How to filter transformer insulating oil?

Purifying transformer insulating oil is crucial for maintaining the performance and longevity of electrical transformers. Over time, transformer oil can become contaminated with water, particulate matter, gases, and sludge, all of which can affect the oil’s insulating properties and, in turn, the transformer’s reliability. Regular oil purification helps remove contaminants, restores the oil's insulating properties, and prevents transformer failure.

Here are the steps and methods involved in purifying transformer-insulating oil:

 1. Preliminary Filtration (Coarse Filtration)

   - Purpose: Remove large debris and particles from the oil, such as dirt, dust, and metal shavings.

   - Method:

     - Mechanical Filtration: Use mesh or paper filters to trap large particles. These are typically coarse filters used as the first step before more advanced treatments.

     - Centrifugal Separation: Some systems use centrifugal force to separate heavier particles from the oil.

   - Tools: Coarse mesh filters, centrifugal oil cleaners.

 2. Water Removal

   - Purpose: Remove dissolved or free water from the transformer oil. Water reduces the oil’s insulating properties and can cause rust and corrosion in transformer components.

   - Methods:

     - Vacuum Dehydration: The oil is heated under a vacuum to lower the boiling point of water, causing the water to evaporate and then be removed through condensation. This is a common method to remove dissolved water from oil.

     - Centrifugal Dehydration: The oil is spun in a centrifuge to separate water droplets from the oil based on their density.

     - Heated Filtration: The oil is passed through a heater, and water is removed via evaporation. This method is especially effective for emulsified water.

   - Tools: Vacuum dehydrators, centrifugal dehydration units, oil heaters.

 3. Removal of Gases (De-gassing)

   - Purpose: Transformer oil can absorb gases like oxygen, nitrogen, and hydrogen, which can compromise the oil's insulating properties. De-gassing removes these dissolved gases.

   - Method:

     - Vacuum Degassing: The oil is placed under a vacuum that lowers the pressure, causing the dissolved gases to escape. This is effective for removing dissolved gases and ensuring the oil maintains its insulating properties.

     - Heat & Vacuum: A combination of heating and applying a vacuum can help release gases from the oil more efficiently.

   - Tools: Vacuum degassing units.

 4. Fine Filtration (Polishing)

   - Purpose: Remove smaller particles, sludge, and contaminants that may not have been removed during the coarse filtration process.

   - Method:

     - Activated Carbon Filtration: Activated carbon is highly effective at removing fine particles, sludge, and some dissolved contaminants.

     - Fine Mesh Filters: Filters with a finer mesh (in the micron range) help to capture smaller particles and impurities.

   - Tools: Activated carbon filters, fine mesh filters.

 5. Acid Neutralization (If Needed)

   - Purpose: Transformer oil can become acidic due to oxidation, and acidic oil can damage the transformer’s insulation and components. Neutralizing acids can restore the oil’s stability.

   - Method:

     - Clay Treatment: The oil is treated with absorbent clay, which removes acidic compounds, oxidation by-products, and other chemical impurities.

     - Alkaline Neutralization: In some cases, mild alkaline solutions may be used to neutralize acids in the oil, though clay treatment is more common.

   - Tools: Clay filtration units, alkaline neutralization systems.

 6. Regeneration / Reconditioning of Transformer Oil

   - Purpose: To restore the oil’s insulating properties by removing oxidation products, dissolved gases, and other impurities that could affect performance.

   - Method:

     - Regenerative Filtration: Some systems use specialized filters (often clay or activated carbon) to regenerate the oil by removing oxidation products, sulfur compounds, and other contaminants.

     - Vacuum Distillation: For high-contamination levels, distillation under vacuum can help separate impurities by boiling point. Lighter oils and contaminants evaporate, leaving behind cleaner oil.

   - Tools: Regenerative filtration units, vacuum distillation systems.

 7. Polishing & Final Purification

   - Purpose: Further clean and remove any residual small particles or contaminants that may still be in the oil.

   - Method:

     - Final Polishing Filters: After the main purification process, oil can pass through a polishing filter to ensure any remaining impurities are removed. These filters are typically fine mesh or use activated carbon.

   - Tools: Polishing filters, fine mesh filters.

 8. Vacuum Drying (If Necessary)

   - Purpose: Some systems use a vacuum drying process to remove any residual moisture from the oil after the purification process. This is particularly important if the oil was heavily contaminated with water.

Method: The oil is subjected to a vacuum environment to evaporate any remaining water and moisture content from the oil. This process is commonly used in vacuum distillation systems.

Tools: Vacuum drying systems.

9. Testing and Quality Control

Purpose: To ensure that the purified transformer oil meets the required specifications for reuse.

Method:

After purification, the oil should be tested for various parameters, such as viscosity, dielectric strength, acid number, water content, and particle count.

A dielectric strength test measures the oil’s ability to insulate and withstand electrical stress.

Water content should be low (typically less than 50 ppm for high-performance transformers).

Particle contamination should be minimal (often measured in microns).

Tools: Dielectric strength testers, viscosity testers, water content testers, particle counters.

10. Reinsertion into the Transformer

Once the oil has been purified and passes the required tests, it can be returned to the transformer. Some systems allow for online oil purification, where oil is purified while the transformer is still in operation, ensuring continuous operation without the need to drain the oil.

Key Methods and Technologies for Transformer Oil Purification:

Vacuum Filtration & Dehydration: One of the most common and effective methods, which involves heating the oil and using vacuum pressure to remove water and gases.

Centrifugal Separation: Effective for removing heavier impurities like sludge and metals.

Activated Carbon Filtration: Used for removing finer contaminants, particularly from used or degraded oils.

Clay Treatment: Removes acidic by-products and regenerates the oil, ensuring its continued performance.

Vacuum Distillation: Applied for advanced oil purification, especially when oil has been heavily contaminated or oxidized.

Benefits of Transformer Oil Purification:

Improved Insulating Properties: Restores the oil’s ability to insulate, preventing electrical faults and transformer failures.

Extended Transformer Life: Regular purification reduces the buildup of harmful contaminants, extending the lifespan of the transformer.

Reduced Maintenance Costs: By reusing the purified oil, you reduce the need for frequent oil changes, lowering overall maintenance costs.

Environmental Benefits: Recycling transformer oil helps minimize the environmental impact of oil disposal and reduces the need for new oil production.

Common Equipment Used for Transformer Oil Purification:

Offline Purifiers: Used when oil is drained from the transformer. The oil is purified externally before being returned.

Online Purifiers: Continuously purify the oil while the transformer is in operation, offering the convenience of constant maintenance without downtime.

Mobile Purifiers: Portable units that can be used in the field for emergency oil purification or on-site servicing.

In summary, purifying transformer insulating oil is a multi-step process that involves filtration, dehydration, degassing, regeneration, and polishing. Regular maintenance of transformer oil ensures optimal performance, prevents transformer failure, and helps reduce operational costs by prolonging the life of the transformer and the insulating oil.