What are the methods for insulating oil filtration?

Filtration and purification of insulating oil (such as transformer oil, switch oil, etc.) is a key link in the maintenance of power equipment. It is mainly used to remove moisture, gas, particulate matter and oxidation products to restore its insulation strength, cooling performance and chemical stability. The following are common insulating oil filtration methods and their technical details:

Vacuum oil filtration method

Principle:

- Reduce the oil viscosity by heating (60~70℃), evaporate the moisture and gas in a vacuum environment (-0.08~-0.095 MPa), and then discharge it by condensation.

Features:

- Deep dehydration: The water content can be reduced to ≤10 ppm (IEC 60422 standard).

- Degassing: Remove dissolved gases (such as O₂, N₂) and improve insulation strength (breakdown voltage ≥60 kV).

- Applicable scenarios: large-capacity processing of transformer oil and high-voltage switch oil.

Key equipment:

- Vacuum separation tank, condenser, Roots vacuum pump, precision filter (1~5μm).

Pressure mechanical filtration

Principle:

- Oil passes through filter paper, filter plate or filter element (such as glass fiber, cellulose) under pressure to intercept solid particles and some free water.

Features:

- High efficiency impurity removal: cleanliness can reach NAS level 6 (ISO 4406 15/13/10).

- Limitations: Unable to remove dissolved water and gas.

- Applicable scenarios: pretreatment or use with a vacuum system.

Filter material type:

- Filter paper (3~10μm), sintered metal filter element (1~5μm), electrostatic filter material (adsorbs colloidal particles).

Centrifugal separation method

Principle:

- Use centrifugal force (speed ≥5000 rpm) to separate pollutants with large density differences (such as water, metal particles).

Features:

- Rapid dehydration: suitable for high water content oil (free water removal rate ＞95%).

- No filter material consumption, but cannot handle dissolved water and tiny particles (<5μm).

- Applicable scenarios: preliminary dehydration of waste insulating oil or separation of metal chips.

Adsorption purification method

Principle:

- Use adsorbents (silica gel, activated alumina, molecular sieves) to remove acidic substances, colloids and pigments in the oil.

Features:

- Reduce acid value: TAN can be reduced to ≤0.03 mg KOH/g (GB/T 7595).

- Regenerate oxidized oil: restore the color and chemical stability of the oil.

- Applicable scenarios: Regeneration of severely aged insulating oil.

Adsorbent selection:

- Activated white clay: highly efficient adsorption of colloids, but requires subsequent filtration.

- Molecular sieves: selective adsorption of water and acidic components.

Coagulation separation method

Principle:

- The oil passes through the hydrophilic coalescence filter element, and the tiny water droplets merge into large water droplets and then settle and separate.

Features:

- Specializes in emulsified water: can be used in conjunction with the vacuum method to improve efficiency.

- Limitations: ineffective for dissolved water.

- Applicable scenarios: switch oil or transformer oil containing emulsified water.

Electrostatic oil purification method

Principle:

- High-voltage electrostatic field charges colloidal particles and oxidation products and adsorbs them by electrodes.

Features:

- Submicron purification: can remove 0.1~1μm particles.

- No filter material, but needs to be combined with other dehydration methods.

- Applicable scenarios: Deep purification of oil used in ultra-high voltage equipment.

Combined process (recommended)

Typical process:

1. Pretreatment: centrifugation or coarse filtration to remove large particles and free water.

2. Vacuum dehydration: deep removal of moisture and gas.

3. Precision filtration: interception of remaining particles (1~3μm).

4. Adsorption treatment (optional): silica gel tower to reduce acid value.

Case:

- Transformer oil regeneration:

- Initial state: water content 50 ppm, particles NAS 10 level, acid value 0.1 mg KOH/g.

- After treatment: water content ≤5 ppm, NAS 5 level, acid value ≤0.03 mg KOH/g.

Selection recommendations

- Small substation: pressure oil filter + adsorption tank (low cost).

- Large transformer: vacuum oil filter + precision filtration + online monitoring (fully automated).

- Severely aged oil: combined process (centrifugal + vacuum + adsorption).

Precautions

- Temperature control: heating does not exceed 80°C to avoid oil oxidation.

- Explosion-proof requirements: An explosion-proof design is required when handling flammable oil.

- Waste residue treatment: filter materials and adsorbents are disposed of by environmental regulations.

The filtration method of insulating oil needs to be selected based on the type of pollution, equipment requirements and economy. Regular oil inspection (such as DGA dissolved gas analysis) can optimize the maintenance strategy.