Transformer oil filtration frequency standard

The frequency of transformer oil filtration should be determined based on the oil quality, equipment operating environment, load conditions and industry standards. The specific filtration frequency standards and influencing factors are as follows:

Conventional filtration frequency reference

(1) Regular filtration (preventive maintenance)

- Newly commissioned transformers:

- Oil sampling should be carried out after the first 72 hours of operation. If the moisture or particulate matter exceeds the standard, filtration is required.

- Normally operating transformers:

- At least once a year, a comprehensive oil quality test (including moisture, dielectric loss, breakdown voltage, etc.) should be carried out, and the decision on whether to filter should be made based on the test results.

- Deep filtration or regeneration treatment (such as vacuum oil filtration) should be carried out every 3 to 5 years.

(2) Filtration under abnormal conditions

When any of the following indicators is detected to be excessive, filtration or treatment must be carried out immediately:

| Test items | Warning threshold | Treatment measures |

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

| Moisture content | >15 ppm (below 110kV) | Vacuum oil filtration and dehydration |

| | >10 ppm (110kV and above) | |

| Breakdown voltage | <30 kV (new oil standard: ≥40 kV) | Filter or ｒｅｐｌａｃｅ oil |

| Dielectric loss factor (tanδ) | >0.04 (90℃) | Adsorption treatment or vacuum oil filtration |

| Particle contamination | NAS 1638 level ≥8 | Precision filtration (1~5 micron filter element) |

Key factors affecting filtration frequency

- Use environment:

- Humid areas: Water penetration is fast, and the inspection cycle needs to be shortened (such as once every 6 months).

- Highly polluted environments (such as dust and chemical areas): particles are easy to invade, it is recommended to install an online oil filter.

- Equipment load:

- Long-term full-load operation: the increase in oil temperature accelerates aging, and annual testing and filtration are required.

- Oil age and aging degree:

- For oil products that have been in operation for more than 10 years, the filtration frequency should be increased even if all indicators are normal.

Differences between different voltage levels

| Voltage level | Test frequency | Filtering standard |

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

| ≤35 kV | Once every 2 years | Filter when breakdown voltage <30 kV or moisture >20 ppm |

| 110~220 kV | Once a year | Filter when breakdown voltage <35 kV or moisture >15 ppm |

| ≥500 kV | Once every 6 months | Treatment is required when breakdown voltage <40 kV or tanδ>0.02 |

According to international standards:

- IEC 60422 "Guide to Maintenance of Mineral Insulating Oil"

- ASTM D6871 (Particulate Matter Detection)

Maintenance recommendations

- Online monitoring: Install sensors for moisture and particulate matter in oil for real-time warning.

- Combined filtration:

- First, remove free water and large particles through a centrifuge, and then use a vacuum oil filter for deep dehydration and degassing.

- Record analysis: Establish an oil quality change trend chart to predict the best time for filtration.

If the transformer load or environment is abnormal (such as coastal salt spray areas), it is recommended to consult a professional organization and customize a maintenance plan.