What should we pay attention to when filtering transformer oil?

Precautions for transformer oil filtration

Transformer oil filtration is a key maintenance measure to ensure the insulation performance and heat dissipation capacity of the transformer. The following are important precautions during the filtration process:

1. Preparation before filtration

- Oil sample testing:

- Test the moisture content (GB/T 7600), dielectric loss (GB/T 5654), and breakdown voltage (IEC 60156) to determine whether filtration is required.

- If the oil contains a large amount of impurities or moisture, it must be pre-treated by sedimentation or centrifugation.

- Equipment inspection:

- Ensure that the oil purifier (vacuum/pressure type) is clean and the filter element is not damaged.

- Check the vacuum pump oil level, heater function, and pipeline sealing.

2. Key control points during the filtration process

(1) Temperature control

- Optimal range: 60~65℃ (too high accelerates oil oxidation, too low affects dehydration efficiency).

- Heating method: Use step-by-step heating to avoid local overheating.

(2) Vacuum control (vacuum oil purifier)

- Requirements: absolute pressure ≤50Pa (high efficiency degassing and dehydration).

- Monitoring: observe the vacuum gauge in real time, and check the sealing if fluctuations are found.

(3) Filter element selection and replacement

- Accuracy matching:

- Particle filtration: β≥200 (5μm filter element, ISO 4406 cleanliness ≤12/9).

- Moisture adsorption: molecular sieve or silica gel filter element (treated to ≤10ppm) can be selected.

- Replacement standard: pressure difference ≥0.3MPa or continuous operation for 100 hours.

(4) Avoid oil contamination

- Pipeline cleaning: flush with qualified oil before use to prevent cross-contamination.

- Environmental protection: operate in a closed environment to prevent dust and moisture from intrusion.

3. Handling of special situations

- Emulsified oil treatment:

- Demulsify first (add demulsifier or centrifuge), then enter the vacuum oil purifier.

- Regeneration of aged oil:

- If the acid value is > 0.1mg KOH/g (GB/T 264), it is necessary to use a clay adsorption or regeneration device.

4. Acceptance criteria after filtration

- Moisture content: ≤10ppm (≤8ppm for transformers above 330kV).

- Breakdown voltage: ≥60kV (new oil standard ≥70kV, GB 2536).

- Dielectric loss (90℃): ≤0.005 (≤0.002 for 500kV transformer).

5. Safety precautions

- Fire prevention measures: stay away from open flames, and equip the site with a fire extinguisher (CO₂ or dry powder).

- Anti-static: The equipment is well grounded, and the flow rate is controlled to <1m/s (to avoid static electricity accumulation).

- Shutdown sequence: turn off the heater first, continue to circulate and cool down to <50℃ before stopping the pump.

6. Maintenance recommendations

- Online monitoring: Install particle/water sensors to monitor oil quality in real time.

- Periodic filtration:

- New oil must be filtered before commissioning.

- Transformer oil in operation should be inspected at least once a year, and any excess should be handled immediately.

Transformer oil filtration requires strict control of temperature, vacuum and cleanliness to ensure that the treated oil meets standards such as DL/T 429.9. For ultra-high voltage equipment, it is recommended to entrust a professional organization to use two-stage vacuum oil filtration technology.