Thermal stability testing of transformer insulation films is essential for verifying the long-term reliability of winding insulation in dry-type and oil-immersed transformers. This article describes the experimental procedures, sample requirements, test items and international standards (IEC, ASTM, ISO, GB, UL, JIS) used to evaluate the thermal endurance of polyester, polyimide, PEN and PPS films. The results provide a scientific basis for thermal class assignment, material selection and quality control in transformer manufacturing.
Technical Overview
Insulation films are among the most critical components in transformer manufacturing, serving as interturn, interlayer and barrier insulation between windings, and between windings and the core. Common materials include biaxially oriented polyethylene terephthalate (PET), polyimide (PI), polyethylene naphthalate (PEN) and polyphenylene sulfide (PPS) films, with nominal thicknesses typically ranging from 25 micrometers to 350 micrometers. Because transformer windings operate continuously at elevated temperatures, often 105 degrees Celsius to 180 degrees Celsius depending on the thermal class defined in IEC 60085 and GB/T 11021, the ability of these films to resist thermal degradation directly determines insulation service life, which is generally expected to reach 20 to 30 years.
Thermal stability experiments simulate decades of service through accelerated aging. Film specimens are exposed to several temperatures well above the intended operating point, and diagnostic properties such as dielectric strength, tensile strength and elongation at break are measured periodically until a defined endpoint criterion is reached. By plotting the logarithm of time-to-endpoint against the reciprocal of absolute temperature in Arrhenius coordinates, a thermal endurance profile is established and extrapolated to long service times, yielding the temperature index (TI) used for thermal classification.
The principal degradation mechanisms investigated in these experiments include:
- Thermooxidative chain scission and progressive reduction of molecular weight, leading to embrittlement
- Hydrolytic degradation of ester-based films such as PET in hot and humid environments
- Relaxation of oriented molecules, causing thermal shrinkage and dimensional instability
- Post-crystallization, microvoid formation and surface oxidation at sustained high temperature
- Chemical interaction between the film and transformer oil or winding varnish during aging
Testing Samples
Polyester (PET) Insulation Films
Biaxially oriented PET film is the most widely used material for layer and wrap insulation in distribution transformers. Typical thicknesses are 25, 50, 75, 100, 125 and 190 micrometers, with a melting point of approximately 255 to 260 degrees Celsius and a glass transition temperature near 78 degrees Celsius. PET films are commonly assigned to thermal class 105 (A) or 120 (E) depending on the qualified insulation system. Specimens are prepared as 15 millimeter wide strips for tensile testing and as square sheets of at least 100 by 100 millimeters for breakdown testing.
Polyimide (PI) Films
Aromatic polyimide films exhibit no melting point, a decomposition onset above 500 degrees Celsius and continuous service capability of roughly 200 to 240 degrees Celsius, with a temperature index that can reach 220. They are preferred for dry-type, traction and special-purpose transformers. Corona-resistant grades filled with inorganic nanoparticles are available for windings driven by power electronics.
PEN and PPS Films
PEN film offers a higher glass transition temperature of about 120 degrees Celsius and significantly better hydrolysis resistance than PET, making it suitable for class 130 to 155 applications. PPS film combines a high crystalline melting point near 280 degrees Celsius with excellent chemical resistance, supporting class 155 to 180 usage.
Composite and Coated Films
Composite structures include polyimide film laminated with FEP or acrylic bonding layers for fusion-bonded insulation, film-aramid paper laminates for enhanced impregnation, and corona-resistant nanocomposite films. Each construction must be evaluated as a complete laminate, since the weakest layer governs thermal endurance.
Sample Preparation and Conditioning
All 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 and ASTM D618. A sufficient number of replicates, typically five to ten specimens per temperature and aging time point, is prepared to allow statistical evaluation, and specimen edges are inspected to exclude mechanical defects that could distort failure data.
Testing Items
A complete thermal stability evaluation of transformer insulation films typically covers the following items:
- Temperature index (TI) and thermal endurance profile (TEP) determined per the IEC 60216 series
- Halving interval (HIC), which describes how service life halves as temperature increases
- Thermal shrinkage in machine and transverse directions after exposure at 150 to 300 degrees Celsius
- Mass loss behavior and decomposition temperatures by thermogravimetric analysis
- Glass transition, melting point and crystallinity changes by differential scanning calorimetry
- Dielectric strength retention after each aging interval
- Tensile strength and elongation at break retention after each aging interval
- Volume resistivity, relative permittivity and dissipation factor before and after aging
- Compatibility with transformer oil and winding varnish under sealed thermal aging
Testing Methods
Long-Term Thermal Aging (IEC 60216, GB/T 11021, UL 746B)
Specimens are aged in forced-draft ventilated ovens with controlled air exchange rates as specified in IEC 60216-4-1. At least three, and preferably four, aging temperatures are selected so that the time to endpoint ranges from approximately 100 hours at the highest temperature to 5,000 hours or more at the lowest. After each aging period, the diagnostic property is measured; the most common endpoint criteria are 50 percent retention of initial tensile strength or 50 percent retention of initial breakdown voltage. Linear regression in Arrhenius coordinates gives the thermal endurance graph, and extrapolation to 20,000 hours yields the temperature index. GB/T 11021 adopts the same methodology for the Chinese market, while UL 746B applies a comparable long-term aging philosophy to derive the relative temperature index (RTI) of polymeric materials.
