What is temperature rise test for a transformer?

Temperature rise in a transformer is the average temperature of the windings and insulation above the existing ambient temperature. The rating or kVA of a transformer is determined by the allowable operating temperature of its insulation or as more commonly referred to in transformer application, the temperature rise above the ambient temperature. The temperature that the insulation reaches depends upon the load and the ambient temperature surrounding the transformer.
Temperature limits for windings in transformers with 220 °C insulation have been established as 210°C by ANSI and NEMA standards. This comes from the average winding temperature rise of 150°C plus a 30°C average ambient plus another 30°C hot spot differential. These limits were established to provide normal life expectancy of the insulation.
Transformers are designed to meet their rated kVA and the temperature test is a means to verify that these limits are met. The temperature rise test simulates the rated load on the transformer at the worst case tap connection. There are a few ways manufacturers can do this test but the most common is the short circuit method. This method is done by shorting one winding while circulating full load current in the other winding. The temperature rise is then determined by using thermocouples placed at hot spots in the windings and measuring cold and hot resistance.
Temperature rise tests are an optional test and are usually not performed unless it is specifically required. The standards permit this test to be omitted if results from an essentially duplicate unit have been done. 
It should be pointed out that loading a transformer in excess of its temperature limits will result in loss of life. When overloading is required, A.N.S.I. C57.96, Guide for Loading dry distribution and Power Transformers provides loss of life information. ANSI guidelines have data showing that each 10°C above the insulation system shortens the life by half. So, it is important to operate the transformer within its temperature limits to obtain maximum life from the transformer.