Transformer end-users face challenging times with aging fleets, extended lead times, increasing loads, distributed generation additions, and changing regulations. One way to increase system capacity is to unleash the capacity of under-utilized transformers by calculating custom ratings using the transformer’s design capabilities. Additional capacity can provide options to increase load and delay capacity increase projects. IEEE C57.91 provides guidance on such methodologies. This guide is undergoing a major revision, including the primary thermal model, and adding the availability of open-source code. The next revision should be published in the 2025–2026 time.
Active Committees/Task Forces of Interest
- IEEE PES Transformers Committee
- IEEE PES Insulation Life Subcommittee (Subcommittee of the IEEE PES Transformers Committee)
- Working Group C57.91 – Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators (Working Group of the IEEE PES Insulation Life Subcommittee) – An IEEE Account is required to participate in this Working Group.
Technical Reports & Applicable Papers or Presentations
Additional resources are available in the IEEE C57.91 bibliography.
- AIEE Transformer Subcommittee, “Preliminary Report on Laboratory Aging Tests on Class A Insulation,” in Transactions of the American Institute of Electrical Engineers, vol. 66, no. 1, pp. 879-883, Jan. 1947, doi: 10.1109/T-AIEE.1947.5059522.
- M. Montsinger and P. M. Ketchum, “Emergency overloading of air-cooled oil-immersed power transformers by hot-spot temperatures,” in Electrical Engineering, vol. 61, no. 12, pp. 906-915, Dec. 1942, doi: 10.1109/EE.1942.6435475.
- J. McNutt, “Insulation thermal life considerations for transformer loading guides,” in IEEE Transactions on Power Delivery, vol. 7, no. 1, pp. 392-401, Jan. 1992, doi: 10.1109/61.108933.
- Swift, T. S. Molinski, R. Bray, and R. Menzies, “A fundamental approach to transformer thermal modeling. II. Field verification,” in IEEE Transactions on Power Delivery, vol. 16, no. 2, pp. 176-180, April 2001, doi: 10.1109/61.915479.