In 10kV distribution systems, oil-immersed transformers are widely used in factories, communities, and substations due to their excellent insulation and heat dissipation performance. However, over years of operation, the insulating oil gradually ages, leading to decreased dielectric strength, increased moisture content, and higher dielectric loss, which can result in transformer failure. This raises a key maintenance question: should you replace the oil or simply filter it?
This article provides a detailed analysis of oil aging mechanisms, chromatographic testing, evaluation criteria, and economic considerations to help maintenance teams make informed decisions and ensure long-term safe operation of power transformers.
1. The Role and Aging Mechanism of Transformer Insulating Oil
Insulating oil is one of the core components of an oil-immersed transformer, providing three essential functions: insulation, cooling, and oxidation protection. Its dielectric strength and chemical stability are directly related to operational safety and reliability.
As operating time increases, transformer oil undergoes degradation due to several factors:
1. Thermal aging: Prolonged high temperatures break down hydrocarbon bonds, producing acids, sludge, and deposits that reduce insulation strength.
2. Oxidative aging: Contact with air causes oxidation, forming acids and aldehydes that increase acidity and darken the oil.
3. Moisture contamination: Poor sealing or humid environments allow water to enter, drastically lowering dielectric strength and increasing the risk of partial discharge.
4. Electrical stress and arcing: Faults or partial discharges decompose oil into gases such as hydrogen, methane, and ethylene, whose concentrations can be detected by gas chromatography.
2. Oil Replacement vs. Oil Filtration: Differences and Applications
Oil filtration uses a vacuum oil purifier or centrifugal device to remove impurities, moisture, and gases from the oil, restoring its physical and electrical properties without changing its chemical composition. It is ideal for mildly aged transformers.
Oil replacement involves draining and refilling the transformer with new insulating oil after vacuum drying. It is recommended for severely aged or oxidized oils or when chromatographic tests indicate potential internal faults.
Comparison summary:
Oil filtration: economical, fast, suitable for mild degradation but cannot remove all oxidation byproducts.
Oil replacement: more expensive and time-consuming but fully restores oil quality and extends equipment lifespan.
3. Chromatographic Analysis: The Scientific Basis for Decision-Making
Dissolved Gas Analysis (DGA) is the most effective diagnostic method for assessing transformer oil condition. By analyzing the type and concentration of dissolved gases, it reveals potential thermal or electrical faults inside the transformer.
Normally, oil contains trace gases such as hydrogen (H₂), methane (CH₄), ethylene (C₂H₄), acetylene (C₂H₂), and carbon dioxide (CO₂). Different combinations and concentrations indicate specific issues:
1. Thermal faults: High oil temperature produces elevated ethylene (C₂H₄) and ethane (C₂H₆) levels.
2. Arcing: The presence of acetylene (C₂H₂) indicates high-energy discharges.
3. Partial discharge: Mainly generates hydrogen (H₂) and small amounts of methane (CH₄).
Chromatographic data should be interpreted alongside key oil quality parameters:
Acid number (mgKOH/g): Indicates oxidation level; values above 0.2 suggest oil replacement.
Breakdown voltage (kV): Reflects insulation strength; below 35kV, filtration or replacement is recommended.
Dielectric loss factor (tanδ): Values over 0.7% indicate severe aging.
If the chromatographic results show only minor deviations and other indicators remain within limits, filtration is sufficient. However, if acetylene is detected or gas levels continue to rise, immediate oil replacement and fault inspection are necessary.
4. Operational Recommendations for Oil Filtration and Replacement
1. Filtration Process Essentials
Use a high-vacuum, double-stage filtration unit to reduce moisture below 10 ppm. Check oil temperature and humidity before filtration, and clean the tank thoroughly. After processing, retest dielectric strength and acid number to verify effectiveness.
2. Oil Replacement Procedure
Completely drain old oil, dry the tank, and evacuate it before refilling. Inject new oil slowly to prevent air bubbles. The new oil must pass electrical and chemical tests—such as dielectric strength, acid number, and gas analysis—before use.
3. Maintenance Cycle and Cost Considerations
Under normal conditions, transformers should undergo oil sampling and DGA testing every 3–5 years. If indicators remain normal, filtration is sufficient. When acidity or gas levels rise, replacement is recommended. A proper maintenance schedule can extend service life by 5–10 years.
5. Comprehensive Evaluation and Maintenance Strategy
Decisions on filtration versus replacement should be based on a combination of chromatographic results, electrical performance, and environmental conditions:
1. Mild aging (Breakdown > 40kV, Acid < 0.1): Filtration is sufficient.
2. Moderate aging (Breakdown 30–40kV, Acid 0.1–0.2): Filter and monitor closely.
3. Severe aging (Breakdown < 30kV, Acid > 0.2 or presence of C₂H₂): Replace oil and inspect the transformer.
Complementary tests—such as infrared scanning, micro-water analysis, and color change monitoring—help provide a more comprehensive assessment of internal transformer health, enabling predictive maintenance.
For 10kV oil-immersed transformers, making the right choice between oil replacement and filtration directly affects equipment reliability and grid safety. Regular oil sampling and chromatographic analysis allow accurate evaluation of oil aging and deterioration, ensuring timely maintenance decisions.
Oil filtration is a cost-effective routine maintenance solution, while oil replacement is a long-term strategy for life extension. Understanding test indicators and oil aging patterns is key to achieving intelligent transformer operation and efficient asset management.
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