IEEE C57.91-2025

Annotation of standard text IEEE C57.91-2025 :

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.

ISBN: 979-8-8557-2268-0, 979-8-8557-3111-8, 979-8-8557-3112-5

Number of Pages: 123

Product Code: STDAPE27988, STD28632, STDPD28632

Keywords: distribution transformer, IEEE C57.91(TM), insulation aging, loading, long-term overloads, loss-of-life, mineral-oil-immersed, power transformer, short-term overloads, step-voltage regulators

Category: Transformers

Draft Number: PC57.91/D11, May 2025 - APPROVED DRAFT