Thermal Stability Testing of Insulation Films for Transformers: Standards, Methods, and Applications

Transformer insulation film thermal stability testing evaluates how polymer films such as polyester (PET), polyimide (PI), PEN, PPS, and composite laminates withstand prolonged thermal stress in transformer windings and insulation systems. Guided by internationally recognized standards including IEC 60216, IEC 60674, ASTM D2305, and UL 746B, this testing establishes temperature indices, aging behavior, and dielectric endurance that directly underpin transformer reliability and service life. This article explains the sample types, test items, test methods, and practical applications involved in a comprehensive thermal stability evaluation of transformer insulation films.

Technical Overview

Insulation films are thin polymeric materials, typically ranging from a few micrometers to several hundred micrometers in thickness, used as turn-to-turn, interlayer, and barrier insulation in transformer windings. Common materials include biaxially oriented polyester (PET) film, polyimide (PI) film, polyethylene naphthalate (PEN) film, polyphenylene sulfide (PPS) film, and laminated composites combining polymer films with aramid fiber substrates. Because these films operate continuously at elevated winding temperatures, their long-term thermal stability is one of the most critical factors determining transformer service life.

Thermal degradation of insulation films proceeds through several mechanisms, including thermo-oxidative chain scission, hydrolysis in the presence of moisture, post-crystallization, embrittlement, and dimensional shrinkage. These changes progressively reduce tensile strength, elongation at break, and dielectric breakdown strength, eventually leading to insulation failure. The rate of degradation follows Arrhenius behavior, meaning service life approximately halves for every 6 to 10 degrees Celsius increase in hotspot temperature.

Thermal stability testing assigns or verifies the thermal class of a film in accordance with IEC 60085, which defines classes such as Class E (120 degrees Celsius), Class B (130 degrees Celsius), Class F (155 degrees Celsius), Class H (180 degrees Celsius), and Class C (above 220 degrees Celsius). The central output of such testing is the temperature index (TI), the temperature at which a diagnostic property reaches a defined endpoint criterion after 20,000 hours of exposure. Testing may be performed by manufacturers, by accredited third-party laboratories, or within research and failure-analysis programs, and equivalent national standards such as GB/T 11026 and GB/T 13542 series are widely applied alongside IEC and ASTM methods.

Testing Samples

Polyester (PET) Insulation Films

Biaxially oriented PET film is the most widely used insulation film in distribution transformers, foil windings, and general-purpose layer insulation. It is typically supplied in thicknesses from approximately 20 to 350 micrometers and carries a nominal thermal classification of Class E or Class B, which thermal stability testing is used to confirm for specific grades and constructions.

Polyimide (PI) Films

Polyimide films offer exceptional thermal endurance, with temperature indices commonly in the range of 200 to 240 degrees Celsius, qualifying them for Class H and Class C applications. Corona-resistant polyimide grades, which incorporate inorganic fillers for use under partial-discharge conditions in high-frequency converters and inverter-fed equipment, are also evaluated for their thermal endurance characteristics.

PEN and PPS Films

PEN films provide higher thermal capability than standard polyester, with typical indices near Class F, while PPS films combine Class H-level temperature capability with strong chemical and oil resistance. Both materials are increasingly used in compact, high-temperature transformer designs and require dedicated thermal endurance data.

Composite Laminates and Coated Films

Layered products such as aramid-fiber-paper and polyester-film laminates, as well as adhesive-coated or resin-coated films, are tested as complete constructions because the interfaces and adhesives often govern thermal performance. Specimens are taken in the as-delivered form to reflect real manufacturing conditions.

Specimen Preparation and Conditioning

Tensile specimens are prepared as rectangular strips per ISO 527-3 or ASTM D882, while dielectric breakdown specimens are cut into sheets of suitable area, commonly 100 by 100 millimeters or larger. Unless otherwise specified, specimens are conditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 24 hours in accordance with ISO 291 or ASTM D618 before baseline measurement, and adequate replicate specimens are prepared at every aging temperature and aging time.

Testing Items

A complete thermal stability evaluation typically includes the following test items:

• Temperature index (TI) and halving interval derived from long-term thermal aging per IEC 60216
• Relative temperature index (RTI) determination in accordance with UL 746B requirements
• Retention of tensile strength and elongation at break after thermal aging
• Retention of dielectric breakdown voltage and dielectric strength after aging
• Mass loss and thermogravimetric decomposition parameters, including onset temperature and the temperature at 5 percent mass loss
• Glass transition temperature and melting behavior by differential scanning calorimetry
• Heat shrinkage and dimensional stability after elevated-temperature exposure
• Thermal endurance under immersion in transformer insulating oil or under combined thermal and electrical stress

Testing Methods

Long-Term Thermal Aging per IEC 60216 and GB/T 11026

The reference method for determining the thermal endurance of insulation films is long-term oven aging in accordance with the IEC 60216 series. Specimens are exposed in forced-ventilation ovens meeting the requirements of IEC 60216-4 at a minimum of three, and preferably four, elevated temperatures selected so that failure times span a suitable range. At each temperature, specimens are withdrawn at programmed intervals and tested for the diagnostic property, most commonly elongation at break or dielectric strength. The time to reach the endpoint criterion, conventionally 50 percent retention of the initial property value, is plotted logarithmically against reciprocal absolute temperature. Statistical evaluation per IEC 60216-3 produces the thermal endurance graph, from which the temperature index and halving interval are extrapolated.

