Transformer Winding Temperature Determination: Standards, Methods, and Applications

Transformer winding temperature determination is a core electrical testing procedure used to verify thermal performance, confirm compliance with IEC, GB, and IEEE standards, and predict the service life of winding insulation systems. This article provides a professional overview of the applicable standards, test sample types, measured parameters, and measurement techniques such as the resistance method, embedded sensors, and fiber-optic hot-spot measurement for oil-immersed and dry-type transformers.

Technical Overview

Transformer winding temperature determination refers to the set of measurement and calculation procedures used to establish the average winding temperature and the winding hot-spot temperature of a transformer under defined loading and cooling conditions. Because the thermal aging of cellulose insulation and resin systems follows an approximately exponential relationship with temperature, the aging rate of oil-paper insulation is widely accepted to roughly double for every increase of about 6 K in the hot-spot temperature, as described in IEC 60076-7. Accurate winding temperature data is therefore essential for verifying design guarantees, validating insulation thermal class, and evaluating permissible loading.

The principal standards governing this test include the IEC 60076 series (particularly IEC 60076-1, IEC 60076-2 for liquid-immersed transformers, IEC 60076-7 for loading guides, and IEC 60076-11 for dry-type transformers), the GB/T 1094 series and GB/T 6451 in China, the European EN 60076 series, IEEE C57.12.00 and IEEE C57.12.90, as well as relevant UL and JIS standards for distribution transformers. Temperature-rise testing is performed by manufacturers, by accredited third-party laboratories, and by utility acceptance teams to demonstrate that the corrected average winding temperature rise and hot-spot rise remain within the guaranteed limits at the reference ambient temperature.

Testing Samples

Oil-Immersed Power and Distribution Transformers

These include sealed, conservator, and automatic gas-filled units ranging from small distribution ratings up to large power transformers with forced-air or forced-oil cooling. Samples may be prototypes for type testing, production units for routine testing, or in-service units evaluated during acceptance, overhaul, or after overload events. For each sample, the test object must be complete with bushings, cooling equipment, and the specified insulating liquid, and the winding resistance must be measured cold before energization.

Dry-Type and Cast-Resin Transformers

Open-wound, VPI (vacuum pressure impregnated), and cast-coil transformers with insulation systems of thermal class 130 (B), 155 (F), or 180 (H) are tested to verify compliance with the tighter temperature-rise limits of IEC 60076-11 and GB/T 1094.11. These units frequently incorporate embedded RTD sensors that allow direct comparison between sensor readings and the resistance-based average winding temperature.

Amorphous-Alloy and Energy-Efficient Transformers

Low-loss designs using amorphous alloy cores are tested to demonstrate that reduced losses do not compromise thermal performance. Results also support compliance declarations under national energy-efficiency regulations and utility efficiency programs.

Specialty Transformers and Reactors

Rectifier, furnace, traction, and wind-turbine transformers, as well as shunt and series reactors, are tested against their product-specific standards, for example IEC 60076-16 and IEC 60076-6, with particular attention to harmonic-rich load currents that increase eddy and stray losses in the windings.

Testing Items

Depending on the standard invoked and the purpose of the test, transformer winding temperature determination typically covers the following parameters:

• Average winding temperature rise of the HV, LV, and tap windings, determined by the resistance method.
• Top-oil and bottom-oil temperature rise for liquid-immersed units.
• Winding gradient, defined as the difference between average winding temperature and average oil temperature.
• Winding hot-spot temperature and hot-spot temperature rise, calculated or directly measured.
• Corrected temperature-rise values referred to the reference cooling medium temperature of the applicable standard.
• Thermal time constants of oil and windings for use in dynamic thermal models and loading guides.
• Verification of insulation thermal class for dry-type transformers.
• Comparison of embedded sensor (RTD or fiber-optic) indications with reference measurement results.

Testing Methods

Loading Circuits for Temperature-Rise Tests

The short-circuit (impedance) method is the most common loading approach: one winding is short-circuited and the other is supplied at a voltage sufficient to dissipate the total losses (sum of load losses corrected to reference temperature and no-load losses). Once the top-oil temperature rise has stabilized, the input is reduced to rated current to establish the winding gradient. For small units, the open-circuit method and the mutual loading (back-to-back) method may be used, with the total loss condition maintained until steady state is reached, typically verified when the rate of top-oil rise is below 1 K per hour.

