IEEE C57.91-2025

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 C57.12.00™ and tested in accordance with IEEE Std C57.12.90™, as well as step-voltage regulators manufactured and tested in accordance with IEEE Std C57.15™. Since a substantial population of transformers and step-voltage regulators with insulation systems rated for a 55 °C average winding temperature rise at rated load are still in service, recommendations that are specific to this equipment are also included. This standard incorporates open-source code hosted on the IEEE Open-Source Platform.

This guide provides recommendations on loading and loading beyond nameplate of mineral-oil-immersed transformers and step-voltage regulators, considering prescribed current and temperature limits and their effect on insulation life. This guide is intended for use by equipment end-users in planning and operating transformers and voltage regulators, utilizing the equipment’s loading capability to its maximum extent while understanding the impacts of utilizing this capability. This guide also provides a practical and theoretical basis for the development of transformer and step-voltage regulator thermal models and end-user continuous and emergency ratings.

Revision Standard - Active. The assumptions, techniques, and methods for estimating the effects of loading above nameplate of mineral-oil-immersed distribution and power transformers, as well as step voltage regulators, are described in this guide. Consequences due to loading above nameplate ratings are discussed, including damage to insulation, production of gases in oil, and ultimately loss of life. In addition, the effects of loading on auxiliary equipment are presented. Different types of loading scenarios, as well as variations in ambient temperature and altitude, are described. The effects of different cooling techniques and their impact on transformer capacity are reviewed. Transformer temperature criteria are also provided. Finally, calculation methods with and without consideration of duct oil effects are provided, along with sample temperature calculations.