Can recycled transformer oil be used again?

Whether the regenerated transformer oil can be used again depends on the thoroughness of the regeneration process and the degree of recovery of the oil's performance. If properly handled, the regenerated oil can reach or approach the new oil standard, but the following key factors need to be strictly evaluated:

I.Conditions for the use of regenerated transformer oil

1. Performance indicators meet the standards

- Electrical properties:

- Dielectric strength ≥50 kV (IEC 60156 standard, 2.5mm electrode gap).

- Dielectric loss factor (tanδ) ≤0.005 (at 90℃).

- Chemical properties:

- Acid value ≤0.01 mg KOH/g (IEC 62021).

- Moisture content ≤10 ppm (Karl Fischer method).

- Physical properties:

- Viscosity (40℃) meets the new oil standard (such as within ±10%).

- Flash point ≥140℃ (to prevent the risk of high temperature volatilization).

2. Complete removal of pollutants

- Solid particles: cleanliness reaches NAS 1638 Class 6 or ISO 4406 15/13/10.

- Dissolved gas: total gas content ≤ 0.1% (avoid partial discharge).

- Oxidation products: no visible precipitation or colloid (pass DL/T 429.7 test).

3. Additive supplement (if necessary)

- Antioxidants (such as DBPC) need to be re-added to 0.3%~0.5% to extend the oil life.

II.Typical application scenarios of recycled oil

1. Non-critical equipment:

- Such as factory transformers, low-voltage distribution transformers (relatively loose requirements on oil quality).

2. Replenishing oil:

- Mix with new oil (compatibility test is required before mixing).

3. Temporary emergency:

- Short-term use in case of sudden oil shortage, and subsequent monitoring of oil quality changes.

III. Limitations and risks of regenerated oil

1. Unusable situations

- Deep aging: acid value > 0.1 mg KOH/g or dielectric strength < 30 kV.

- Severe pollution: PCB (polychlorinated biphenyls) or metal particles (such as copper chips) exceeding the standard.

- Risk of oil mixing: Mixed oils with unverified compatibility are prone to precipitation.

2. Potential risks

- Degradation of insulation performance: Residual polar compounds may cause partial discharge.

- Accelerated oxidation: The antioxidant capacity of regenerated oil may be lower than that of new oil, and the detection cycle needs to be shortened.

IV. Comparison of regeneration processes

| Regeneration method | Treatment effect | Applicable oil conditions | Limitations |

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

| White clay adsorption | Remove pigments, colloids, improve color | Mildly oxidized oil (acid value <0.05) | Cannot reduce acid value |

| Vacuum distillation | Remove moisture, light hydrocarbons | High moisture oil (>1000 ppm) | High temperature may damage additives |

| Hydrorefining | Deep desulfurization, saturated olefins, close to new oil | Severely aged oil | High equipment cost, requires professional factories |

| Ion exchange | Remove acidic compounds, reduce tanδ | High acid value oil (>0.08) | Slow processing speed |

V. Recommendations for use

1. Strict testing:

- Full testing is required after regeneration (according to IEC 60296 or ASTM D3487 standards).

2. Step-by-step trial:

- First, try it in a low-load transformer for 3 to 6 months and monitor the changes in oil quality.

3. Mixing taboos:

- Avoid mixing with silicone oil or ester oil (precipitation may occur).

Recycled transformer oil can be used again, but it must meet the electrical, chemical, and physical performance standards and pass compatibility verification. For key equipment (such as high-voltage main transformers), it is recommended to use new oil first; if recycled oil is used, the testing cycle needs to be shortened to once every 3 months. The quality of recycled oil selected by the hydrofining or vacuum-white clay combined process is more reliable.