Test service for compressive permanent deformation rate of immersed transformer oil

Compression Permanent Deformation Rate Testing Service for Elastomeric Seals in Immersed Transformer Oil – Validating Long‑Term Sealing Performance for Brazilian Power Transformers

As an ISO/IEC 17025 accredited independent testing laboratory, we offer specialised compression permanent deformation (compression set) rate testing services for elastomeric seals, gaskets, and insulating pads that operate while fully immersed in transformer oil. In high‑voltage power transformers, rubber components (e.g., O‑rings, flange gaskets, and bushing seals) are continuously exposed to hot, pressurised mineral or ester‑based insulating oils. Over time, compression set – the irreversible deformation remaining after a compressive load is released – can lead to oil leakage, moisture ingress, and reduced insulation performance, directly affecting transformer reliability and safety. Our test protocols measure the permanent deformation of elastomeric specimens after compressive loading in a temperature‑controlled oil bath, following ABNT NBR standards, ASTM D395 (Standard Test Methods for Rubber Property – Compression Set), ISO 815 (Rubber, vulcanised or thermoplastic – Determination of compression set), IEC 60216 (Thermal endurance of insulating materials), and IEC 60590 (Test methods for insulating oil‑immersed transformers). Our reports are recognised by ANEEL (electricity regulator), INMETRO (product certification), and major Brazilian utilities and transformer manufacturers for quality assurance and equipment life‑cycle management.

Elastomeric Components We Regularly Test in Transformer Oil

Our test lab accommodates a range of rubber and elastomeric seals commonly used in oil‑filled transformers. Typical test specimens include:

• O‑rings and circular seals – for tank covers, bushings, and valve connections
• Flange gaskets and flat sealing sheets – for oil‑cooling radiators and pipe joints
• Insulating pads and spacer blocks – used in winding supports and core clamping
• Custom‑moulded profiles – for cable entry and sensor ports
• Gasket materials – NBR (nitrile), HNBR, FKM (fluorocarbon), EPDM, and silicone rubber

Compression Set in Oil – Constant Deflection Method (ASTM D395 / ISO 815)

• Specimen preparation and initial thickness measurement – We cut standard test specimens (typically cylindrical discs or O‑ring segments) from the supplied seal material. The initial thickness (t₀) is measured at four points around the circumference using a calibrated micrometer (accuracy ±0.01 mm) under a standard load (22 kPa). Specimens are conditioned at 23 ± 2 °C for 24 hours before testing.
• Compression assembly and oil immersion – The specimens are placed between polished metal plates and compressed to a defined deflection – typically 25 % of the original thickness (for O‑rings) or 15 % for flat gaskets – using a mechanical jig with spacers. The entire assembly is then fully immersed in a stainless‑steel pressure vessel containing pre‑heated transformer oil (mineral oil or synthetic ester, at a specified temperature, typically 100 °C, 120 °C, or 140 °C) for a standard duration of 22, 70, or 168 hours, per ASTM D395 Method B. The oil temperature is controlled to ±1 °C using a thermostatic bath.
• Recovery and thickness measurement – After the immersion period, the specimen is removed from the oil, quickly blotted of excess oil, and allowed to cool on a non‑absorbent surface at 23 °C for 30 minutes. The compressed thickness is then released, and the specimen is left to recover for 30 minutes. The final thickness (t₁) is measured at the same four points. The compression set (CS) is calculated as: CS (%) = [(t₀ – t₁) / (t₀ – tₛ)] × 100, where tₛ is the thickness of the spacer (compressed height). We report the average and standard deviation of at least five replicate specimens.
• Effect of oil type and ageing – We perform the test in both fresh transformer oil and aged oil (pre‑oxidised per IEC 61125) to simulate in‑service conditions. The difference in compression set between fresh and aged oil is reported as an oil‑compatibility factor, which is critical for Brazilian utilities that use reclaimed or long‑service oils.

Constant‑Load Compression Set (ASTM D395 Method A / ISO 815‑2)

• Compression under fixed load in hot oil – For certain seal designs, we apply a constant compressive stress (e.g., 5 MPa) rather than a fixed deflection. The specimen is placed between the plates and loaded with a calibrated weight, then immersed in the oil bath at the test temperature. After the specified period, the load is removed, the specimen is cooled, and the thickness recovery is measured. The compression set is expressed as a percentage of the original thickness reduction. This method better simulates the bolted flange loads in real transformers.
• Stress relaxation measurement – In parallel, we monitor the decay of the applied load over time (using a load cell) and report the residual stress as a percentage of the initial stress. A high residual stress (≥ 70 % after 168 hours) indicates good sealing force retention, which is essential for preventing oil leaks in Brazilian substations.
• Effect of multiple heating/cooling cycles – We subject the assembly to 5 thermal cycles (from 23 °C to 120 °C and back) under constant load, and then measure the compression set after the last cycle. This simulates the thermal cycling experienced by transformers during daily load variations.

