What is the purpose of transformer oil filtration?

The main purpose of transformer oil filtration is to maintain or restore the physical, chemical and electrical properties of the insulating oil and ensure safe, stable and efficient operation of the transformer. Specific goals include the following aspects:

1. Improve insulation performance

- Remove moisture: Moisture will significantly reduce the dielectric strength (breakdown voltage) of the oil and increase the risk of short circuit or partial discharge. After filtration, the moisture content in the oil must be controlled at ≤15 mg/L (new oil) or ≤20 mg/L (operating oil).

- Remove impurities: Contaminants such as metal particles and fibers may form conductive channels, and filtration can reduce particle contamination (such as reaching NAS level 8 or below).

2. Eliminate dissolved gases

- Fault gases: Overheating or discharge inside the transformer will produce gases such as hydrogen (H₂), methane (CH₄), acetylene (C₂H₂), etc., which are dissolved in the oil. Filtration (especially vacuum degassing) can reduce the gas content and avoid:

- Insulation failure caused by bubble discharge

- Gas accumulation leads to abnormal pressure (such as gas relay action).

3. Delay oil aging

- Remove oxidation products: During operation, the reaction between oil and oxygen will generate acidic substances and sludge, resulting in:

- Increased acid value (standard requirement ≤0.1 mg KOH/g).

- Sludge blocks the heat dissipation channel and affects the cooling effect.

- Filtration measures: Remove aging products through adsorbents (such as silica gel, activated alumina) or mechanical filtration.

4. Ensure heat dissipation efficiency

- Clean the oil circuit: impurities and sludge will adhere to the windings, cores and radiators, hindering the oil flow circulation. After filtration, the oil fluidity is restored to ensure the heat dissipation capacity of the transformer.

5. Prevent internal faults

- Reduce partial discharge and arc: Pure insulating oil can effectively suppress discharge and reduce the failure rate of transformers.

- Extend equipment life: Regular filtration can slow down the aging of solid insulating materials (such as insulating paper).

6. Meet the commissioning or overhaul standards

- Before injecting new oil: Even new oil may contain impurities introduced during transportation or storage, and needs to be filtered to meet the standards (such as breakdown voltage ≥50 kV).

- Post-overhaul treatment: After replacing parts or overhauling, the oil needs to be filtered to ensure that no contaminants remain.

Comparison of key filtration indicators

| Parameters | Common problems before filtration | Standards after filtration |

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

| Breakdown voltage | <30 kV | ≥50 kV (new oil) / ≥45 kV (operating oil) |

| Moisture content | >30 mg/L | ≤15 mg/L (new oil) / ≤20 mg/L (operating oil) |

| Dielectric loss factor (90℃) | >0.05 | ≤0.005 (new oil) / ≤0.02 (operating oil) |

| Acid value | >0.1 mg KOH/g | ≤0.03 mg KOH/g (new oil) / ≤0.1 mg KOH/g (operating oil) |

| Gas content | >3% (volume) | ≤1% (≤0.5% required for 500 kV) |

When is filtration necessary?

1. Regular maintenance: according to oil test results (such as excessive moisture and acid value).

2. Post-fault processing, such as internal discharge and overheating, leading to oil quality deterioration.

3. Before injecting new oil, ensure that the oil quality meets the standards.

4. After maintenance or an oil change, prevent installation contamination.

The core purpose of transformer oil filtration is to maintain the insulation, cooling and arc extinguishing performance of the oil, which is directly related to the safe operation life of the transformer. By removing moisture, gas, impurities and aging products, faults can be effectively prevented and operation and maintenance costs can be reduced.