Transformer Oil Filtration Requirements

Transformer oil filtration is an important measure to maintain transformer insulation performance and extend oil life. Its core requirements include equipment selection, temperature control, moisture and impurity treatment, vacuum management, etc. The following are specific technical specifications and operating requirements:

1. Filter equipment selection

- Vacuum oil filter: suitable for deep dehydration and degassing, and can efficiently remove trace moisture and dissolved gas.

- Pressure oil filter: mainly used for filtering mechanical impurities (such as particles and fibers).

- Precision filter element: The filtration accuracy is usually required to be ≤5μm (even 1μm) to ensure that the particulate matter meets the standard (such as the IEC 60726 standard).

2. Temperature control

- Optimal range: 50°C~65°C (maximum not exceeding 70°C).

- Reason: When the temperature is too low, the viscosity is high and the filtration efficiency is low; when the temperature is too high, the oil oxidation will be accelerated and the additives will be destroyed.

- Special working conditions: When vacuum filtering oil, 70°C~80°C is allowed for a short time (the time needs to be strictly monitored).

3. Key indicator requirements

| Parameters | Standard requirements | Test methods |

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

| Water content | ≤15 ppm (UHV transformer) or ≤30 ppm (conventional) | ASTM D1533 (Karl Fischer method) |

| Gas content | ≤1% (volume ratio) | IEC 60567 |

| Breakdown voltage | ≥60 kV (new oil) or ≥50 kV (running oil) | IEC 60156 |

| Dielectric loss factor | ≤0.005 (at 90°C) | IEC 60247 |

| Particle contamination | NAS 1638 Class 6 or below or ISO 4406 15/12/10 | Laser particle counting method |

4. Vacuum dehydration requirements

- Vacuum degree: ≤0.5 kPa (absolute pressure), with spray or film technology to enhance the degassing effect.

- Time: Cycle filtration until the index is qualified (usually multiple cycles are required, 4~8 hours per cycle).

5. Operation process

1. Pretreatment: Take samples to test the initial oil quality (water, gas, acid value, etc.).

2. Heating: Slowly increase the temperature to 50°C~65°C to avoid local overheating.

3. Filtration:

- First, remove large particles through a pressure oil filter, and then use a vacuum oil filter to dehydrate and remove micro particles.

- Regularly ｒｅｐｌａｃｅ the filter element (immediately ｒｅｐｌａｃｅ when the pressure difference alarm is triggered).

4. Testing: Take samples and retest every 2 hours until the index is stable and qualified.

5. Oil filling: Let it stand for more than 12 hours after filtration, and inject it into the transformer after confirming that there are no bubbles.

6. Safety and environmental protection

- Fire prevention: Keep away from open flames, and equip fire extinguishing equipment (such as CO₂ fire extinguishers) on site.

- Explosion-proof: The vacuum system needs to be grounded to avoid static sparks.

- Waste oil treatment: It cannot be discharged directly and needs to be recycled or professionally treated (refer to GB 14542).

7. Standard basis

- International standards: IEC 60422, ASTM D3487 (new oil specifications).

- Domestic standards: GB/T 14542 (maintenance guidelines for operating transformer oil), DL/T 1096 (oil filter acceptance specifications).

Precautions

- Avoid repeated heating: Too many filtration times of the same batch of oil may cause performance degradation.

- Ambient humidity: When the relative humidity is greater than 80%, filtration is suspended to prevent secondary intrusion of moisture.

- Online filtration: For transformers in operation, live oil filtration can be used, but the flow rate must be controlled (≤5% tank volume/hour).

By strictly following the above requirements, the insulation strength, chemical stability and cooling performance of the transformer oil can be restored, ensuring the safe operation of the transformer.