What Is Average Winding Temperature?

Average winding temperature is the mean thermal state of a transformer's or rotating machine's conductor windings, measured as the bulk temperature averaged over the entire winding volume rather than at any single localized point. It is determined during load testing by the resistance method, in which the winding resistance measured while the machine is under load is compared to a reference resistance measured at a known ambient temperature, and the difference is converted to a temperature rise through the known temperature coefficient of resistance for the conductor material. Average winding temperature serves as a primary indicator of thermal loading and as the reference parameter against which insulation class temperature limits are specified.

The concept is central to the thermal design and lifetime management of power transformers, motors, and generators. Electrical insulation degrades irreversibly when subjected to sustained elevated temperatures, so standards bodies including IEEE and IEC have codified insulation thermal classes with specific average winding temperature limits that, when respected, provide a baseline insulation service life of approximately 20,000 hours under continuous operation at the rated temperature.

Temperature Rise and Insulation Classes

Transformer and motor temperature ratings are expressed as temperature rise above a specified ambient, typically 40°C. Dry-type transformers are available in standard average winding temperature rises of 80°C, 115°C, and 150°C, corresponding to insulation classes B, F, and H respectively, while liquid-filled transformers use standard rises of 55°C and 65°C. The Maddox Transformer resource on transformer temperature ratings summarizes how these classes map to absolute winding temperature limits: an 80°C rise class corresponds to a maximum average winding temperature of 120°C when added to the 40°C ambient, while a 150°C rise class corresponds to 190°C. The IEC 60085 standard and IEEE C57.12 series formalize the class definitions and the test procedures for verifying compliance.

Hot-Spot Temperature and Its Relationship to Average Winding Temperature

The hot-spot temperature is the highest temperature at any localized point within the winding, occurring at specific axial and radial positions where cooling is least effective and current density is highest. Because it governs the rate of insulation aging, the hot-spot temperature is the thermally critical parameter in transformer lifetime estimation. Standards such as IEEE C57.91 specify that for a transformer rated at 65°C average winding temperature rise, the hot-spot temperature may be 15°C to 25°C higher than the average, depending on the design. The erlphase paper on transformer thermal loading and protection explains how the hot-spot is estimated from average winding temperature using thermal models that account for core losses, load losses, and the cooling arrangement of the transformer.

Thermal Aging and Load Management

Insulation aging follows an Arrhenius relationship: for oil-paper insulation, the rate of degradation approximately doubles for each 6°C to 10°C increase in hot-spot temperature above the rated value. This relationship allows engineers to calculate the loss of insulation life caused by overload events and to manage transformer loading schedules accordingly. IEEE C57.91 provides a thermal aging model and defines the concept of normal insulation life as 65,000 hours at a hot-spot temperature of 110°C for thermally upgraded paper, or 20,000 hours at 110°C for standard kraft paper. The Copper Development Association technical note on transformer temperature rise and efficiency discusses how lower average winding temperatures, achieved through higher conductor cross-section, reduce both thermal aging and load losses simultaneously, improving both lifetime and efficiency.

Applications

Average winding temperature measurement and management has applications across multiple areas of power engineering, including:

• Power transformer nameplate rating and factory acceptance testing
• Motor and generator insulation class selection and thermal design
• On-load tap-changer and protection relay settings based on thermal capacity
• Life-cycle management and planned overload scheduling for substation transformers
• Energy-efficient transformer design targeting reduced winding temperature rise