Thermal Resistance Determination of Transformer Insulation Materials: Standards, Methods, and Temper

Thermal resistance determination of transformer insulation materials evaluates the long-term thermal endurance of papers, films, varnishes, and laminates, yielding the temperature index (TI) and thermal class that govern safe transformer design and service life. Testing is performed through accelerated thermal aging in accordance with IEC 60216 and GB/T 11026 in accredited third-party laboratories, supporting material qualification for both dry-type and oil-immersed transformers. This article outlines the test principles, sample types, key test items, methods, and typical applications of thermal endurance evaluation.

Technical Overview

In the field of electrical insulation, thermal resistance—more precisely termed thermal endurance or heat resistance—describes the capability of an insulating material to retain its dielectric and mechanical properties during prolonged exposure to elevated temperature. For transformers, this property directly determines the hot-spot temperature the insulation system can tolerate, and therefore the unit's permissible loading, expected service life, and thermal class assignment. The determination is not a measurement of steady-state heat conduction, but an endurance evaluation expressed through the temperature index (TI), relative temperature index (RTI), and thermal endurance profile (TEP).

Thermal degradation of insulation generally follows an Arrhenius-type relationship in which the logarithm of the time to reach a defined failure criterion is linearly proportional to the reciprocal of absolute temperature. Cellulosic materials degrade mainly by depolymerization, oxidation, and hydrolysis, reducing the degree of polymerization (DP) from an initial 1,000–1,200 down to approximately 200 at end of life. Synthetic polymers age through oxidation, chain scission, and post-curing reactions, while composite systems additionally suffer interfacial deterioration.

The governing framework comprises IEC 60216 (Electrical insulation materials—Thermal endurance properties), its Chinese counterpart GB/T 11026, and the classification scheme of IEC 60085, which defines thermal classes 105(A), 120(E), 130(B), 155(F), 180(H), 200, 220, and 250. Polymeric materials intended for equipment certification are additionally assessed under UL 746B, which assigns relative temperature indices.

Testing Samples

Thermal endurance testing can be applied to virtually every solid and film insulation used in transformer construction. Representative sample categories include the following:

Cellulosic Insulation

Kraft insulating paper, crepe paper, thermally upgraded paper, and pressboard used in oil-immersed transformers. Specimens are aged either in air or in sealed tubes containing transformer oil to reproduce in-service conditions, and degradation is tracked through tensile strength, breakdown voltage, and degree of polymerization.

Polymer Films and Aramid Products

Polyester films, polyimide films, aromatic polyamide (aramid) papers, and pressboards used in dry-type and special-purpose transformers. Films are typically tested as narrow tensile strips or as dielectric specimens with a defined electrode geometry.

Winding Wire Enamels

Enamel coatings on round and rectangular conductors are evaluated using twisted-pair specimens for dielectric breakdown, together with mandrel-bend or tensile tests on the coating, combining the winding wire test methods of IEC 60851 with the thermal endurance procedures of IEC 60216-2.

Impregnating Resins, Varnishes, and Laminates

Epoxy and unsaturated polyester casting resins, solventless impregnating varnishes, and laminated sheets, rods, and tubes based on epoxy-glass, polyester, or phenolic substrates. Free-cast plaques or impregnated fabric substrates serve as test specimens depending on the diagnostic property under study.

Testing Items

Core deliverables of a thermal resistance determination program include:

• Temperature Index (TI): the temperature at which the selected end-point criterion is reached after a specified time, conventionally 20,000 hours.
• Relative Temperature Index (RTI): the index assigned by comparison with a reference material of known performance, as applied under UL 746B.
• Thermal Endurance Profile (TEP): the complete graphical and numerical relationship between aging temperature and time to end point.
• Halving interval (HIC): the temperature interval in kelvin corresponding to a halving of time to end point, characterizing the slope of the Arrhenius line.
• Thermal class verification against the designations of IEC 60085.
• Retention curves of diagnostic properties: breakdown voltage, tensile strength, elongation at break, flexural strength, mass loss, and, for cellulose, degree of polymerization.

