IEEE C57.91 – Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators

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.