Can transformer oil be used if it has impurities?

Whether transformer oil can continue to be used when it contains impurities depends on the type and content of the impurities. Minor contamination may not affect operation temporarily, but severe contamination can lead to reduced insulation performance, overheating of equipment, and even failure. The following is a detailed analysis:

1. Risks of transformer oil containing impurities

| Impurity type | Allowable limit | Hazards of exceeding the standard |

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

| Solid particles (metal, dust) | NAS ≤6 level (ISO 4406) | Wear insulation paper, block oil channels, and cause partial discharge |

| Moisture | ≤10ppm (more stringent for ultra-high voltage transformers) | Reduce insulation strength and accelerate oil aging |

| Carbide (arc product) | No visible black suspended matter | Conductive, causing oil degradation or even breakdown |

| Fiber impurities (paper scraps, cotton wool) | No visible suspended matter | Block oil pump, poor heat dissipation |

If impurities exceed the standard, it may lead to:

- Reduced insulation performance → Inducing short circuit or discharge

- Reduced heat dissipation efficiency → Transformer overheating and shortened life

- Mechanical wear → Damage to key components such as windings and cores

2. Standards for judging whether transformer oil can continue to be used

✅ Can be used for short-term emergencies (need to be handled as soon as possible):

- Slight impurities (NAS level 7~8), and dielectric strength ≥40kV/2.5mm

- Water content ≤30ppm (non-UHV transformer)

- No visible carbonized particles or sludge

❌ Must be shut down immediately for processing:

- Oil color is black, turbid or has obvious suspended matter

- Dielectric strength <30kV/2.5mm (minimum requirement of national standards)

- NAS level >9 (serious particle pollution)

- Water content >50ppm (may cause insulation breakdown)

3. Temporary emergency measures (if operation is necessary)

If the transformer cannot be shut down immediately, but the oil quality is poor, the following measures can be taken:

1. Install a temporary oil filter (such as an online oil filter) to reduce the impurity content.

2. Monitor oil temperature and partial discharge, and stop the operation immediately if abnormality is found.

3. Shorten the inspection cycle (such as weekly sampling and testing).

Note: This is only a stopgap measure. Long-term use of contaminated oil will seriously damage the equipment!

4. Final solution: Thoroughly purify transformer oil

If the oil quality is not up to standard, professional treatment is required:

1. Mechanical filtration (removal of solid particles) → Plate and frame oil filter (20~50μm)

2. Vacuum dehydration and degassing (removal of moisture and air) → Vacuum oil filter

3. Adsorption treatment (removal of colloid and carbide) → Activated clay/silica gel

4. Precision filtration (1~5μm filter element) → Ensure NAS ≤6 level

Standard after treatment:

- Dielectric strength ≥50kV/2.5mm (new oil standard)

- Water content ≤10ppm

- NAS cleanliness ≤6 level

5. Conclusion

- A small amount of impurities (NAS ≤6 level) → Can continue to use, but need to be monitored regularly.

- Moderate pollution (NAS 7~9 level) → Short-term emergency use, filter oil as soon as possible.

- Severe contamination (NAS >9, dielectric strength <30kV) → Must shut down for oil change or regeneration.

Recommendation: Transformer oil impurity problems should be "discovered and dealt with early" to avoid small losses! Regular inspections (1~2 times a year) can effectively prevent failures.