Thermogravimetric Analysis (ISO 11358, ASTM E1131)
Specimens of 3 to 10 milligrams are heated at 10 degrees Celsius per minute from ambient temperature to 800 degrees Celsius, first in nitrogen and then in air. The onset temperature of decomposition, the temperature at 5 percent mass loss, the maximum rate degradation temperature and the residual char yield are recorded. The nitrogen run reflects intrinsic thermal stability, while the air run reveals susceptibility to oxidation.
Differential Scanning Calorimetry (ISO 11357, ASTM D3418)
DSC scans measure the glass transition, cold crystallization, melting behavior and degree of crystallinity before and after aging. Shifts in these transitions, such as a rising glass transition or growing crystallinity, provide early evidence of thermochemical change before mechanical properties deteriorate noticeably.
Thermal Shrinkage (IEC 60674-2, ASTM D1204)
Square specimens of approximately 100 by 100 millimeters are held for 30 minutes at selected temperatures such as 150, 200 or 250 degrees Celsius, then reconditioned and remeasured. Shrinkage is calculated separately for the machine and transverse directions. Typical specification limits are below 2 percent for PET film at 150 degrees Celsius and below 0.5 percent for polyimide film at 200 degrees Celsius, with project specifications governing the exact values.
Dielectric Strength (IEC 60243-1, ASTM D149, GB/T 1408)
Breakdown strength is measured using sphere or cylindrical electrodes, either in air or immersed in insulating oil to prevent flashover. A minimum of ten breakdowns is performed per condition and evaluated statistically, including Weibull analysis where required. Comparing breakdown strength before and after each aging interval quantifies the dielectric endurance of the film.
Mechanical Property Retention (ASTM D882, ISO 527-3)
Strips of 15 millimeter width are tested in tension at a crosshead speed appropriate to the film thickness. Tensile strength and elongation at break are recorded for unaged and aged specimens; retention values are the primary aging endpoints in many specifications.
Dielectric Parameters After Aging (IEC 62631-2-1, ASTM D150)
Relative permittivity and dissipation factor are measured over a frequency range from power frequency up to 1 megahertz, and volume resistivity is determined according to IEC 62631-3-1. Increases in dissipation factor after aging often indicate oxidation products or moisture uptake within the film.
Oil Compatibility Aging
For oil-immersed applications, film specimens are aged in sealed tubes or beakers containing uninhibited transformer oil. After exposure, both the film properties and the oil condition, including acid number and dissipation factor, are evaluated with reference to IEC 60422 to confirm that no harmful interaction occurs.
Applications
Thermal stability test data for insulation films are used throughout the transformer industry and adjacent power electronics sectors:
- Thermal class assignment and insulation system qualification for dry-type transformers per IEC 60076-11 and for oil-immersed transformers per IEC 60076
- Material selection for traction transformers, rail transit equipment and electric vehicle charging systems where high ambient temperatures and overload cycles are common
- Layer and interturn insulation design for high-frequency planar transformers in photovoltaic inverters, wind converters and switch-mode power supplies
- Incoming quality control at transformer manufacturers and film converters, using shrinkage, breakdown and mechanical checks against JIS C 2318, GB/T 13542 and comparable film product standards
- Research and development of new film materials, including corona-resistant nanocomposites and high-temperature laminates
- Aging life modeling and residual life assessment of transformers already in service
- Technical support for certification programs conducted by accredited third-party laboratories
FAQ
What is the temperature index (TI) of an insulation film?
The TI is the temperature in degrees Celsius, derived from the thermal endurance relationship, at which the material retains a specified property level for 20,000 hours. It is reported together with the halving interval (HIC), typically 8 to 12 degrees Celsius, which describes how much the temperature can increase before the endurance time is cut in half.
Why is 20,000 hours used as the reference time?
IEC 60216 adopted 20,000 hours as the conventional reference point for thermal endurance characterization because it can be reached by controlled extrapolation while remaining representative of long-term behavior. Combined with the halving interval, the 20,000-hour TI can be extrapolated to 200,000 hours or more for transformer design life calculations.
How long does a complete thermal endurance experiment take?
Depending on the material, a full multi-temperature aging campaign usually takes from six months to more than one year, because the lowest aging temperature alone must run for at least 5,000 hours. Shorter screening tests, such as TGA, DSC and shrinkage measurements, can be completed within a few days.
Which failure criterion is applied during aging?
The most common criteria are 50 percent retention of the initial tensile strength or 50 percent retention of the initial dielectric breakdown voltage. Some specifications use a fixed absolute elongation limit instead. The criterion must match the actual failure mode expected in the end application, and it is always stated in the test report.
Does thermal stability testing also cover oil-immersed transformer conditions?
Yes. For oil-immersed designs, sealed-tube aging in transformer oil is performed in addition to air aging. The film is evaluated for property retention, and the oil is checked for acid number and dissipation factor changes, confirming chemical compatibility under thermal stress.
What representative results indicate good thermal stability?
Indicative benchmarks include a PET film shrinkage below 2 percent at 150 degrees Celsius, a polyimide decomposition onset above 500 degrees Celsius with shrinkage below 0.5 percent at 200 degrees Celsius, and retention of more than 50 percent of dielectric strength after long-term aging at the class temperature. Final acceptance values are always defined by the applicable product standard or customer specification.