Determination of Compression Deformation After Immersion in Transformer Oil: Testing Standards, Meth

Compression deformation after immersion in transformer oil is a critical performance test for elastomeric sealing materials used in oil-immersed electrical equipment, evaluating how well rubber gaskets and seals retain their elastic recovery after long-term exposure to hot insulating oil under compressive load. This test, typically performed per GB/T 7759.1, ISO 815-1, and ASTM D395 in combination with oil immersion protocols, helps manufacturers predict long-term sealing reliability in transformers, reactors, and bushings. In this article, we explain the testing principle, sample requirements, procedures, calculation methods, and typical applications of this essential quality control test.

Technical Overview

Sealing components in oil-immersed power equipment—such as transformer flange gaskets, O-rings, valve seals, and cable penetration seals—must maintain reliable sealing performance over service lives of 20 years or more. During operation, these elastomeric components are continuously subjected to two simultaneous stresses: mechanical compression between flanges and chemical swelling or degradation caused by hot mineral transformer oil. The combination of these factors causes progressive loss of elastic recovery, a phenomenon known as compression set or compression deformation.

The determination of compression deformation after transformer oil immersion evaluates the ability of a rubber material to recover after being compressed for a defined period while fully immersed in transformer oil at an elevated temperature. A high compression set value indicates that the seal has permanently deformed, which can lead to oil leakage, moisture ingress, and ultimately insulation failure of the equipment. The test therefore combines elements of fluid resistance testing (per GB/T 1690, ISO 1817, ASTM D471) with compression set testing (per GB/T 7759.1, ISO 815-1, ASTM D395), and is commonly specified in technical requirements for transformer sealing components.

The test is typically performed by accredited third-party laboratories as well as in-house quality control departments of rubber product manufacturers and transformer OEMs, using unused mineral insulating oil conforming to IEC 60296 or GB 2536 as the immersion medium.

Testing Samples

Vulcanized Rubber Sealing Products

Nitrile butadiene rubber (NBR) is the most widely used material for transformer sealing applications due to its excellent oil resistance. Other tested materials include fluororubber (FKM), ethylene propylene diene monomer (EPDM) for specific applications, silicone rubber, and composite cork-rubber gaskets. Samples may be supplied as standard test buttons or cut from finished sealing products.

Standard Compression Set Specimens

Two standard specimen geometries are commonly used. Type A specimens are cylindrical buttons with a diameter of 29.0 mm and a height of 12.5 mm, while Type B specimens are smaller cylinders with a diameter of 13.0 mm and a height of 6.3 mm. Specimens must be flat, parallel, and free from surface defects such as bubbles, cracks, or mold defects that could affect measurement accuracy.

Finished Sealing Components

When testing actual products such as O-rings, gaskets, or sealing strips, the compression set can be determined by measuring the cross-sectional thickness or overall height before and after the test, provided the geometry permits accurate and repeatable thickness measurement.

Testing Items

The core deliverables of the test include the following measurable parameters:

• Compression deformation (compression set) after immersion in transformer oil at elevated temperature, expressed as a percentage
• Original specimen thickness, compressed thickness, and recovered thickness after release
• Volume change and mass change of the rubber after oil immersion (optional supplementary indicators)
• Hardness change before and after oil immersion (optional supplementary indicator)
• Tensile strength and elongation at break retention after oil aging (optional, when a full material assessment is required)

Testing Methods

Test Apparatus and Materials

The test setup consists of a compression set fixture comprising two parallel steel plates and steel spacers that limit the compression to a defined percentage (typically 25%), a temperature-regulated oil bath or air-circulating oven with an oil immersion vessel, a thickness gauge or dial indicator capable of measuring to 0.01 mm or better, and unused mineral transformer oil conforming to IEC 60296 or GB 2536. The volume of oil should be at least 15 to 20 times the total volume of the specimens to ensure the fluid composition is not significantly altered by extractables during the test.

Specimen Conditioning and Initial Measurement

Specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for at least 16 hours prior to testing. The original thickness of each specimen is then measured at a minimum of three points, and the median value is recorded as the initial thickness.