Resistance Method (Reference Method)

The resistance method specified in IEC 60076-1 and IEC 60076-2 is the reference technique for average winding temperature. The cold winding resistance R1 is measured at a known temperature θ1. Immediately after shutdown at the end of the heating period, the hot resistance R2 is measured and the average winding temperature θ2 is calculated as:

θ2 = (R2 / R1) × (θ1 + k) − k

where the material constant k equals 235 for copper and 225 for aluminum. Because windings cool rapidly after de-energization, the first resistance reading must be taken as quickly as possible using four-terminal micro-ohmmeters or automated fast-resistance instruments, and any delay must be corrected by extrapolating the cooling curve back to the instant of shutdown using the semi-logarithmic graphical procedure defined in IEC 60076-2. The measured average winding temperature rise is then corrected to the reference ambient temperature of the invoked standard.

Embedded Sensor Methods

Platinum resistance detectors (Pt100 RTDs per IEC 60751) and thermocouples per IEC 60584 are installed in winding ducts, between discs, or in the insulating liquid. RTDs provide the routine monitoring interface for protection and cooling control, while laboratory-grade Class A sensors improve correlation studies between direct indications and resistance-based results. Sensor readings must never replace the resistance method for acceptance purposes, but they are indispensable for continuous surveillance and for validating thermal models.

Fiber-Optic Hot-Spot Measurement

Fiber Bragg grating (FBG) and fluorescent-decay fiber-optic probes, embedded directly in the winding during manufacture, enable genuine direct hot-spot measurement with complete immunity to electromagnetic interference. They are increasingly specified for large power transformers, traction transformers, and research programs aimed at calibrating the hot-spot factor H against measured data.

Hot-Spot Simulation and Online Indication

Thermal-image winding temperature indicators (WTIs) combine measured top-oil temperature with a heating element driven by a current transformer replica of the load current, producing an online estimate of the hot-spot temperature used for alarm, trip, and cooling-stage control. The hot-spot rise above ambient may also be calculated as top-oil rise plus the hot-spot factor H multiplied by the winding gradient, with H typically between 1.1 and 1.3 for distribution and small power transformers, consistent with IEC 60076-7.

Infrared Thermography

Infrared thermography is applied to dry-type units, enclosures, and accessible connections to detect localized overheating and to support qualitative assessments. It supplements, but does not substitute for, the standardized resistance-based determination of average winding temperature.

Applications

Transformer winding temperature determination supports a wide range of engineering and commercial objectives:

• Type testing and routine testing during design verification and factory acceptance of new transformers.
• Independent compliance verification by accredited third-party laboratories for tendering, certification, and market access under IEC, EN, GB, UL, and JIS frameworks.
• Loading capability studies and dynamic overload assessment using the thermal models of IEC 60076-7 and IEEE C57.91.
• Insulation aging analysis and loss-of-life estimation for life-extension and asset-management decisions.
• Validation of electromagnetic-thermal simulation models and cooling-system design improvements.
• Condition assessment after through-faults, prolonged overloads, cooling failures, or suspected blocked oil ducts.
• Calibration of online monitoring systems, WTIs, and fiber-optic instrumentation against reference measurements.

FAQ

What is the difference between average winding temperature and hot-spot temperature?

Average winding temperature, obtained by the resistance method, represents the bulk thermal state of the entire winding. The hot-spot temperature is the highest local temperature, usually near the top of the winding, and drives insulation aging. The hot-spot is estimated from the top-oil temperature plus the hot-spot factor multiplied by the winding gradient, or measured directly with fiber-optic probes.

What temperature-rise limits apply?

For oil-immersed transformers with ON cooling under IEC and GB standards, the typical limits are a 60 K top-oil rise and a 65 K average winding rise above the reference ambient. For dry-type transformers under IEC 60076-11, the average winding rise limits depend on thermal class: 60 K for class 105 (A), 75 K for class 120 (E), 80 K for class 130 (B), 100 K for class 155 (F), and 125 K for class 180 (H). Specific contracts may specify different or split-level guarantees.

Why must the hot resistance be measured immediately after shutdown?

Windings and adjacent oil cool significantly within the first minutes after de-energization, so any delay causes the measured temperature to understate the true value. Fast automated resistance instruments combined with the standardized extrapolation of the cooling curve correct for this delay and preserve measurement traceability.

Can the test be performed at reduced or partial losses?

Yes. If the available test power cannot supply total losses, the test may be run at reduced input and the oil rises corrected using the procedures in IEC 60076-2, provided the winding gradient stage is performed at rated current. The associated measurement uncertainty must be evaluated and reported.

Is winding temperature measurement possible on an energized, in-service transformer?

Direct measurement in service requires factory-installed fiber-optic probes, since conventional RTDs and thermocouples cannot be connected to energized windings for safety and insulation reasons. For most units, online hot-spot indication relies on WTI devices and analytical models calibrated against factory temperature-rise test data.