It's been years since I worked with bushings, but if memory serves me right.
There is no difference between a bushing used on a distribution transformer and a power transformer.
The transformer manufacture usually goes through the bushing manufacture's catalog and selects a bushing that meets the mechanical requirements, operating voltage, operating current and test voltages of the transformer being designed.
Since power transformers operate at higher power and are usually tested at higher voltages, bushings used on power transformers tend to be larger than bushings used on distribution transformer.
I'm not sure if distribution class and power class is the correct terminology, but in the USA, typically, bushings on power transformers will be higher BIL than those on distribution transformers. At 12.5 kV, power transformer bushings are 110 kV BIL and distribution transformer bushings are 95 kV BIL.
Regardless if the transformer is classified as power or for distribution type, the bushings are specified to operate safely within electrical and mechanical withstand values. The main electrical parameters are:
o Maximum continuous operating current.
o Basic insulation level (BIL)
o Creepage distance of the bushing
In polluted environment, the minimum creepage distance may be the driving parameter to avoid flashover. Therefore, larger bushing with higher BIL may be required by the minimum values stipulate in the standards.
Below, is the minimum dielectric insulation level per ANSI/IEEE Std C57 for distribution transformer and Class I power transformers.
