Transformer insulation film temperature resistance testing evaluates the ability of polymer films such as PET, PEN, PPS, and polyimide to withstand long-term thermal stress in transformer windings and insulation systems. By determining the temperature index (TI), heat-shrinkage behavior, and retention of dielectric and mechanical properties after thermal aging, manufacturers can verify thermal class compliance with IEC 60216, IEC 60085, and UL 746B requirements. Accredited third-party laboratories provide these tests to support product qualification, quality control, and international market access for transformer materials.
Technical Overview
Insulation films are among the most critical components in transformer construction, serving as layer insulation, interturn insulation, barrier insulation, and wrapping materials in windings. Because transformers operate continuously at elevated temperatures, the long-term thermal performance of these films directly determines the service life, safety, and reliability of the entire insulation system. Prolonged exposure to heat causes polymer films to undergo physical and chemical degradation, including chain scission, oxidation, embrittlement, crystallization changes, and dimensional shrinkage, all of which reduce dielectric strength and mechanical toughness.
Temperature resistance testing quantifies this degradation through accelerated thermal aging followed by measurement of residual properties. According to IEC 60085, electrical insulation materials are assigned thermal classes such as 105, 120, 130, 155, 180, 200, 220, and 250, corresponding to the maximum continuous operating temperature in degrees Celsius. The thermal endurance framework defined in the IEC 60216 series, together with the Relative Temperature Index (RTI) approach in UL 746B, provides internationally recognized methods for establishing these ratings experimentally. GB/T 13542 and GB/T 11026 define the corresponding Chinese national requirements for electrical insulating films and their thermal endurance evaluation.
Testing Samples
Polyethylene Terephthalate (PET) Insulation Films
Biaxially oriented PET films are the most widely used insulation films in transformers, offering good dielectric strength, tensile strength, and cost efficiency. They are typically qualified for thermal classes 120 to 130, with certain stabilized grades reaching class 155. Testing verifies their thermal endurance, shrinkage behavior, and property retention after prolonged heat exposure.
Polyimide Insulation Films
Polyimide films provide exceptional thermal stability, remaining serviceable at temperatures up to 200–240°C and above, and are used in high-temperature and high-reliability transformer insulation systems. Their thermal endurance characterization focuses on long-term aging behavior, since they exhibit no melting point and degrade only at very high temperatures.
PEN and PPS Insulation Films
Polyethylene naphthalate (PEN) films offer a higher glass transition temperature and better hydrolysis resistance than PET, while polyphenylene sulfide (PPS) films combine high temperature resistance with inherent flame retardancy. Both materials are tested to confirm their suitability for class 155 to 180 applications in demanding transformer environments.
Composite and Coated Insulation Films
Multi-layer structures, such as PET films bonded with polyimide layers, adhesive-coated films, and fluoropolymer-coated films, are tested as complete systems. The weakest interface or adhesive layer often governs thermal performance, so aging tests evaluate delamination, bond strength retention, and edge integrity in addition to bulk film properties.
Testing Items
Typical testing items for assessing the temperature resistance of transformer insulation films include:
- Temperature Index (TI) and Relative Temperature Index (RTI) determination
- Thermal endurance profiling and Arrhenius lifetime extrapolation
- Tensile strength and elongation at break retention after thermal aging
- Dielectric strength (electric strength) before and after heat aging
- Insulation resistance and volume resistivity after thermal exposure
- Heat shrinkage and dimensional stability at elevated temperature
- Glass transition temperature (Tg) by DSC
- Melting point and melting behavior by DSC
- Thermal decomposition temperature and mass loss by TGA
- Thermal shock and thermal cycling resistance
- Visual assessment of embrittlement, cracking, discoloration, and delamination after aging
Testing Methods
Long-Term Thermal Aging per IEC 60216 and UL 746B
Specimens are exposed in forced-ventilation ovens at a minimum of three, and preferably four, aging temperatures selected so that the highest temperature produces endpoint failure within approximately 100–500 hours while the lowest temperature requires several thousand hours. At each temperature, samples are withdrawn at predetermined intervals and tested against an endpoint criterion, most commonly the retention of 50% of the initial elongation at break or tensile strength measured per ISO 527-3, ASTM D882, or IEC 60674-2. The logarithm of the time to endpoint is plotted against the reciprocal of absolute temperature (Arrhenius plot), and regression analysis is used to extrapolate the temperature at which the material survives a specified lifetime, typically 20,000 hours. This extrapolated temperature defines the Temperature Index, reported together with the half-life interval (HIC) indicating the thermal endurance confidence.
