IEEE C57.91-2011

This guide provides recommendations for loading mineral-oil-immersed transformers and step-voltage regulators with insulation systems rated for a 65 ¿¿C average winding temperature rise at rated load. This guide applies to transformers manufactured in accordance with IEEE Std C57.12.001 and tested in accordance with IEEE Std C57.12.90, and step-voltage regulators manufactured and tested in accordance with IEEE Std C57.15. Because a substantial population of transformers and step-voltage regulators with insulation systems rated for 55 ¿¿C average winding temperature rise at rated load are still in service, recommendations that are specific to this equipment are also included.