, the windings sit in a liquid (mineral oil or ester) that insulates and cools. In a dry-type transformer there is no liquid at all: the windings are encapsulated in cast resin and cooled by air. For the liquid itself, see mineral oil or ester.
You can also classify by number of phases (single or three-phase; three-phase dominates distribution), by cooling mode (ONAN, ONAF and so on), or by special characteristics (a K-rated transformer for data center harmonics). But the three axes above are enough to place the vast majority of needs.
In short: oil-immersed for outdoor sites, higher ratings and the best cost; dry-type for indoor installation and fire safety; an MV or HV substation when you need a turnkey assembly. The right choice follows the power, the environment and the budget: our configurator guides you, or browse our products.
General overview; the final choice depends on your installation and is validated against your specification.
An oil-immersed transformer uses a liquid (mineral oil or ester) to insulate and cool; a dry-type has no liquid at all (resin and air). Dry-type is favoured indoors for fire safety; oil-immersed for outdoor sites and higher ratings, usually at lower cost.
Distribution transformers supply end users by stepping medium voltage down to low voltage. Power transformers are larger units working on transmission and sub-transmission at high voltage.
The direction depends on how it is connected. A given unit is used one way for its application; for generation injected into the grid (step-up), it must be designed for that flow.
A substation integrates the transformer with the switchgear and protections into a complete assembly: MV for medium voltage, HV for high voltage.