When is it necessary to filter transformer oil?

Transformer oil filtration (or purification) is a key measure to maintain the insulation performance and heat dissipation capacity of the transformer. Filtration is usually required in the following situations:

Common scenarios where transformer oil filtration is required

1. Pretreatment before new oil injection

- Newly purchased transformer oil may contain trace amounts of moisture, particles or gas. It must be filtered to qualified standards (such as moisture ≤10ppm, particle size ≤NAS 8 level) before injection into the equipment.

2. Regular preventive maintenance

- According to the operating time (such as every 3 to 5 years) or oil quality test results, filter the oil regularly to delay aging, by the requirements of "DL/T 596 Preventive Test Procedure for Power Equipment".

3. Oil deterioration index exceeds the standard

- Exceeding the standard of moisture: When the moisture content in the oil is >15ppm (for transformers above 330kV) or >30ppm (below 110kV), it is necessary to filter and dehydrate (such as using a vacuum oil filter).

- Increased dielectric loss value (tanδ): tanδ＞0.5% (at 90℃) indicates that the insulation performance of the oil has decreased and needs to be filtered or regenerated.

- Reduced breakdown voltage: Treatment is required when the breakdown voltage is <30kV (the standard for new oil is ≥40kV).

- Particle contamination: Excessive particle size (such as NAS level 10 or above) may block the oil channel or cause partial discharge.

4. After transformer maintenance or failure

- After internal operations such as core inspection and winding replacement, the oil may be mixed with impurities or moisture and needs to be fully filtered.

- After a failure (such as arc discharge, overheating), carbon particles or acidic substances are generated in the oil, which need to be filtered and adsorbed.

5. Abnormal dissolved gas in oil (DGA)

- If the total hydrocarbon (CH₄, C₂H₄, etc.) or hydrogen (H₂) content exceeds the standard, the oil may need to be filtered and degassing treatment is required.

Core goals of filtering transformer oil

1. Remove pollutants

- Water: evaporate and separate by vacuum heating (60~70℃).

- Particles: intercepted by 1~5μm precision filter element.

- Gas: vacuum degassing reduces oxygen and nitrogen content.

2. Restore the basic properties of oil

- Make indicators such as breakdown voltage, dielectric loss, acid value (≤0.1mg KOH/g) return to the qualified range.

3. Extend the service life of oil

- Reduce the accumulation of oxidation products and avoid sludge formation.

Common filtering methods

1. Vacuum oil filter

- Combined with heating, vacuum dehydration and precision filtration, it is suitable for deep dehydration and degassing (water content can be reduced to below 5ppm).

2. Pressure oil filter

- Mechanically filter particles through filter paper or filter element, suitable for rapid removal of solid impurities.

3. Adsorption treatment

- Add adsorbents such as silica gel and activated alumina to remove acidic substances (used when the acid value is high).

4. Centrifugal separation

- For oils with high water content and large differences in impurity density (less used for transformer oil).

Operation precautions

1. Pre-filtration detection

- First take samples for testing (water content, dielectric loss, breakdown voltage, etc.), determine the type of pollution and then choose the filtration scheme.

2. Control process parameters

- The temperature does not exceed 70℃ during vacuum oil filtration to avoid oil oxidation; the vacuum degree is maintained above -0.095MPa.

3. Prevent secondary pollution

- The oil filter pipeline and oil storage tank must be clean and dry, and the oil must be left to stand for 24 hours after filtration before sampling and re-inspection.

4. Environmental protection requirements

- Waste oil and filter elements must be treated as hazardous waste to avoid direct discharge.

Special cases where filtration is not required

- If all indicators of oil quality testing are qualified (such as moisture <10ppm, dielectric loss <0.2%), and there are no other abnormalities, no additional filtration is required.

The timing of transformer oil filtration should be based on oil test data as the core basis, combined with the equipment operation status for comprehensive judgment. Regular maintenance can significantly reduce the risk of transformer failure, and thorough filtration after failure is the key step to restoring equipment health. For ultra-high voltage transformers (such as above 500kV), oil quality requirements are more stringent, and a higher precision processing process is required.