Distribution trf spec varies from country to country in finer details;
For example in American continent the following ANSI stds cover various types of Distribution trfs:
ANSI C57.12.20,21,22,23,24,25&26 Oil filled trfs
ANSI C57.12.50,51,52,55 Dry type trfs .
For Indian Sub continent ,CBIP stds cover detailed spec for distribution trfs.