Since the bushing BIL is higher than the windings, this indicates that if anything, the bushings are the weakest link, not the windings.
There are a lot of reasons out there for doing this (some of them are pretty strange sounding, such as protection from animals). A couple of the best reasons are that they are cheap insurance methods (not much price difference stepping up to next BIL level for a bushing), and the fact that they are in a harsher environment than the transformer windings. The windings are protected by at least a metal tank, so that outside air and moisture have minimal leak into the enclosure. In oil filled tanks, the protection level is even greater. However, bushings are exposed to the elements, which can impact the creep level and insulation-to-ground can be impacted negatively. In particularly harsh environments, where there is greater level of contamination in the air, it would be easier for the transformer to short to ground since the conductivity of air could be greater, or a build up of contamination on the porcelain of the bushing could lead to creepage short. The last reason is that customers would prefer to have a smaller distribution of spare parts in their inventory. If they can use the same bushings on all of their transformers, they would only have to keep a couple on hand, rather than keeping several of each BIL.
The reason we normally keep BIL of bushings one step higher than that of windings is that we want more safety margin in bushings as any failure in condenser bushings can lead to a fire unlike a winding failure.Standards also now recognise this and new IEC on bushings calls for 10 % higher AC test voltage than normal Ac test voltage for that voltage class of windings.So double margin is obtained in Bushing insulation level.
