Dimensional Stability Testing of Transformer Insulation Films: Methods, Standards, and Acceptance Cr

Dimensional stability testing of transformer insulation films is a critical quality control procedure that evaluates heat shrinkage and dimensional change in polyester, polyimide, and aramid insulating materials used in transformer windings. This article explains the testing samples, test items, standardized methods based on IEC, ASTM, and GB standards, typical acceptance criteria, and practical applications, helping manufacturers and engineers select reliable insulating films and qualify materials through accredited third-party laboratories.

Technical Overview

Transformer insulation films are thin polymeric materials—most commonly biaxially oriented polyester (PET), polyimide, polyphenylene sulfide (PPS), and aramid-based papers—that provide interlayer, interturn, and barrier insulation in distribution transformers, power transformers, dry-type transformers, and other electrical apparatus. Because these films are subjected to sustained elevated operating temperatures and thermal excursions during transformer service, their ability to retain original dimensions—known as dimensional stability—directly affects winding tightness, insulation coordination, and long-term reliability.

Excessive or anisotropic shrinkage of an insulation film can cause wrinkling, loosening of winding layers, reduction of creepage and clearance distances, localized mechanical stress, and even dielectric breakdown. Dimensional stability verification is therefore a mandatory receiving inspection and incoming quality control item for transformer manufacturers, and a routine qualification test performed by accredited third-party laboratories during material certification.

The evaluation is primarily conducted as a heat shrinkage test, in which specimens are exposed to a specified elevated temperature for a defined duration, and the resulting linear dimensional changes in the machine direction (MD) and transverse direction (TD) are measured and expressed as a percentage of the original gauge length. Relevant framework standards include IEC 60674-2 (plastic films for electrical purposes – methods of test), ASTM D1204 (linear dimensional changes of nonrigid thermoplastic sheet or film at elevated temperature), ASTM D5219, and the Chinese national standard series GB/T 13542 covering electrical insulating films.

Testing Samples

Biaxially Oriented Polyester (PET) Films

BOPET films in thicknesses typically ranging from 25 µm to 350 µm are the most widely used insulation films for Class E (120 °C) and Class B (130 °C) transformer insulation systems. Because these films are biaxially stretched during manufacturing, they contain frozen-in internal stresses that relax upon heating, producing measurable MD and TD shrinkage. Specimens are usually sampled from at least three positions across the film web width to capture cross-direction variability.

Polyimide Films

Polyimide films rated for Class H (180 °C), Class C (220 °C), and above are used in high-temperature dry-type transformers, traction transformers, and aerospace applications. These films exhibit very low but still specification-relevant shrinkage at elevated temperature, and testing is typically performed at higher exposure temperatures (for example 200 °C to 300 °C) with correspondingly tighter acceptance limits.

Aramid Paper and Fiber-Based Insulation

Calendered and uncalendered aramid papers, used as layer insulation and winding barriers in oil-immersed and dry-type transformers, are evaluated for thermal shrinkage in accordance with the IEC 60641 series and related material standards. Testing under both dry and conditioned humidity states may be required, since cellulosic and aramid fiber structures can exhibit hygroscopic dimensional change in addition to thermal shrinkage.

Composite Laminates and Coated Films

Laminated structures such as PET film laminated with aramid paper, polyimide-PET composites, and fluoropolymer- or epoxy-coated films must be tested as complete laminates. The differential shrinkage between constituent layers is a particular concern, since it can cause curling, delamination, or warping that compromises automated winding processes and insulation performance.

Testing Items

The dimensional stability verification program for transformer insulation films typically includes the following test items:

• Heat shrinkage in the machine direction (MD) after exposure to a specified elevated temperature
• Heat shrinkage in the transverse direction (TD) after exposure to a specified elevated temperature
• Dimensional change after thermal cycling or thermal aging representative of service conditions
• Moisture-related dimensional change (hygroscopic expansion) for fiber-based and paper-containing materials
• Warpage, curling, blistering, and visual defect assessment after thermal exposure
• Verification of thickness and width conformity relative to specified dimensional tolerances

Depending on the insulation system qualification scope, dimensional stability testing may be complemented by related characterization such as tensile strength and elongation (IEC 60674-2 or ISO 527-3), breakdown voltage and electric strength (IEC 60243), and thermal endurance assessment, since these properties collectively determine the suitability of the film for a given thermal class.

Testing Methods

Specimen Preparation and Conditioning

Representative specimens, commonly square sections of approximately 100 mm × 100 mm or rectangular strips cut from the roll width, are prepared with clean, sharp cutting tools to avoid edge defects that could introduce artificial shrinkage. Two accurate gauge marks spaced 100 mm apart are applied in each principal direction (MD and TD), typically using a fine marker, punch marks, or scribed lines. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 291 or ASTM D618 until mass equilibrium is reached.

