Based on their structural and technical characteristics, box-type substations are classified into the following categories:
Assembled type: High- and low-voltage switchgear assemblies and the transformer are housed within a metal enclosure that includes an internal operating corridor. Due to their large size, these substations are rarely used today.
Combined unit type: Instead of using pre-assembled switchgear units, high- and low-voltage control and protection components are installed directly into the enclosure to form an integrated unit. Designed for maintenance-free operation, this type lacks an operating corridor and features a compact enclosure; it is also known as the European-style or standard box-type substation.
Integrated type: High-voltage control and protection mechanisms are simplified, and the high- and low-voltage distribution components are housed together with the transformer core and windings inside the transformer's oil tank as a single unit. This results in a much smaller footprint-comparable to that of an oil-immersed transformer of the same capacity and approximately one-third the size of the European-style version. This type is also known as the American-style or compact box-type substation.