Oil Temperature and Pressure Variation – Simulating Real Transformer Conditions

• Compression set at elevated temperatures (100 °C, 120 °C, 140 °C) – We systematically test the same material at three different oil temperatures to generate a temperature‑compression set curve. The increase in compression set with temperature is expressed as a temperature coefficient (% per 10 °C). For Brazilian transformers, a value below 5 % per 10 °C is generally acceptable for reliable long‑term sealing.
• High‑pressure oil immersion (up to 0.5 MPa) – For sealed‑tank transformers, we perform the compression set test in a pressurised oil vessel (using nitrogen overpressure) to simulate the 0.2‑0.5 MPa operating pressure. The compression set under pressure is compared to atmospheric pressure results; a difference greater than 3 % indicates that the seal material is pressure‑sensitive.
• Effect of oil moisture content – We condition the test oil to two different moisture levels: < 10 ppm (dry) and 50 ppm (wet), representing normal and high‑moisture conditions. The compression set is measured under both conditions, and the increase (typically 2‑5 %) is reported. This data helps Brazilian transformer maintenance teams plan oil‑drying schedules.
• Long‑term ageing up to 1,000 hours – For critical transformers, we offer extended tests at 120 °C for 336, 500, and 1,000 hours to predict the sealing life. The compression set data is plotted against log(time), and the time to reach a critical set (e.g., 50 %) is extrapolated. This life‑estimation is invaluable for asset managers of Brazilian power transmission networks.

Post‑Test Evaluation – Physical and Chemical Property Changes

• Hardness change (Shore A) after compression set test – We measure the Shore A hardness of the specimen before and after the test (per ASTM D2240). An increase of more than 5 points indicates oil‑induced hardening, while a decrease indicates swelling and plasticisation. Both are reported as indicators of oil compatibility.
• Volume and mass change – We weigh the specimen and measure its volume (by immersion in water or by geometric calculation) before and after the test. The percentage weight gain and volume swell are reported. A volume swell of less than 10 % is generally acceptable for NBR seals in mineral oil.
• Tensile strength and elongation retention – For a subset of specimens, we perform a tensile test (ASTM D412) on dumbbell samples cut from the same material after the oil immersion (without compression). The retention of tensile strength and elongation at break is calculated. A retention below 70 % indicates significant oil degradation.
• Microscopic inspection for cracks and surface degradation – We examine the surface of the tested specimens under a digital microscope (50‑200×) for any micro‑cracks, blisters, or surface deposits. The condition is rated on a 0‑5 scale (0 = no change, 5 = severe degradation). Photographs are included in the final report.

Report Acceptance & Compliance with Brazilian Utility and Transformer Standards

All compression permanent deformation tests are conducted under our ISO/IEC 17025:2017 accreditation, using calibrated thickness gauges, temperature controllers, pressure vessels, and tensile/durometer instruments traceable to INMETRO. Our final reports provide: test conditions (oil type, temperature, pressure, duration, deflection/load), individual and average compression set values, statistical summaries (standard deviation, coefficient of variation), hardness, mass, and volume change data, tensile retention (when requested), microscopic images, a comparison of results across different oil ageing or moisture levels, and a clear pass/fail evaluation against your specified criteria (e.g., maximum compression set ≤ 40 % after 168 hours at 120 °C for NBR O‑rings). We also supply an expanded uncertainty (k=2) for the compression set measurement. These reports are widely accepted by ANEEL for equipment reliability assessments, by INMETRO for transformer component certification, and by Brazilian power utilities and transformer manufacturers for incoming material qualification, maintenance planning, and life‑cycle analysis. Bilingual (Portuguese/English) versions are available to facilitate regulatory submissions and support your engineering and procurement teams. With our rigorous compression permanent deformation testing, you can confidently select elastomeric materials that maintain sealing integrity in hot transformer oil, preventing costly oil leaks and ensuring the reliable operation of Brazil’s power grid.