Transformer oil filtering method

Transformer oil filtering methods mainly include physical filtration, vacuum dehydration and degassing, adsorption purification and other technologies, aiming to remove moisture, gas, solid particles and soluble impurities in the oil and restore the insulation performance and chemical stability of the oil. The following are common oil filtering methods and applicable scenarios:

Mechanical filtration method

(1) Pressure oil filter

- Principle:

The oil is pressurized by the oil pump to pass through the filter paper or filter element (such as cellulose, glass fiber, metal mesh, etc.) to intercept solid particles (such as metal chips, fibers, carbides).

- Filtration accuracy:

- Ordinary filter paper: 5~10μm

- Precision filter element: 1~3μm (up to NAS 1638 level 6)

- Applicable scenarios:

- Removal of free impurities (such as preliminary filtration before new oil injection or during maintenance).

- Disadvantages: Unable to remove moisture and dissolved gases.

(2) Centrifugal separation method

- Principle:

Use centrifugal force to separate water and particles with higher density (such as free water and metal particles) from the oil.

- Applicable scenarios:

- A large amount of free water or impurities is precipitated in the oil (such as initial treatment after a failure).

- Disadvantages:

- Unable to remove dissolved water and tiny particles.

Vacuum oil filtration method (core method)

(1) Vacuum dehydration and degassing

- Principle:

After the oil is heated to 50~65℃, it is atomized or forms a film in a vacuum tank (pressure <1kPa), and the water and gas (O₂, N₂, etc.) evaporate and are pumped out by the vacuum pump.

- Key equipment:

Vacuum oil filter (including heater, vacuum pump, condenser, precision filter element).

- Effect:

- Water content ≤10ppm, gas content ≤0.1%, breakdown voltage ≥50kV.

- Applicable scenarios:

- Deep purification (maintenance of operating oil, regeneration of damp oil).

(2) Thin film evaporation technology

- Principle:

The oil forms an extremely thin oil film in the vacuum tank, which prolongs the degassing and dehydration time and improves efficiency.

- Advantages:

More thorough than the spray type, suitable for ultra-high voltage transformer oil.

Adsorption purification method

(1) Molecular sieve/silica gel adsorption

- Principle:

The oil flows through a 3Å molecular sieve or an activated alumina filter bed to absorb water and acidic substances.

- Applicable scenarios:

- Mildly damp oil or online micro-water control (such as transformer respirator).

(2) Activated clay treatment

- Principle:

The clay adsorbs colloids and acidic compounds (such as sludge) in the oil.

- Note:

Subsequent fine filtration is required to remove the clay residue.

Electrostatic oil purification method

- Principle:

High-voltage electrostatic field charges impurities and adsorbs them on the electrode.

- Applicable scenarios:

- Remove submicron particles (0.1~1μm).

- Disadvantages:

Complex equipment and high maintenance costs.

Combined treatment process

(1) Vacuum + adsorption + filtration

- Process:

Centrifugal separation → Heating vacuum dehydration → Molecular sieve adsorption → Precision filtration.

- Applicable scenarios:

Severely contaminated oil (such as water + particulate matter + oxidation products).

(2) Hot oil circulation drying

- Method:

The oil is heated to 70℃ and circulated, and dehydrated, degassed and filtered simultaneously through the oil filter.

- Applicable scenarios:

Comprehensive treatment of the transformer body when it is damp.

Precautions

1. Temperature Control: Heating should not exceed 65℃ to prevent oil oxidation.

2. Vacuum Tightness: System Leakage will reduce dehydration efficiency.

3. Filter replacement: ｒｅｐｌａｃｅ immediately when the pressure difference alarm sounds to avoid secondary contamination.

4. Safe operation:

- Prevent oil spraying during vacuum oil filtration.

- Explosion-proof measures are required during electrostatic oil purification.

Transformer oil filtration needs to ｓｅｌｅｃｔ mechanical filtration, vacuum dehydration, adsorption, or combined processes according to the type of pollution. Vacuum oil filter is the core equipment for comprehensive treatment of moisture, gas and particles, whilethe  precision filtration + adsorption method is suitable for online maintenance. Regular oil filtration can extend the life of the transformer. It is recommended to test the oil quality (moisture, pressure resistance, gas content) at least once a year.