Larger buildings may be supplied with a medium voltage electricity supply, and will required a substation or mini-sub. What is the main item of equipment contained in these?

A transformer is the primary component in a substation or mini-sub, used to step down the medium voltage (typically 11kV, 22kV, or 33kV) to the low voltage (415V/240V) needed for building distribution. Transponders are communication devices, transducers convert energy forms, and converters change AC to DC - none of these are the main equipment in distribution substations.