Thermal Analysis Methods
Differential Scanning Calorimetry (DSC) determines the glass transition temperature and melting characteristics, providing insight into short-term thermal limits and crystalline structure. Thermogravimetric Analysis (TGA) measures decomposition onset temperature and mass-loss behavior under nitrogen or air, characterizing intrinsic thermal stability. These methods are performed in accordance with ASTM E1356, ASTM E1131, or the corresponding ISO test procedures.
Heat Shrinkage and Dimensional Stability Testing
Film specimens of defined dimensions are conditioned and then exposed to specified temperatures, such as 150°C or 180°C for 30 to 60 minutes, in accordance with IEC 60674-2 or GB/T 13542. Dimensional changes in the machine and transverse directions are measured, and shrinkage values are compared against product specification limits, since excessive shrinkage can expose winding conductors and compromise insulation coordination.
Post-Aging Electrical Testing
Aged and unaged specimens undergo dielectric breakdown testing per IEC 60243-2 or ASTM D149 using standardized electrode assemblies, often with the film immersed in insulating oil to prevent flashover. Retention of dielectric strength, expressed as a percentage of the initial value, serves as both a supplementary endpoint criterion for thermal endurance studies and an acceptance metric for qualification testing.
Applications
Temperature resistance testing of insulation films supports design, qualification, and quality assurance across a wide range of electrical equipment, including:
- Power and distribution transformers, including layer and interphase insulation in windings
- Dry-type and cast-resin transformers with high operating temperature requirements
- Instrument transformers such as current and voltage transformers
- High-frequency switch-mode power supply transformers and inductive components
- Reactors, chokes, and magnetic components in power electronics
- Electric vehicle charging modules, onboard chargers, and drive inverters
- Electric motors and generators utilizing film-based slot and phase insulation
- Capacitors, cables, and flexible printed circuits employing polymer dielectric films
FAQ
What is the difference between the temperature index and the thermal class?
The temperature index (TI) is an experimentally determined temperature, derived from accelerated aging data, at which the material retains a defined property level after a specified lifetime, usually 20,000 hours. The thermal class is the standardized classification assigned to the insulation system or material based on this temperature. A material with a TI of 155, for example, can be classified for class 155 (class F) applications.
How long does thermal endurance testing take?
A full IEC 60216 thermal endurance program typically requires several months, because the lowest aging temperature must run until the endpoint is reached, which can exceed 5,000 hours. Shorter screening programs using fewer aging points or comparison testing against a reference material with a known RTI can provide earlier indications of thermal capability.
What endpoint criterion is used for insulation films?
For flexible films, the most common endpoint is a 50% reduction in elongation at break, since elongation decreases sharply as the film embrittles. Tensile strength retention and dielectric strength retention are also frequently used, either individually or in combination, depending on the specification and application requirements.
How many samples are required for testing?
Each aging temperature requires a statistically sufficient number of specimens, typically at least five per withdrawal interval for tensile testing, with multiple withdrawal intervals per temperature. Laboratories generally request several square meters of film, or a defined number of specimen strips at the specified thickness, to complete a full thermal endurance characterization.
Which standards govern this testing?
The primary references are the IEC 60216 series for thermal endurance determination, IEC 60085 for thermal classification, IEC 60674 for plastic films for electrical purposes, UL 746B for long-term RTI evaluation, and GB/T 13542 together with GB/T 11026 for Chinese national compliance. Supporting test methods include ISO 527-3, ASTM D882, IEC 60243-2, and ASTM D149.