Thermogravimetric Analysis (TGA)

TGA, performed per ISO 11358 or ASTM E1131, provides rapid characterization of decomposition behavior. Small film specimens are heated at controlled rates, for example 2, 5, and 10 kelvin per minute, under nitrogen or air, and mass loss is recorded continuously. The decomposition onset temperature and the temperature at 5 percent mass loss allow comparison between materials, while multi-heating-rate kinetic methods such as the Flynn-Wall-Ozawa approach give estimated activation energies and support accelerated screening of thermal endurance. TGA does not replace long-term aging but is a valuable complementary and quality-control tool.

Differential Scanning Calorimetry (DSC)

DSC testing per ISO 11357 is used to measure the glass transition temperature and melting behavior of films and to detect aging-induced changes in crystallinity. Shifts in these transitions after thermal exposure provide insight into post-crystallization, relaxation of orientation, and early-stage degradation before mechanical embrittlement becomes apparent.

Mechanical and Electrical Endpoint Testing

Tensile testing per ISO 527-3 or ASTM D882 and dielectric breakdown testing per IEC 60243-1 or ASTM D149 are the two principal diagnostic methods applied to aged specimens. Elongation at break is generally the most sensitive indicator of thermal embrittlement in films and is therefore frequently selected as the primary endpoint criterion, with tensile strength and breakdown voltage retained as confirmatory properties.

Heat Shrinkage and Dimensional Stability

Shrinkage testing per IEC 60674-2 or ASTM D2305 measures dimensional change after a defined exposure, such as 30 to 60 minutes at temperatures between 150 and 300 degrees Celsius, depending on the material. Excessive shrinkage in service can displace insulation edges, expose conductors, and concentrate electrical stress, so dimensional stability limits are specified for most film grades.

Immersion and Combined-Stress Aging

For films used in oil-immersed transformers, aging is also performed while fully immersed in mineral oil or alternative ester-based insulating liquids at elevated temperature, followed by dielectric testing of the aged film and assessment of oil compatibility. Combined thermal-electrical aging, in which specimens simultaneously carry electrical stress at elevated temperature, is used for films intended for high-frequency, high-voltage applications such as inverter-fed traction systems.

Applications

Thermal stability testing of insulation films supports design, qualification, and quality control across the transformer industry and adjacent sectors:

• Oil-immersed power and distribution transformers, where films provide interlayer and barrier insulation between winding sections
• Dry-type and cast-resin transformers, in which films operate in air at Class F and Class H temperatures
• Instrument transformers, including current and voltage transformers requiring compact, high-reliability insulation
• Reactors, inductors, and high-frequency power transformers in switch-mode converters and renewable energy systems
• Electric vehicle traction systems and charging infrastructure, where films endure high temperatures and steep electrical stress gradients
• Rotating machine insulation, motor slot liners, and magnet wire wrap applications using similar film materials
• Forensic and condition assessments of films sampled from in-service or aged transformers

FAQ

What is the temperature index of common insulation films?

Typical values are approximately 120 to 130 degrees Celsius for standard PET films, around 155 degrees Celsius for PEN films, approximately 160 to 180 degrees Celsius for PPS films, and 200 to 240 degrees Celsius for polyimide films. The exact temperature index must always be verified by testing for the specific grade, thickness, and construction used.

Why is elongation at break used as the primary diagnostic property?

Elongation at break declines sharply as thermal aging causes chain scission and embrittlement, making it the most sensitive and reproducible early indicator of degradation in thin films. It typically reaches the 50 percent retention endpoint well before tensile strength or dielectric strength shows comparable change.

How long does a full thermal endurance test take?

A complete IEC 60216 program, involving multiple aging temperatures run to endpoint, commonly requires twelve to twenty-four months because the lowest aging temperature must reach failure before extrapolation is valid. Rapid screening with TGA and short-term aging can provide indicative results within days to weeks.

What is the difference between the IEC temperature index and the UL relative temperature index?

The IEC temperature index per IEC 60216 is defined as the temperature corresponding to the endpoint criterion after 20,000 hours of aging. The UL 746B relative temperature index is assigned through long-term aging programs using a longer extrapolation basis and comparison against a reference material with established performance, and it is commonly required for components used in equipment certified to North American requirements.

Can thermal stability testing be performed on films removed from in-service transformers?

Yes. Retained tensile and dielectric properties, mass loss, and DSC or molecular-weight indicators measured on sampled films can be compared against baseline data to estimate remaining thermal life and support asset-management or failure-investigation decisions.

Where should these tests be performed?

For type certification, product qualification, and regulatory submission, testing should be carried out by accredited third-party laboratories operating in compliance with ISO/IEC 17025 and equipped with ovens, dielectric test sets, mechanical test frames, and thermal analysis instrumentation traceable to relevant IEC, ASTM, GB, and UL standards.