IEEE C57.91-2025: A Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators
- Augusto Moser
- Jun 15
- 4 min read
Updated: Jun 16
Introduction
The IEEE C57.91-2025 standard, titled Draft Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators, provides comprehensive recommendations for safely operating mineral-oil-immersed distribution and power transformers as well as step-voltage regulators above their nameplate ratings. Developed by the Transformers Committee of the IEEE Power and Energy Society, this 2025 draft consolidates earlier guides (including the 2011 edition of C57.91, the withdrawn C57.92, and C57.115) into a single, unified document. It emphasizes the thermal behavior of 65 °C rise insulation systems and equips users with practical tools to maximize equipment utilization while managing risks such as insulation aging, gas evolution in oil, and stress on auxiliary components.
The guide is intended for equipment owners, operators, and planners who want to push transformers and regulators harder during peak demand or emergencies—without compromising long-term reliability.
Key Concepts of the IEEE C57.91-2025 Guide for Loading Transformers
Scope and Purpose
The IEEE C57.91-2025 Guide for Loading Transformers covers mineral-oil-immersed transformers manufactured to IEEE Std C57.12.00 and tested per IEEE Std C57.12.90, plus step-voltage regulators per IEEE Std C57.15. It explains how to evaluate the effects of overloads on insulation life, temperature limits, and overall transformer integrity. Key topics include:
Consequences of loading beyond nameplate (e.g., accelerated aging, bubble formation, mechanical weakening).
Different loading categories: normal estimated life, planned loading beyond nameplate (PLBN), long-time emergency (LTE), and short-time emergency (STE).
Influence of ambient temperature, altitude, cooling type (ONAN, ONAF, OFAF, etc.), and voltage/frequency variations.
Requirements for bushings, tap-changers, leads, and other auxiliaries (Annex B).
Our Solutions
The HV Assets Care platform is a complete solution for data analysis and diagnosis. Includes all recommended IEEE standard methods, including the Duval Triangle and Pentagon, integrated into an Asset Management Panel. In addition, it provides a Health Index with individual scores for the creation of the Asset Ranking and the IEEE thermal model applied to sensors for analysis of the life and thermal limits of the transformer. For more information, click here.
Transformer Insulation Life
Insulation aging is modeled using Arrhenius-based equations that relate hottest-spot temperature to the rate of cellulose degradation. The guide introduces a per-unit life (PUL) concept and an aging acceleration factor (FAA):
where θhs is the winding hottest-spot temperature in °C. This allows precise calculation of equivalent aging over any load cycle and percent loss of life (%LoL) using the equivalent aging factor FEQA. The reference life is 180,000 hours at 110 °C hottest-spot temperature (per IEEE Std C57.12.00), but users can select alternative end-of-life criteria (e.g., tensile strength retention or degree of polymerization) from Annex I.
Thermal Models for Loading Calculations
Clause 7 presents the core technical innovation: a set of governing differential equations and simplified thermal models that predict key temperatures (top-oil, bottom-oil, duct-oil, average winding, and hottest-spot) under dynamic conditions. These models incorporate:
Oil flow and viscosity changes with temperature.
Variable ambient temperature.
Load-dependent losses.
Duct-oil effects during transients (oil in winding cooling ducts heats faster than tank-top oil).
The models support both steady-state and transient analyses, making them suitable for continuous operation, planned overloads, and emergency scenarios.
Benefits of Using the Thermal Model
Adopting the thermal model described in Clause 7 delivers tangible, real-world advantages for transformer management:
Accurate Transient Temperature Prediction Traditional steady-state assumptions often underestimate hottest-spot temperatures during sudden load increases. The model captures rapid duct-oil temperature rise and the complex fluid dynamics inside windings, giving operators reliable hottest-spot forecasts even under highly variable loads.
Precise Insulation Aging and Loss-of-Life Assessment By integrating the aging acceleration factor with time-step calculations, users can quantify exactly how much insulation life is consumed during any 24-hour (or longer) load profile. This turns subjective “overload risk” into objective numbers, enabling data-driven decisions on when and how much to overload.
Support for Dynamic and Emergency Loading Scenarios The model handles all loading categories—normal, PLBN, LTE, and STE—while respecting manufacturer limits on hottest-spot temperature, top-oil temperature, and auxiliary equipment. It is equally effective for distribution transformers, large power units, and step-voltage regulators.
Adaptability to Real Operating Conditions Ambient temperature swings, altitude derating, partial cooling outages (Annex H), and cold-load pickup (Annex F) are all directly incorporated. This flexibility is especially valuable in modern grids with renewable integration and fluctuating demand.
Risk Mitigation and Asset Optimization Early identification of bubble inception risks (Annex A), mechanical weakening during faults, or excessive stress on bushings and tap-changers helps prevent unexpected failures. Utilities can safely defer capital expenditures by extending the useful life of existing transformers.
Basis for Custom Ratings and Specifications Clause 10 uses the thermal model as the foundation for developing or revising transformer ratings. Annex C provides worked examples for sizing new units or re-rating existing ones, streamlining procurement and upgrade projects.
Consistency Across Equipment Types and Lifecycles The same equations apply to both new and aging fleets, distribution and power transformers, and regulators—eliminating the need for separate legacy methods and ensuring uniform application across an organization.
In short, the thermal model transforms transformer loading from an empirical art into an engineering science. It gives operators confidence to utilize full equipment capability while clearly understanding—and managing—the trade-offs in insulation life and reliability.
Conclusion
IEEE C57.91-2025 is more than a loading guide—it is a practical framework for maximizing the value of mineral-oil-immersed transformers and step-voltage regulators in today’s demanding power systems. By combining clear risk awareness, updated insulation-life science, and a robust thermal model, the standard empowers users to operate equipment closer to its physical limits safely and economically. Whether planning daily operations, responding to emergencies, or developing future ratings, the thermal model provides the quantitative foundation needed for informed, confident decision-making. For complete implementation details, refer to the full standard and its informative annexes.
