IEEE C57.12.00-2015 IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
6.5 Tank pressure requirements 6.5.1 Maximum under rated conditions Tank pressure under rated conditions for sealed transformers shall not exceed two atmospheres (203 kPa) absolute pressure unless requirements of applicable sections of the ASME Boiler and Pressure Vessel Code (BPVC) are met. 13 6.5.2 Limits and tests Specific pressure limits and tests are included in standards applying to particular types of transformers. 6.6 Liquid insulation system 6.6.1 Insulating liquids Transformers shall be filled with a suitable insulating liquid such as the following: a) Mineral oil. New, unused mineral oil shall meet the requirements of ASTM D3487. NOTE 1—IEEE Std C57.106™ [B19] provides information concerning the acceptance and maintenance of mineral oil, including dielectric test breakdown criteria according to oil application, age, and test method. b) Less-flammable hydrocarbon fluid. New, unused less-flammable hydrocarbon fluid shall meet the requirements of ASTM D5222.
7. Short-circuit characteristics 7.1 Requirements 7.1.1 General Liquid-immersed transformers shall be designed and constructed to withstand the mechanical and thermal stresses produced by external short circuits under the conditions specified in 7.1.3, 7.1.4, and 7.1.5. The external short circuits shall include three-phase, single line-to-ground, double line-to-ground, and line-to- line faults on any one set of terminals at a time. Multi-winding transformers shall be considered to have system fault power supplied at no more than two sets of unfaulted terminals rated greater than 35% of the terminal kVA of the highest capacity winding. For other fault conditions, the requirements shall be specified by those responsible for the application of the transformer. Short-circuit withstand capability can be adversely affected by the cumulative effects of repeated mechanical and thermal overstressing produced by short circuits and loads above the nameplate rating. It is not feasible to continuously monitor and quantitatively evaluate the degrading effects of such duty; short- circuit tests, when required, should be performed prior to placing transformer(s) in service. The intention here is not that every transformer be short-circuit tested to demonstrate adequate construction. When specified, short-circuit tests shall be performed as described in IEEE Std C57.12.90.IEEE C57.12.00 pdf download.