Can the filtered transformer oil still be used?

Whether the filtered hydraulic transformer oil can continue to be used depends on the oil state, pollution type, filtering effect and equipment requirements. The following is a detailed analysis:

1. The difference between transformer oil and ordinary hydraulic oil

Transformer oil (insulating oil) is not only used for lubrication and pressure transmission, but more importantly, for its electrical insulation and cooling performance. Therefore, its purity, dielectric strength, water content and other requirements are stricter than those of ordinary hydraulic oil.

Key judgment indicators

Whether the filtered transformer oil can be reused requires testing of the following core parameters:

- Dielectric strength (breakdown voltage)

- Standard requirements: New oil is usually ≥30kV (GB/T 507 or IEC 60156 standard), and operating oil is ≥20kV.

- Requirements after filtration: If the breakdown voltage is restored to above 25kV, it can be considered for use; otherwise, it needs further processing or scrapping.

- Water content (ppm)

- Allowable range: Generally required ≤30ppm (ultra-high voltage transformers require ≤15ppm).

- Note: trace moisture will significantly reduce the insulation performance, and vacuum dehydration or molecular sieve adsorption is required.

- Acid value (TAN)

- Critical value: new oil ≤ 0.03mg KOH/g, running oil ≤ 0.1mg KOH/g. Exceeding this value indicates serious oxidation and an oil change is required.

- Particle contamination

- Solid particles will reduce insulation and need to be filtered to NAS 1638 Class 6 or below (or ISO 4406 15/12/9).

- Dielectric loss factor (tanδ)

- Reflects the polarization loss of the oil. Tanδ≤0.05 (90℃) is qualified. If it is too high, regeneration is required.

Treatment methods for different pollution situations

| Pollution type | Can it be filtered and restored? | Treatment methods |

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

| Solid particles | ✅ Recoverable | High-precision filtration (1~3μm filter element) |

| Water | ✅ Partially recoverable | Vacuum dehydration, coalescence separation |

| Light oxidation | ⚠️ Partially recoverable | Adsorbent (white clay, silica gel) treatment |

| Severe oxidation/acidification | ❌ Irreversible | Oil change or professional regeneration |

| Mixed with other oils | ❌ Irreversible | Must be replaced |

When must it be replaced?

- The dielectric strength is continuously lower than 20kV and cannot be increased after dehydration.

- Acid value > 0.15mg KOH/g, or sludge is obvious (oxidation product precipitation).

- The dielectric loss factor (tanδ) exceeds the standard, indicating that the insulation performance of the oil has deteriorated.

- The oil is dark black and has a pungent smell (such as a burnt smell).

Conclusion

- Can be reused: If the key indicators, such as dielectric strength and water content, meet the standards after filtration, and there is no chemical pollution (such as mixed oil, rancidity).

- Must be replaced: If the oil is severely aged (oxidized, high acid value), the dielectric properties are irreversibly reduced, or there are unknown pollutants.

Note: Failure of transformer oil insulation may cause equipment failure or even an explosion. It is recommended to operate under professional guidance and make conservative decisions rather than risky reuse.