Necessity of oil filtration for power transformers

Filtration and purification of power transformer oil is a key maintenance measure to ensure safe operation of transformers, extend equipment life and maintain grid stability. Its necessity is mainly reflected in the following aspects:

Direct safety risks: hazards of unpurified transformer oil

1. Deterioration of insulation performance

- Moisture: When the moisture content in the oil is ≥50 ppm, the breakdown voltage may ｄｒｏｐ by 30%~50%, causing partial discharge or insulation breakdown.

- Particles: The accumulation of particles ≥5 μm causes electric field distortion. 1g of impurities in a 500 kV transformer can increase the local electric field strength by 200%.

- Case: A 220 kV transformer had an inter-turn short circuit after running for 3 years due to excessive moisture in the oil (120 ppm), with direct losses exceeding $5 million.

2. Overheating and accelerated aging

- Oxidation products: When the acid value is ≥0.1 mg KOH/g, the life of the oil-paper insulation system is shortened by 50% (IEEE Std 62-1995).

- Dissolved gas: The accumulation of H₂, C₂H₂, etc. may trigger the malfunction of the gas relay or cause an explosion (such as the Indian power grid accident in 2019).

Mandatory requirements of regulations and standards

| Standard system |             Key indicators           |         Allowable limits         | Consequences of non-compliance   |

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

| IEC 60422            | Breakdown voltage (kV)       | ≥30 (operating oil)             | Forced shutdown for maintenance  |

| GB/T 7595           | Water content (ppm) | ≤35 (330 kV and above)                | Accelerated insulation degradation|

| ASTM D1816       | Dielectric loss factor (90℃)  | ≤0.5% | Energy efficiency reduction, heating risk |

| CIGRE TB 445      | Particle count (ISO 4406)      | ≤15/12 (UHV transformer) | Blocked oil channel, cooling failure |

Economic benefit analysis

1. Cost comparison

- Filtration maintenance cost: about $5,000-20,000/year (depending on transformer capacity).

- Accident loss cost:

- Small transformer failure: $100,000+ (including power outage loss).

- 500 kV main transformer replacement: over $3 million + grid stability risk.

2. Life extension effect

- Regular purification can extend the life of the transformer from 20 years to 40 years (EPRI research data).

- When the acid value is controlled below 0.03 mg KOH/g, the winding aging rate is reduced by 60%.

Purification technology selection and scenario adaptation

1. Daily maintenance (online monitoring + mild pollution)

- Equipment: circulating oil filter car with particle/moisture sensor (such as Parker IC30).

- Frequency: quarterly inspection, start filtering when the water content is ≥25 ppm.

2. Moderate pollution (water/particles exceed the standard)

- Technology: vacuum dehydration + precision filtration (such as GlobeCore CMM-4R).

- Target: Water content ≤10 ppm, NAS ≤6 level.

3. Severe aging (high acid value, dark color)

- Technology: white clay adsorption + vacuum degassing (such as Dürr EcoPower).

- Regeneration effect: acid value is restored to ≤0.01 mg KOH/g, close to the new oil standard.

Industry practice consensus

- Preventive maintenance: US DOE statistics show that 75% of transformer failures can be avoided through oil purification.

- Condition maintenance: In smart grids, oil data (such as micro-water, DGA) have been included in the transformer health rating system.

Conclusion

Power transformer oil filtration is not "optional maintenance" but a necessary measure to ensure the safety of the power grid. From the perspectives of technology, regulations and economy, regular purification can reduce the risk of insulation failure by more than 90%, which is one of the most cost-effective asset management methods. For key transformers, a combination strategy of online purification + offline deep treatment is recommended.