This article has a training video for Unit Substation (USS) - Transformer in COMPAS Go. Unit Substations consist of a Primary, Transformer and Secondary.  High Voltage enters through the Primary section and exits through the Secondary. There is always a transformer between the Primary and Secondary. 

USS Transformer Types

Dry Type Transformers: Primarily used in indoor clean environment (office buildings, schools, etc.), but can be installed outdoors with proper weather enclosures. Lighter weight than liquid filled & cast coil. Must eliminate moisture from coils prior to energizing, more susceptible to airborne contaminants.

Cast Coil Transformers: Primarily used where reliability is important (healthcare, datacenters) and damp/dusty/dirty environments. Usually most expensive upfront cost but has longer life than VPI with greater fan overload and higher short circuit strength.

Oil Filled Transformers: Primarily used in outdoor applications. Smaller footprint than dry-type. Higher standard BIL than Dry type transformers. Some cost associated with maintenance to ensure proper fluid.