Testing Methods

Long-Term Aging Procedure (IEC 60216 / GB/T 11026)

Specimens are exposed in forced-ventilation aging ovens meeting the temperature uniformity and airflow requirements of the IEC 60216-4 series, typically at four aging temperatures selected so that the lowest temperature produces end-point failure after approximately 5,000 hours and the highest after about 100 to 500 hours. At each temperature, groups of specimens are withdrawn at pre-calculated intervals and the diagnostic property is measured under the conditions specified in IEC 60216-2. The time to reach the end-point criterion—commonly 50% retention of the initial breakdown voltage or tensile strength—is determined for each temperature by interpolation.

Data Evaluation and Extrapolation

Following the statistical procedures of IEC 60216-3, the logarithm of time to end point is plotted against the reciprocal of absolute temperature to establish the Arrhenius regression line. Extrapolating the line to 20,000 hours yields the temperature index, while the slope provides the halving interval. Confidence limits, typically at the 95% level, are reported alongside the TI to demonstrate the statistical validity of the extrapolation, and the complete dataset constitutes the thermal endurance profile of the material.

Diagnostic Property Measurement

Dielectric strength is determined on aged specimens using breakdown tests with appropriate electrode systems; tensile strength and elongation at break are measured on strip or dumbbell specimens; and mass loss is determined gravimetrically. For cellulose insulation aged in oil, sealed-tube exposure followed by viscometric determination of the average degree of polymerization provides a direct chemical measure of aging, with values near 200 conventionally marking the limit of mechanical integrity.

Rapid Screening by Thermogravimetric Analysis

As a complementary tool, thermogravimetric analysis under ASTM D3850 and ASTM E1640 provides rapid estimates of degradation onset temperature and apparent activation energy. These results are suitable for formulation screening and comparative ranking but are not accepted as substitutes for the full long-term aging determination required for thermal class assignment.

Applications

Thermal resistance determination data underpin every stage of transformer insulation engineering:

• Qualification of new insulation materials and complete insulation systems for distribution and power transformers.
• Design verification of dry-type transformers operating at classes F and H, where resin and laminate endurance limits the permissible hot-spot temperature.
• Life assessment of oil-immersed transformers through cellulose aging models calibrated with measured endurance data.
• Technical support for upgrades from class A systems to thermally upgraded cellulose, aramid hybrids, or higher-temperature liquid dielectrics.
• Supplier quality control, incoming material acceptance, and comparative evaluation of alternative formulations.
• Generation of technical documentation for certification of transformer components in international markets.

FAQ

Question: What is the difference between TI and RTI?

Answer: The temperature index is derived directly from a full Arrhenius aging campaign performed on the material itself, whereas the relative temperature index is assigned by comparing the candidate material with a reference material of established performance tested in parallel, the approach commonly applied under UL 746B.

Question: How long does a complete determination take?

Answer: Because the lowest aging temperature must run until end-point failure occurs, typically around 5,000 hours, a complete IEC 60216 test program generally requires twelve to eighteen months, including statistical evaluation and reporting.

Question: Why is 50% retention commonly used as the end point?

Answer: For many properties, 50% retention corresponds to the level at which dielectric or mechanical performance under electrical stress, thermal cycling, and vibration can no longer be reliably guaranteed, making it the conventional engineering criterion specified in IEC 60216-2.

Question: Can testing be performed on complete transformer insulation systems rather than individual materials?

Answer: Yes. Model coils or complete insulation systems can be aged under combined thermal and electrical stress, and the resulting data support thermal class assignment of the entire system rather than a single constituent material.

Question: What sample quantities are required?

Answer: Requirements depend on the selected diagnostic property. Tensile aging of paper requires multiple strip specimens per withdrawal interval, while twisted-pair breakdown testing of wire enamels requires dozens of pairs per aging point to obtain statistically valid failure distributions.