How to filter the dirty transformer oil?

Transformer oil filtration method

Transformer oil contamination (including water, particles, colloid, etc.) will reduce insulation performance and heat dissipation efficiency, and its purity needs to be restored through system filtration. The following are professional and practical filtration steps and precautions:

Preparation before filtration

1. Detect oil quality

- Visual inspection: observe the color (normally light yellow and transparent). If it is black, turbid, or has suspended matter, it needs to be treated.

- Professional testing (recommended):

- Breakdown voltage (standard ≥35kV/2.5mm, if <30kV, it needs to be treated immediately);

- Water content (required ≤20ppm, can be tested by coulometric method);

- Acid value (≤0.1mg KOH/g);

- Particle size (NAS 1638 grade ≤8).

2. ｓｅｌｅｃｔ the filtering method

| Pollution type | Recommended method |

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

| Free water + large particles | Centrifugal separation + precision filtration |

| Dissolved water + trace particles | Vacuum dehydration + adsorption filtration |

| Colloidal/acidic substances | White clay adsorption + regeneration treatment |

Detailed explanation of filtering methods

1. Mechanical filtration (removal of solid particles)

- Equipment: Plate and frame oil filter, precision oil filter trolley (with 3-5μm filter element).

- Steps:

Pump the dirty oil into the oil filter and intercept the particles through multiple layers of filter paper or polymer filter element;

Circulate and filter until the oil is transparent (particle size meets the standard);

Note: Filter paper/filter element needs to be replaced regularly (stop when the pressure difference is >0.3MPa).

2. Centrifugal separation (for water and heavy particles)

- Equipment: disc centrifuge (speed ≥ 6000 rpm).

- Steps:

1. Heat the oil to 50-60℃ (reduce viscosity);

2. Start the centrifuge and adjust the oil inlet flow (100-300L/h);

3. Detect the water content after separation (need ≤15ppm).

3. Vacuum dehydration (deep dehydration and degassing)

- Equipment: vacuum oil filter (vacuum degree ≤0.095MPa).

- Steps:

1. The oil is sprayed into the vacuum tank, and the water evaporates and is pumped out by the vacuum pump.

2. Use a coalescing filter element to enhance the dehydration effect;

3. The breakdown voltage after treatment should be ≥50kV.

4. Adsorption purification (treatment of colloid/acidic substances)

- Materials: activated alumina, silica gel or kaolin (dosage 3-5% oil weight).

- Steps:

1. Add the adsorbent to the oil and stir it in a cycle for 6-8 hours;

2. Filter to remove the adsorbent residue;

3. Note: After adsorption, the acid value and dielectric loss factor (tanδ) need to be tested.

Operation precautions

1. Safety regulations

- Open flames are strictly prohibited at the filtration site, and the oil temperature is controlled below 80°C;

- The centrifuge/vacuum pump must be grounded to prevent static sparks.

2. Prevent secondary pollution

- All pipes and containers are flushed with new oil before use.

- Ambient humidity is <60% to avoid mixing of moisture in the air.

3. Parameter monitoring

- Real-time monitoring of oil temperature, vacuum degree, inlet and outlet oil pressure;

- Sample every 2 hours to test breakdown voltage and water content.

Acceptance criteria after filtration

| Index | Qualification criteria |

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

| Breakdown voltage | ≥45kV/2.5mm (new oil ≥60kV) |

| Water content | ≤10ppm (required for ultra-high voltage equipment) |

| Acid value | ≤0.05mg KOH/g |

| Particle size | NAS 1638 ≤6 levels |

Situations where oil cannot be filtered and needs to be changed

New oil should be replaced in any of the following situations:

- Breakdown voltage <30kV and repeated filtration is ineffective;

- Acid value >0.2mg KOH/g;

- Sludge content >0.1% (volume ratio);

- Dissipation factor tanδ >2% (at 90℃).

Daily maintenance recommendations

1. Regular filtration: Cycle filtration every 3 years or when the breakdown voltage drops by 10%.

2. Sealed oil tank: Install desiccant (such as silica gel) in the respirator;

3. Online monitoring: Install an oil moisture sensor and particle counter.

The above methods can effectively restore the performance of transformer oil. If the oil quality is seriously deteriorated or the equipment is of ultra-high voltage level (such as above 500kV), it is recommended to entrust a professional oil treatment company to adopt deep regeneration processes such as molecular sieve adsorption + low-temperature vacuum dehydration.