Transformers are rated based on the power output they are capable of delivering continuously at a specified rated voltage and frequency under “usual” operating conditions without exceeding prescribed internal temperature limitations.
Insulation is known to deteriorate with increases in temperature, so the insulation chosen for use in transformers is based on how long it can be expected to last by limiting the operating temperature.
The temperature that insulation is allowed to reach under operating conditions essentially determines the output rating of the transformer, called the kVA rating.
Power transformers have been loosely grouped into three market segments based on size ranges. These three segments are:
Small power transformers: 500 to 7500 kVA
Medium power transformers: 7500 to 100 MVA
Large power transformers: 100 MVA and above
Credit: http://electrical-engineering-portal.com/what-is-transformer-rating-based-on
