What to do if there are impurities in the transformer oil?

Removing impurities from transformer oil is the key to ensuring the transformer's insulation performance, cooling efficiency and long-term stable operation. Impurities mainly include moisture, solid particles, gas (such as air) and degradation products (such as acid, colloid). The following are the purification methods of the system:

1、Physical filtration (removal of solid particles)

- Equipment:

- Plate filter: Use filter paper or filter cloth to intercept particles (the accuracy is usually 1-10 microns).

- Vacuum filter: Combined with suction filtration to improve efficiency, suitable for highly polluted oil.

- Steps:

- Pass the oil through a multi-stage filter (coarse filtration → fine filtration) and ｒｅｐｌａｃｅ the filter material regularly.

- In case of severe pollution, it can precipitate for 24 hours before filtering.

2. Vacuum dehydration and degassing (removal of moisture and gas)

- Principle: Heat the oil in a vacuum environment to reduce the boiling point of water to evaporate it, and extract dissolved gas at the same time.

- Equipment: Vacuum oil filter (such as a two-stage vacuum oil filter).

- Parameters:

- Temperature: 50-65℃ (avoid overheating and oxidation).

- Vacuum: ≤0.1 kPa (absolute pressure).

- Treatment time: Circulate until the moisture is ≤10 ppm (IEC 60422 standard).

- Effect: Free water, dissolved water and air can be removed simultaneously.

3. Adsorption treatment (removal of colloid, acid and trace moisture)

- Adsorbent:

- Activated alumina: removes acidic substances.

- Silica gel or molecular sieve: deep dehydration (moisture can be reduced to less than 5 ppm).

- Activated clay: adsorbs colloid and pigment (subsequent filtration is required).

- Method:

- Percolation method: let the oil flow slowly through the adsorbent-filled column.

- Stirring method: add the adsorbent to the oil, stir and filter (need to control the time to prevent secondary pollution).

4. Centrifugal separation (rapid treatment of large amounts of contaminated oil)

- Applicable scenarios: oil containing a large amount of free water or large particles (such as sudden water inflow into the transformer).

- Equipment: high-speed centrifuge (separation of impurities with a density difference of ≥ 0.1 g/cm³).

- Limitations: unable to remove dissolved water and fine particles.

5. Electrostatic oil purification (high-precision purification)

- Principle: using an electrostatic field to adsorb charged impurities on electrodes.

- Advantages: can handle submicron particles (<0.1 microns) and colloids.

- Applicable: deep purification of ultra-high voltage transformer oil.

6. Chemical regeneration (treatment of aged oil)

- Steps:

1. Acid washing: Use sulfuric acid or phosphoric acid to remove oxidation products (concentration must be strictly controlled).

2. Alkali neutralization: Add sodium hydroxide solution to neutralize residual acid.

3. Water washing: remove saponification, and then vacuum dehydration.

- Note: Chemical regeneration requires professional operation and may change the performance of oil products.

Standard purification process (taking a vacuum oil filter as an example)

1. Pretreatment: coarse filtration to remove visible impurities.

2. Heating: oil temperature rises to 50-60℃ (reduces viscosity).

3. Vacuum dehydration: spray degassing and evaporating water in a vacuum tank.

4. Fine filtration: through 1-5 micron precision filter.

5. Testing: measure breakdown voltage (≥50 kV/2.5mm), moisture (≤15 ppm), acid value (≤0.1 mg KOH/g).

Key points

- Safety: Oil heating needs to prevent an explosion, and the vacuum system must be well sealed.

- Compatibility: Avoid using filter materials that react with oil products (such as certain synthetic fibers).

- Environmental protection: waste adsorbents and filter residues are treated as hazardous waste.

- Online monitoring: install a moisture sensor or particle counter in oil for real-time monitoring.

When is an oil change necessary instead of oil purification?

- Excessive sludge: colloid content > 0.1%.

- Severe oxidation: acid value > 0.3 mg KOH/g.

- Electrical performance failure: breakdown voltage still does not meet the standard after repeated treatment.

For large power transformers, it is recommended to use a vacuum oil filter + adsorbent combined treatment regularly and follow IEEE Std 637 or GB/T 14542 standards.