Thermal Exposure Procedure

Initial distances between gauge marks are measured to the nearest 0.5 mm or better using a calibrated steel rule, optical comparator, traveling microscope, or non-contact video measurement system. Specimens are then placed flat, without constraint and without contact between adjacent specimens, in a force-ventilated oven maintained at the specified test temperature with a tolerance of ± 2 °C or tighter. Typical exposure conditions referenced in material specifications and in the test methods of IEC 60674-2, ASTM D1204, and GB/T 13542 include:

• PET insulation films: 150 °C for 30 minutes (alternative conditions such as 120 °C or 155 °C for 30 to 60 minutes may be specified by agreement)
• Polyimide films: 200 °C to 300 °C for 30 minutes to 2 hours depending on thermal class
• Aramid papers and composites: conditions per IEC 60641 or the applicable material specification

After the exposure period, specimens are removed, reconditioned to room temperature in the standard atmosphere, and the final distances between gauge marks are remeasured.

Calculation and Expression of Results

The percentage dimensional change (shrinkage) in each direction is calculated as follows:

Shrinkage (%) = (L₀ − L₁) / L₀ × 100, where L₀ is the original distance between gauge marks before exposure and L₁ is the distance after heat treatment. A positive value indicates shrinkage and a negative value indicates expansion. Results are reported individually and as the mean for each direction (MD and TD), together with the test temperature, exposure duration, and any visual observations such as wrinkling, curling, blistering, or discoloration.

Acceptance Criteria

Acceptance limits are defined by the applicable material specification, the transformer manufacturer's incoming inspection standard, or the agreement between supplier and purchaser. Commonly applied reference limits include:

• PET films for Class B insulation systems: MD shrinkage ≤ 3.0 % and TD shrinkage ≤ 1.5 % after 150 °C for 30 minutes, with tighter grades limited to ≤ 1.5 % in MD
• Polyimide films: shrinkage typically ≤ 1.0 %, with premium grades limited to ≤ 0.2 % to 0.5 % depending on test temperature
• Aramid papers: shrinkage limits per the applicable IEC 60641 material sheet, generally within a few percent
• No visible wrinkling, blistering, delamination, or severe curling after exposure

Calibration of the oven temperature, traceability of dimensional measurement instruments, documented conditioning, and full test records are essential for results to be accepted in supplier qualification and certification reports issued by accredited third-party laboratories.

Applications

Dimensional stability testing of insulation films supports quality assurance and material qualification across a broad range of electrical equipment applications, including:

• Oil-immersed distribution and power transformers, where films serve as layer, interturn, and electrostatic (capacitive) barrier insulation
• Dry-type cast resin and VPI transformers using PET, polyimide, and aramid insulation systems
• Instrument transformers, reactors, and special-purpose traction and rectifier transformers
• Wire and cable wrap insulation and winding components for rotating machines
• Insulation system certification and thermal class verification programs
• Supplier audits, incoming material inspection, and failure analysis investigations

By verifying that insulation films retain their dimensions under thermal stress, manufacturers ensure tight, wrinkle-free windings, stable insulation distances, and predictable long-term performance of the complete transformer insulation system.

FAQ

Why is dimensional stability so important for transformer insulation films?

Insulation films are wound under tension together with conductors. If the film shrinks unevenly at operating temperature, the winding can loosen, wrinkles can form, and insulation clearances can be reduced, creating localized electrical and thermal stress points that may lead to partial discharge and premature dielectric failure. Dimensional stability testing detects these risks before the material enters production.

What is the difference between MD and TD shrinkage?

MD (machine direction) is the direction in which the film moves during manufacturing, and TD (transverse direction) is perpendicular to it. Biaxially oriented films carry different frozen-in stresses in each direction, so shrinkage must be measured and reported separately for both directions. Excessive anisotropy between MD and TD shrinkage is itself a cause of warping and curling.

Which standards govern the test method?

The core framework comes from IEC 60674-2 for plastic films for electrical purposes, ASTM D1204 for linear dimensional changes of thermoplastic films at elevated temperature, and the GB/T 13542 series for electrical insulating films. Aramid and cellulosic materials are covered by the IEC 60641 series. Conditioning atmospheres follow ISO 291 or ASTM D618.

Can testing be performed in-house, or is a third-party laboratory required?

Transformer manufacturers commonly perform dimensional stability testing as part of incoming inspection using calibrated ovens and measuring instruments. For new supplier qualification, insulation system certification, or dispute resolution, test reports from accredited third-party laboratories with traceable calibration and recognized accreditation scopes carry greater technical authority.

How does moisture affect the results?

Polymeric films such as PET and polyimide are largely non-hygroscopic, so moisture influence is minimal, but fiber-based aramid and cellulosic materials can swell or shrink with humidity changes. Standardized pre-conditioning at 23 °C and 50 % relative humidity ensures comparable, reproducible results regardless of the ambient history of the sample.

What should a complete test report include?

A compliant report should identify the material, thickness, and batch, describe specimen geometry and gauge marks, state conditioning and exposure parameters (temperature, time, atmosphere), tabulate individual and mean MD and TD shrinkage values, record visual observations after exposure, reference the applied test standard, and include instrument calibration and laboratory accreditation details where applicable.