Immersion and Compression Procedure

The specimens are placed between the lubricated plates of the compression fixture with spacers set to achieve the specified compression, typically 25% of the original thickness. The assembled fixture is then fully immersed in transformer oil inside a thermostatically controlled bath. Commonly specified test conditions include 100 °C or 125 °C for durations of 24 hours, 70 hours, 168 hours, or extended periods up to 1000 hours for long-term life assessment. The selected temperature and duration should reflect, or be more severe than, the actual service conditions of the sealing component.

Recovery Measurement

Upon completion of the test period, the fixture is removed from the oil bath and the specimens are released from compression within a short, standardized time. After a recovery period—typically 30 minutes at standard laboratory temperature, with the specimen kept away from direct airflow—the recovered thickness is measured in the same manner as the original thickness.

Calculation of Compression Deformation

The compression deformation after oil immersion is calculated using the following relationship:

Compression set (%) = (t0 − tr) / (t0 − ts) × 100

where t0 is the original specimen thickness, tr is the recovered thickness after release, and ts is the spacer thickness representing the compressed height. A result of 0% indicates complete elastic recovery, while 100% indicates total permanent deformation. For transformer sealing applications, typical acceptance limits range from below 20% to below 50% depending on the material class, test severity, and equipment specification.

Applications

The test is widely applied across the electrical power equipment and rubber products industries:

• Quality control and incoming inspection of gaskets and O-rings used in power transformers, instrument transformers, and reactors
• Material qualification and formulation comparison for oil-resistant rubber compounds intended for oil-immersed service
• Type testing and routine testing of transformer sealing components against technical specifications from power utilities and equipment manufacturers
• Failure analysis of leaking transformer flanges, valve seals, bushing gaskets, and cable box seals
• Long-term aging studies and service life prediction for seals in distribution transformers, tap changers, radiator valves, and oil-conservator systems
• Verification of compatibility between new sealing materials and alternative insulating liquids such as ester-based transformer oils
• Support for product certification, tenders, and technical documentation submitted by manufacturers to power grid operators

FAQ

Why is compression deformation tested in oil rather than in air?

Transformer seals operate while immersed in hot insulating oil, which can cause swelling, extraction of plasticizers and additives, and chemical degradation of the rubber network. Testing in oil reproduces the combined thermal, chemical, and mechanical stresses of real service conditions, providing a far more realistic prediction of sealing performance than an air-oven test alone.

What test temperature and duration should be selected?

The test conditions should match or exceed the operating temperature of the equipment in which the seal is installed. Common choices are 100 °C or 125 °C for durations of 24 hours to 168 hours for quality control purposes, while extended durations of 500 to 1000 hours are used for life assessment. The selected conditions must always be agreed between the purchaser and supplier and documented in the test report.

What compression percentage is applied?

A compression of 25% of the original specimen thickness is the standard deflection used in GB/T 7759.1, ISO 815-1, and ASTM D395 Method B. Other compression ratios may be specified when the actual assembly compression of the finished seal differs significantly from the standard value.

How is the test result interpreted?

A lower compression set percentage indicates better elastic recovery and long-term sealing capability. Excessive compression set means the seal will not maintain sufficient contact pressure after relaxation, leading to potential oil leakage paths. Acceptance criteria depend on the product standard or purchase specification, but values below 50% under severe conditions and below 20% under moderate conditions are commonly required.

Can finished gaskets be tested instead of molded buttons?

Yes. When the geometry of the finished component permits accurate thickness measurement before and after testing, the compression deformation can be determined directly on the product. Test reports should clearly state whether results were obtained on standard specimens or finished parts, as the two are not directly interchangeable.

Which standards govern this test?

The primary standards are GB/T 7759.1 and ISO 815-1 for compression set determination, ASTM D395 for rubber compression set methods, GB/T 1690, ISO 1817, and ASTM D471 for the resistance of rubber to liquids, and IEC 60296 or GB 2536 for the specification of the transformer oil used as the immersion medium. Testing should be performed by accredited third-party laboratories or qualified in-house facilities with calibrated equipment and traceable measurement standards.