What are the requirements for transformer oil filtration?

The filtration requirements for transformer oil mainly include key indicators such as oil cleanliness, moisture content, gas content, dielectric strength, etc., and must be combined with equipment type and operating standards. The following are specific requirements and common specifications:

1. Key indicators of filtered oil

(1) Cleanliness (particle contamination)

- International standards (ISO 4406 or NAS 1638):

- New oil or oil after overhaul: usually requires ≤ NAS 1638 Class 6 (or ISO 4406 15/12/9).

- UHV (above 500kV) equipment: may require ≤ NAS Class 5.

- Particle size control: The filtration accuracy generally needs to reach 1~5 μm (a high-precision filter element can reach 0.5 μm).

(2) Moisture content

- Allowable limit:

- New oil: ≤ 10 ppm (mg/kg).

- Operating oil: ≤ 15~20 ppm (the higher the voltage level, the stricter the requirements, such as 500kV requires ≤15 ppm).

- Treatment method: The vacuum oil filter can remove moisture to less than 5 ppm.

(3) Dielectric strength (breakdown voltage)

- Minimum requirements (IEC 60296 standard):

- New oil: ≥ 30 kV/2.5mm (untreated oil may only be 15~20 kV).

- Filtered oil: ≥ 50~60 kV/2.5mm (after high vacuum oil filtration, it can reach more than 70 kV).

(4) Gas content (dissolved gas)

- Total gas content: ≤ 1% (volume ratio), UHV equipment requires ≤ 0.5%.

- Acetylene (C₂H₂): ≤ 1 ppm (fault gas needs to be analyzed separately).

2. Filtration equipment and process requirements

(1) Equipment selection

- Vacuum oil filter: suitable for deep dehydration and degassing (water content can be reduced to below 5 ppm, gas content ≤0.1%).

- Pressure oil filter: used for particle filtration (precision 1~5 μm), but the dehydration capacity is weak.

- Centrifugal oil filter: suitable for large flow processing, but has limited effect on tiny particles and water.

(2) Filtration process

- Pretreatment: coarse filtration (10~20 μm) to remove large particles and protect precision filter elements.

- Multi-stage filtration:

- Primary filtration: 5 μm (remove suspended matter).

- Secondary filtration: 1~3 μm (improve cleanliness).

- Vacuum dehydration: temperature control at 50~60℃ (to avoid oil aging).

- Online monitoring: real-time detection of moisture, particle size, and dielectric strength.

3. Industry norms reference

- National standards:

- GB/T 7595-2017 (quality standard for transformer oil in operation).

- DL/T 429.9-2014 (method for determination of particle size of oil for power use).

- International standards:

- IEC 60422 (Guidelines for maintenance of mineral insulating oil).

- ASTM D3487 (performance standard for transformer oil).

4. Precautions

- Filter element replacement: The filter element needs to be replaced when the pressure difference exceeds 0.3 MPa or the pollution alarm is triggered.

- Oil temperature control: The oil temperature should be kept at 40~60℃ during filtration (too high will accelerate oxidation).

- Prevention of secondary pollution: Pipelines and oil storage tanks need to be clean and dry (below NAS Class 6).

The core of transformer oil filtration is "cleanliness + dryness", and the appropriate process needs to be selected according to the voltage level and equipment type. For example:

- 500kV transformer: high vacuum oil filtration is required (water content ≤10 ppm, NAS Class 5).

- 10kV distribution transformer: pressure filtration (water content ≤20 ppm, NAS Class 7) is sufficient.

It is recommended to test the oil sample before and after filtration to ensure that it meets the standards before injecting it into the equipment.