Oil filtration procedure of power transformer

The oil filtration procedure for power transformers is crucial for maintaining the efficiency, reliability, and longevity of the transformer. Transformer oil acts as both an insulating material and a coolant. Over time, the oil can become contaminated with moisture, dirt, gases, and oxidation products, which can degrade the performance of the transformer and even lead to failure if not addressed.

Here is an overview of the oil filtration procedure for power transformers:

 1. Preparation for Filtration

Before starting the oil filtration process, it is essential to ensure that all equipment and systems are in place and operational.

 Key Steps:

- Check the Oil Level: Ensure that the oil level in the transformer is within acceptable limits. Low oil levels may indicate leaks or insufficient oil.

- Check for Oil Contamination: Perform an initial inspection to determine the type of contaminants (e.g., moisture, particulate matter, gas, acids) present in the oil.

- Prepare Equipment:

   - Oil filtration unit: The filtration unit could be a vacuum oil purifier, filtration rig, or high-efficiency oil filtration system.

   - Filter elements: Depending on the contaminant type, appropriate filter elements (e.g., fine mesh filters, coalescing filters, activated carbon filters) should be prepared.

 2. Initial Inspection and Sampling

Before starting the filtration process, it is important to conduct an initial analysis of the oil to determine the extent of contamination.

 Key Steps:

- Oil Sampling: Collect samples of the transformer oil to measure key parameters such as:

   - Moisture content (e.g., ppm - parts per million)

   - Acidity (often expressed as the Total Acid Number, TAN)

   - Dielectric strength (insulation capability)

   - Particulate contamination (e.g., metal or dirt particles)

   - Gas content (for monitoring the presence of gases such as hydrogen, methane, or carbon monoxide, which could indicate problems like overheating or electrical arcing).

- Visual Inspection: Check the oil’s color, clarity, and presence of visible particles. Dark and opaque oil often indicates the presence of contaminants, such as oxidation or moisture.

 3. Isolation and Shutdown (if needed)

In some cases, the transformer may need to be taken offline to perform filtration, depending on the type of filtration system used and the operational requirements.

 Key Steps:

- Power Shutdown: Ensure that the transformer is de-energized and isolated from the electrical grid.

- Cooling: If the transformer has been operating, let it cool down to avoid handling hot oil, which can be hazardous.

- Safety Measures: Ensure all safety protocols are followed, such as ensuring there are no electrical hazards and using appropriate personal protective equipment (PPE).

 4. Filtration Process

The actual oil filtration process can vary depending on the method being used. Below are the main methods of oil filtration:

 a. Vacuum Oil Filtration (Most Common)

   - Purpose: To remove moisture, gases, and particles from the transformer oil.

   - Working Principle:

     1. Vacuum Dehydration: The transformer oil is heated slightly (usually to around 40°C to 60°C) and passed through a vacuum chamber. The vacuum lowers the pressure, causing moisture in the oil to evaporate at a lower temperature.

     2. Moisture Removal: The moisture in the oil evaporates and is removed through condensation in the vacuum unit.

     3. Filtration: The oil passes through a filter (typically a fine mesh or coalescing filter) to remove particulate matter, sludge, and other solid contaminants.

     4. Gas Removal: Some systems also feature degasification units that remove dissolved gases from the oil, which can improve its insulating properties.

     5. Oil Reconditioning: The filtered and moisture-free oil is then returned to the transformer.

   - Equipment: A typical vacuum oil purifier will include:

     - Vacuum pump to create the vacuum.

     - Heaters to slightly warm the oil.

     - Filtration units to trap solid contaminants.

     - Degassing and moisture separation sections to remove gases and water.

 b. High-Efficiency Oil Filtration (For Solid Contaminants)

   - Purpose: To remove fine particles, sludge, and solid contaminants from the oil.

   - Working Principle: The oil passes through high-efficiency filters (e.g., depth filters, paper filters, or activated carbon filters) that capture even fine particulate matter and oxidation by-products.

   - Equipment: The oil is typically pumped through multi-stage filtration units to progressively remove finer and finer particles.

 c. Coalescing Filters (For Water Removal)

   - Purpose: To remove water from the oil.

• Working Principle: Coalescing filters work by combining small water droplets into larger droplets that can then be easily separated from the oil. This process helps remove emulsified water.
• Equipment: Coalescing filters are often used in conjunction with other filtration methods like vacuum systems or pressure filters.

d. Activated Carbon Filtration (For Acidity and Gas Removal)

• Purpose: To remove acidity and gases dissolved in the transformer oil.
• Working Principle: Activated carbon filters adsorb gases (such as hydrogen, methane, or carbon dioxide) and acid compounds from the oil.
• Equipment: Some vacuum oil filtration units include activated carbon filters to reduce the acidity and remove dissolved gases.

5. Post-Filtration Analysis

Once the oil has been filtered, it is important to perform a post-filtration analysis to ensure that the oil is now clean and free from contaminants.

Key Steps:

• Check Moisture Content: Ensure that the moisture level in the oil has been reduced to acceptable levels (usually less than 20 ppm for transformer oil).
• Check Acidity: Verify that the Total Acid Number (TAN) is within acceptable limits.
• Check Dielectric Strength: The dielectric strength of the oil should be tested to ensure that the oil still provides adequate insulation for the transformer.
• Check for Particulates: Perform a particle count to verify that the oil is free from harmful particulate contamination.
• Gas Content: Analyze the gases present in the oil using gas chromatography to check for any abnormal gases that may indicate internal issues within the transformer.

6. Refill and Final Inspection

After filtration, the clean oil is refilled into the transformer, and the unit is monitored to ensure proper functioning.

Key Steps:

• Refill Oil: Add the cleaned oil back into the transformer to bring the oil level up to the recommended level.
• Seal and Inspect: Ensure that the oil filling process is sealed properly, and check for any leaks or issues before energizing the transformer again.
• Monitor the Transformer: After re-energizing the transformer, monitor its performance, including oil temperature, pressure, and voltage.

7. Disposal of Contaminated Oil

• Waste Oil Disposal: The used, contaminated oil should be disposed of properly according to local environmental regulations. In some cases, the oil can be re-refined, or it may need to be safely disposed of as hazardous waste.

Frequency of Oil Filtration

• Routine Oil Maintenance: Transformer oil filtration is typically performed on a periodic basis (e.g., every 2-5 years), but it may be done more frequently if there are signs of contamination or degradation.
• Triggered Filtration: If the transformer shows signs of overheating, excessive moisture, or a ｄｒｏｐ in dielectric strength, oil filtration should be performed immediately.

The oil filtration procedure for power transformers is vital for maintaining the health of the transformer and ensuring its longevity. By removing contaminants like moisture, particulate matter, and dissolved gases, oil filtration improves the insulating properties and cooling efficiency of the oil. The filtration process may include methods such as vacuum dehydration, coalescing filtration, and activated carbon filtration, depending on the contaminants present. Regular monitoring and post-filtration analysis help ensure that the oil meets the required standards for safe and efficient operation of the transformer.