The purpose of transformer oil filtration

The core purpose of transformer oil filtration is to remove pollutants in the oil by physical or chemical means, restore or maintain its insulation, cooling and arc extinguishing performance, and ensure the safe and stable operation of the transformer. Specific goals can be divided into the following categories:

Core Function Maintenance

1. Restore insulation strength

- Problem: Water and particulate matter will reduce the breakdown voltage of the oil (for example, when the water content increases from 10 ppm to 50 ppm, the breakdown voltage may ｄｒｏｐ by 40%).

- Measures: After filtration, the breakdown voltage can be increased from ≤30kV to ≥60kV (standard oil cup test).

2. Ensure cooling efficiency

- Problem: Suspended particles will block the oil channel and reduce thermal conductivity (the thermal conductivity of contaminated oil can ｄｒｏｐ by 15~20%).

- Measures: After filtration, the kinematic viscosity of the oil is restored to ≤9.5mm²/s (40℃).

Targeted treatment of pollutants

| Pollutant type | Hazard | Filtration target | Typical method |

|----------------|--------------------------|---------------------------------|---------------------------|

| Free/dissolved water | Induces aging of oil-paper insulation | Moisture ≤15ppm (new oil standard ≤10ppm) | Vacuum dehydration (-95kPa, 60℃) |

| Solid particles | Cause partial discharge | Particle size ≤NAS 1638 level 6 | Centrifugal separation + β≥200 precision filtration |

| Dissolved gas | Generates bubbles to reduce dielectric strength | H₂≤50ppm, C₂H₂≤1ppm | Vacuum degassing (thin film evaporation technology) |

| Oxidation products | Increase acid value (＞0.1mg KOH/g) | Acid value ≤0.01mg KOH/g | White clay adsorption/hydrogenation treatment |

Special scenario requirements

1. Anti-aging and prolong life

- Add antioxidants (such as T501, concentration 0.3~0.5%) during filtration to make the oxidation induction period ≥300h (ASTM D2112).

2. Environmental compliance

- Remove PCB (polychlorinated biphenyls) to ≤2ppm (EU Directive 2011/65/EU requirements).

3. UHV equipment

- Ultra-clean treatment: particle size control ≤0.5μm (DL/T 432 standard).

Economic benefits

- Direct savings: The cost of 1 ton of recycled oil is about 30~50% of new oil (taking 25 oil as an example, new oil is about ￥12,000/ton, and the cost of recycling is ￥4,000~6,000/ton).

- Indirect benefits: Avoid transformer failures caused by oil deterioration (the maintenance cost of a medium-sized transformer is about ￥500,000).

Comparison with international standards

- IEC 60422 specifies the treatment cycle and index limit of transformer oil in operation.

- ASTM D3487: specifies the performance requirements of mineral insulating oil.

Through precise filtration, the service life of transformer oil can be extended from the conventional 5 to 8 years to more than 15 years (such as the practical case of Japan's J-POWER). For harsh environments such as wind farms, it is recommended to conduct comprehensive oil treatment every 2 years, while indoor substations can be extended to 5 years.