Capacity (or rating) of a transformer is limited by the temperature that the insulation can tolerate. Ratings can be increased by reducing core and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it will withstand higher temperatures.
A physically larger transformer can dissipate more heat, due to the increased area and increased volume of oil.
A transformer is only as strong as its weakest link, and the weakest link is the paper insulation, which begins to degrade around 100 °C. This means that a transformer must be operated with the “hottest spot” cooler than this degradation temperature, or service life is greatly reduced.
Reclamation typically orders transformers larger than required, which aids in heat removal and increases transformer life.
As size increases, 1 kilovoltampere (kVA) means 1,000 voltamperes, 1 megavoltampere (MVA) means 1 million voltamperes. Large generator step-up (GSUs) may be rated in hundreds of MVAs.
A GSU transformer can cost well over a million dollars and take 18 months to 2 years or longer to obtain.
Each one is designed for a specific application. If one fails, this may mean a unit or whole plant could be down for as long 2 years, costing multiple millions of dollars in lost generation, in addition to the replacement cost of the transformer itself.
This is one reason that proper maintenance is critical.
Reference: Transformers: Basics, Maintenance, and Diagnostics – U.S. Department of the Interior Bureau of Reclamation