Interfacial Tension

Interfacial Tension

Interfacial Tension Analysis of Transformer Oil

Interfacial tension is the force per unit length existing at the interface between two immiscible liquids, such as oil and water. It is expressed in dynes per centimeter (dyne/cm) or milli-Newtons per meter (mN/m). The IFT value indicates the molecular attraction between the two liquids and can reveal the presence of polar contaminants and degradation products in the oil.

Importance of Interfacial Tension in Transformer Oil

1.    Contaminant Detection: A decrease in IFT indicates the presence of contaminants such as acids, sludge, and other polar compounds resulting from oil degradation.

2.    Oil Quality Assessment: High IFT values suggest good oil quality, while low values indicate contamination or aging.

3.    Maintenance Planning: Regular IFT testing helps in planning maintenance activities by providing early warning signs of oil degradation. Early detection of oil degradation through IFT analysis allows for timely maintenance, preventing costly transformer failures

4.     Safety and Reliability: Maintaining high IFT values ensures the transformer operates safely and reliably, reducing the risk of electrical failures and fire hazards.

The most common method for measuring IFT is the Du Noüy Ring Method, as specified in the ASTM D971 standard1.

1. Equipment Needed:

• Tensiometer: An instrument used to measure the force required to detach a ring from the surface of the liquid.
• Platinum-Iridium Ring: A ring made of platinum-iridium alloy, used in the measurement process.
• Sample Container: A clean container to hold the oil sample and distilled water.

Procedure:

• Sample Preparation: Collect a representative oil sample from the transformer. Ensure the sample is free from air bubbles and contaminants.
• Temperature Control: Measure the temperature of the oil sample. The test is typically conducted at a standard temperature of 25°C.
• Measurement:
• Fill the sample container with distilled water and place the oil sample on top, forming a distinct interface.
• Immerse the platinum-iridium ring into the oil-water interface.
• Slowly lift the ring using the tensiometer until the maximum force required to detach the ring from the interface is recorded.
• Calculation: The tensiometer measures the force, which is then used to calculate the interfacial tension in dynes per centimeter or milli-Newtons per meter.

Interpreting Results

• High IFT Values: Indicate good oil quality with minimal contamination. New transformer oil typically has an IFT value above 40 dyne/cm.
• Low IFT Values: Suggest the presence of contaminants such as acids, sludge, or other polar compounds. Values below 25 dyne/cm indicate significant contamination and potential oil degradation.

Low interfacial tension (IFT) in transformer oil can be caused by several factors, often indicating contamination or degradation of the oil. Here are some common causes:

1. Oxidation: Over time, transformer oil can oxidize, especially when exposed to high temperatures and oxygen. This process produces acidic compounds and sludge, which lower the IFT1.
2. Water Contamination: Water is one of the most common contaminants in transformer oil. It can enter the oil through leaks, condensation, or during maintenance. Water contamination significantly reduces the IFT.
3. Acidic Compounds: The formation of acidic compounds due to oxidation or contamination can lower the IFT. These acids are polar in nature and disrupt the oil-water interface3.
4. Sludge Formation: Degradation products such as sludge, which result from the breakdown of oil and insulation materials, can lower the IFT. Sludge is often a sign of advanced oil degradation.
5. Polar Contaminants: Substances like dissolved varnishes, cleaning solvents, and other organic materials can lower the IFT. These contaminants are often introduced during maintenance or from external sources.
6. Thermal Degradation: High operating temperatures can cause thermal degradation of the oil, producing various degradation products that lower the IFT.
7. Particulate Contamination: Solid particles, such as dust, metal particles, and other debris, can also lower the IFT by disrupting the oil-water interface.

Improving the interfacial tension (IFT) of old transformer oil involves several steps aimed at removing contaminants and degradation products. Here are some effective methods:

1. Oil Purification

Purification processes such as vacuum dehydration and degassing can significantly improve the IFT of transformer oil. These methods remove water, gases, and particulate matter from the oil. For instance, using a double-stage vacuum transformer oil purifier can enhance the IFT to above 40 mN/m and reduce water content to below 5 ppm.

2. Oil Regeneration

Oil regeneration involves removing acidic compounds, sludge, and other degradation products. This can be achieved through:

• Adsorption: Using adsorbents like activated clay or Fuller’s earth to remove polar contaminants and acids.
• Chemical Treatment: Adding chemicals that neutralize acids and precipitate contaminants.

3. Filtration

High-efficiency filtration systems can remove solid particles and sludge from the oil. This helps in restoring the oil’s dielectric properties and improving its IFT.

4. Degassing

Degassing units remove dissolved gases from the oil. This process is crucial as gases can form bubbles under high voltage, leading to electrical discharges and further degradation of the oil.

5. Adding Inhibitors

Adding oxidation inhibitors can help in slowing down the degradation process of the oil. These inhibitors prevent the formation of acidic compounds and sludge, thereby maintaining a higher IFT.

6. Regular Monitoring and Maintenance

Regular testing and monitoring of the oil’s IFT can help in early detection of contamination and degradation. This allows for timely intervention and maintenance, preventing significant deterioration of the oil.

7. Replacing Oil

In cases where the oil is severely degraded and purification or regeneration is not effective, replacing the old oil with new, high-quality transformer oil may be necessary.

By implementing these methods, you can effectively improve the interfacial tension of old transformer oil, ensuring the reliable and efficient operation of your transformers.