A transformer is electrically modeled as a device which linearly changes the current/voltage relationship of electrical power flowing through it. Real-life transformers contain wire which gets hot when electricity flows through it, so the most important aspect of modeling transformer activity is tracing heat flow out of the transformer core and into the surroundings.
We get the simplest model of a transformer by assuming it has a certain internal volume made out of metal (copper and iron) and a certain surface area for heat transfer. We assume further that the heat generated inside the transformer originates in its center and diffuses out by conduction. Since the conductive medium possesses both heat capacity and conduction, the transformer core will possess a thermal time constant that sets the time scale for the response of the core to the "prompt" heat being generated inside it.
The outer surface of the transformer will possess a certain amount of area and a certain amount of thermal resistance and these two characteristics then determine the rate at which heat will get transferred away into the surroundings for any given temperature of the surface. This is going to yield a second time constant for long-term thermal equilibration of the transformer as a whole and its surroundings.
The critical design parameter is the peak temperature in the inner core of the transformer, because it is that heat which degrades over time the insulation on the wire from which the coils are wound, causing the transformer to fail. The two operating conditions which must be considered are 1) transient power surges, which occur on timescales too fast for the core to dissipate the excess heat to the surroundings, and 2) long-term steady-state core temperature, to which the insulation will be exposed over the useful lifetime of the transformer.
