Introduction
Interfacial Tension (IFT) measures the amounts of dissolved polar contaminants and oxidation by-products in electrical insulating fluids like mineral oil. A high value, usually for new insulating oil indicates the absence of undesirable polar contaminants. The IFT test adds an advantage to being an early indicator of sludge formation in the oil-insulating fluid well before any sludge formation occurs.
Where do polar contaminants originate from?
Understanding the origins of polar contaminants in transformer oil is crucial for maintaining long-term health. The insulation surrounding transformer windings is typically comprised of oil and cellulose insulating materials which over time together with various factors contribute to the degradation of this insulation, leading to the formation of polar contaminants. These factors include:
Oxidation: Oxidation processes occur naturally over time due to exposure to oxygen in the air. This leads to the formation of polar impurities within the insulation material and oils. These impurities can increase the conductivity of the insulation, compromising its effectiveness.
Moisture Absorption: As the cellulose insulation degrades, it becomes more prone to absorbing moisture and the presence of high levels of water in the insulation material and oil further exacerbates the formation of polar impurities and increases conductivity.
Electrical Stress: Under normal operation, transformers are subjected to electrical stress, which can accelerate the degradation of insulation materials. This stress can lead to partial discharges and breakdowns within the insulation, contributing to the formation of polar contaminants.
Temperature: High temperatures, often experienced within transformers during operation, can also accelerate the degradation of insulation materials. This can promote the formation of polar impurities and increase the likelihood of insulation failure.
How to test for Interfacial Tension?
ASTM D971 is a standard established by the American standardization organization ASTM International which pertains to the evaluation of the aging condition of transformer oils through the measurement of interfacial tension (IFT) when in contact with water [1].
This involves measuring the surface tension where the liquid oil sample interfaces with water. The mutual attraction between water and polar contaminants in the fluid causes a convergence at the interface point leading to a reduction in interfacial tension.
ASTM D3487 is a standard test method used to determine the interfacial tension of oils against water through the Du Nouy ring method [2]. This test method provides a measure of the cleanliness of the oil, with lower interfacial tension values typically indicating higher levels of contamination or degradation.
How to interpret Interfacial Tension limits?
ASTM D3487 prescribes a minimum of 40 mN/m for new mineral oil for use in electrical apparatus [2]. The surface tension can also be indicated by dynes/cm where 1 mN/m = 1 dyne/cm.
Other authorities like SDMyers Inc. consider mineral oil with an IFT ≥ 32 mN/m as Acceptable (AC) with values between 32 mN/m and 28 mN/m considered Questionable (QU). Values of IFT < 28 mN/m are considered Unacceptable (UN) suggesting that the oil has formed significant sludge and is damaging the paper insulation if no external contamination is an issue. Oil reclamation is recommended [3].
The Interfacial Tension can be used in a health index as applied in the Transformer Age Index Model to assess the health of transformer insulating oil.
What can be done to Improve oil condition?
Once the oil reaches a value that is below the limits as prescribed in the standards there are a few ways in which the condition can be improved.
The reclamation of oil is the restoration of usefulness by the removal of contaminants and products of degradation such as polar, acidic, or colloidal materials from used electrical insulating liquids by chemical or adsorbent means such as Fuller’s Earth [4].
The reconditioning of oil is the removal of insoluble contaminants, moisture, and dissolved gases from used, electrical insulating liquids by mechanical means, such as vacuum processing or filtering [4].
Conclusion
Basically, Interfacial Tension is the measure of the presence and amount of soluble contaminants and oxidation products in the insulating oil where a decreasing value indicates an increase in contaminants and or oxidation products. This may also be an early alert to the formation of sludge in the oil. If the values go below limits it is an indication of contamination and may require oil reconditioning or reclamation.
References
- ASTM D971, ASTM Standard D971, “Standard Test Method for Interfacial Tension of Insulating Liquids Against Water by the Ring Method”, ASTM International, DOI: 10.1520/D0971-20
- ASTM D3487, “Standard Specification for Mineral Oil Used in Electrical Apparatus,” ASTM International, DOI: 10.1520/D3487-16E01
- SD Myers, Understanding Your Test Results | SDMyers
- IEEE Std C57.106, “IEEE Guide for Acceptance and Maintenance of Insulating Mineral Oil in Electrical Equipment,” DOI: 10.1109/IEEESTD.2016.7442048