How to deal with dirty transformer oil?

When the transformer oil is contaminated, it must be filtered and purified professionally to remove pollutants such as moisture, gases, and solid particles, thereby restoring its insulation and cooling properties. The following are detailed filtering steps and methods:

Preparation before filtering

1. Oil sample testing

- Take samples for testing, and measure the moisture content (ppm), gas content (%), particle contamination (ISO 4406/NAS 1638 level) and dielectric strength (kV).

- Key indicator reference:

- Moisture ≤10 ppm (new oil standard)

- Particle cleanliness ≤NAS 1638 Class 6

- Dielectric strength ≥50 kV (2.5mm gap)

2. ｓｅｌｅｃｔ oil filtering equipment

- Vacuum oil filter: efficient dehydration and degassing (recommended).

- Pressure oil filter: removes solid particles (must be matched with a precision filter element).

- Coalescing separation equipment: specializes in moisture removal (optional).

Transformer oil filtration steps

1. Coarse filtration (pretreatment)

- Purpose: Remove large particles of impurities (such as metal chips, fibers).

- Equipment: Plate and frame oil filter or centrifuge with 100~200 micron filter element.

- Operation:

 Release the dirty oil from the bottom of the transformer and circulate it through the coarse filtration equipment.

Observe the filter element pressure difference. ｒｅｐｌａｃｅ the filter element when the pressure difference is greater than 0.3 MPa.

2. Vacuum dehydration and degassing

- Equipment: Vacuum oil filter (vacuum degree ≥-0.095 MPa).

- Key parameters:

- Oil temperature: 50~60℃ (heating reduces viscosity and improves dehydration efficiency).

- Vacuum time: usually 4~8 hours (depending on water content).

- Operation:

The oil is heated by the heater and sprayed into the vacuum tank, and the water and gas evaporate.

The evaporated water vapor is liquefied by the condenser and discharged.

Real-time monitoring of water content in oil (target ≤10 ppm).

3. Fine filtration (removal of micron-sized particles)

- Equipment: precision oil filter (1~5 micron filter element).

- Operation:

The oil passes through a high-efficiency filter element with β₅≥200.

Circulate and filter until the particle size meets the standard (such as ISO 4406 15/13/10).

4. Decolorization (optional)

- Applicable scenarios: oil is severely oxidized (dark yellow or brown).

- Method:

- White clay adsorption: add activated white clay (3%~5% oil weight), stir and filter.

- Ion exchange resin: remove polar compounds and pigments.

Post-filtration inspection and oil injection

1. Oil quality re-inspection

- Detection of water content, dielectric strength, and acid value (≤0.01 mg KOH/g).

- Particle counter to verify cleanliness.

2. Vacuum oil filling

- Use a vacuum oil filter to inject purified oil into the transformer and vacuum it at the same time (to avoid residual bubbles).

- After oil filling, let it stand for 24 hours and test the oil quality again.

Precautions

1. Safety regulations

- The transformer needs to be powered off during filtering, and the oil temperature should be ≤70℃ to prevent aging.

- Ex d IIB T4 certified equipment should be used in explosion-proof environments.

2. Filter element maintenance

- Clean the coarse filter element every 8 hours, and ｒｅｐｌａｃｅ the fine filter element when the pressure difference is ＞0.15 MPa.

3. Environmental protection requirements

- Waste filter elements and adsorbents should be treated as hazardous waste (such as white clay containing oil, which needs professional recycling).

Common problem solutions

- Problem 1: Low dehydration efficiency

Cause: Insufficient vacuum or too low oil temperature.

Solution: Check the sealing of the vacuum pump and raise the temperature to 60℃.

- Problem 2: Particles repeatedly exceed the standard

Cause: The internal residue of the transformer is not cleaned up.

Solution: Flush the transformer oil tank before filtering.

Equipment recommendation

- High cost performance: Chongqing Tongrui ZYD series vacuum oil filter.

- High precision: Pall ZODIAC® transformer oil filter.

- Explosion-proof type: Eaton VR series (ATEX certified).

Through the above steps, the transformer oil can be restored to a standard close to new oil (IEC 60422). If the oil is severely aged (acid value > 0.1 mg KOH/g), it is recommended to ｒｅｐｌａｃｅ it with new oil directly.