Transformer Oil Purifier
Chongqing YUNENG Oil-Filter Manufacturing Co., Ltd. specialises in the design and manufacture of advanced vacuum transformer oil purifiers for utilities, transformer manufacturers, substations, power plants, renewable energy projects, and industrial facilities around the world. Our purification systems employ high vacuum dehydration, efficient degassing technology, and multi-stage precision filtration to effectively remove free water, dissolved moisture, gases, and microscopic particles from transformer oil.
YUNENG has over 20 years of experience in manufacturing, with a professional team of 168 employees and an annual production capacity of more than 1,000 transformer oil filtration systems. YUNENG has become a trusted supplier to customers in over 100 countries and regions. Our product line includes transformer oil purifiers, transformer oil regeneration machines, portable oil filtration units, vacuum oil purifiers, dry air generators, and transformer oil testing equipment, providing comprehensive solutions for transformer oil maintenance across the entire equipment lifecycle.
A transformer oil purifier is an important maintenance device used to remove moisture, dissolved gases, solid particles, sludge, and other impurities from the insulating oil of power transformers. Over time, transformer oil degrades due to the effects of oxidation, ingress of moisture, and contamination, leading to reduced dielectric strength, poor cooling performance, and accelerated insulation ageing. Regular filtration of transformer oil restores the oil’s electrical and physical properties, improves transformer reliability, and reduces maintenance costs and unexpected outages.
The purified oil achieves much improved breakdown voltage (BDV), lower moisture content, reduced dissolved gas levels, and improved insulation performance, helping to extend transformer oil service life and the operating life of valuable electrical equipment. Oil purification is a cost-effective alternative to replacing the oil completely and reducing the environmental impact in many maintenance applications.
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Why Transformer Oil Needs Purification
Transformer oil is essential for both electrical insulation and heat dissipation. However, even high-quality insulating oil gradually becomes contaminated during normal operation. Moisture, dissolved gases, and solid particles reduce its dielectric performance and accelerate the aging of transformer insulation. Regular oil purification removes these contaminants, restores oil quality, and helps prevent costly equipment failures.
Problem: Water enters transformer oil through aging seals, breather systems, condensation, or maintenance activities. Even a small amount of moisture can significantly reduce the oil’s dielectric strength and accelerate cellulose insulation aging.
Why It Matters: High moisture content increases the risk of partial discharge, insulation breakdown, and transformer failure.
Purification Solution: High-vacuum dehydration technology removes both free water and dissolved moisture, restoring the oil’s insulating performance.
Problem: Electrical faults such as partial discharge, overheating, and arcing generate gases including hydrogen, methane, and acetylene that remain dissolved in transformer oil.
Why It Matters: Excessive dissolved gases indicate developing internal faults and reduce insulation reliability.
Purification Solution: Vacuum degassing effectively extracts dissolved gases, helping restore oil quality and improve operational safety.
Problem: Metal wear debris, carbon deposits, dust, and aging insulation fibers gradually accumulate in transformer oil.
Why It Matters: These particles create conductive paths, increase electrical stress, clog cooling channels, and accelerate insulation deterioration.
Purification Solution: Multi-stage precision filtration removes particles down to micron-level sizes, ensuring cleaner oil and more reliable transformer operation.
Problem: Moisture, gases, particles, and oxidation products all contribute to lower Breakdown Voltage (BDV), reducing the oil’s insulating capability.
Why It Matters: Low BDV increases the likelihood of flashover, electrical discharge, and unexpected transformer outages.
Purification Solution: By simultaneously removing moisture, dissolved gases, and contaminants, a transformer oil purifier significantly improves BDV, allowing the oil to meet or exceed industry operating standards and ensuring safe, reliable transformer performance.
What Contaminants Can Be Removed from Transformer Oil?
During long-term operation, transformer oil is exposed to heat, oxygen, electrical stress, and environmental contamination. As a result, various impurities gradually accumulate in the oil, reducing its insulating performance and cooling efficiency. A high-vacuum transformer oil purifier is designed to remove these contaminants through dehydration, degassing, and multi-stage precision filtration, helping restore the oil to a safe operating condition.
| Contaminant | Source | Impact on Transformer | Removed by Vacuum Oil Purifier |
| Free Water | Condensation, leaking seals, maintenance exposure | Significantly reduces dielectric strength (BDV), increases insulation failure risk | ✅ Yes |
| Dissolved Moisture | Absorbed from humid environments and aging insulation | Accelerates cellulose insulation aging and partial discharge | ✅ Yes |
| Dissolved Gases | Partial discharge, overheating, electrical arcing | Indicates internal faults and reduces insulation reliability | ✅ Yes |
| Air Bubbles | Oil filling, maintenance, leaks, low-pressure operation | Causes localized electrical discharge and lowers insulation performance | ✅ Yes |
| Solid Particles | Dust, metal wear debris, insulation fibers, carbon deposits | Creates conductive paths, blocks oil circulation, increases wear | ✅ Yes |
| Carbon Residues | Electrical arcing and overheating | Increases conductivity and promotes flashover | ✅ Yes |
| Sludge & Oxidation By-products | Long-term oil oxidation | Restricts cooling, blocks oil passages, accelerates oil degradation | ✅ Partially |
| Organic Acids | Oxidation of insulating oil | Corrodes metal components and accelerates paper insulation aging | ⚠ Limited |
| Volatile Contaminants | Light hydrocarbons and volatile decomposition products | Affect oil stability and insulation characteristics | ✅ Yes |
| Large Mechanical Impurities | Rust, welding debris, external contamination | Causes filter blockage and mechanical wear | ✅ Yes |
How a Transformer Oil Purifier Works
A transformer oil purifier restores the insulating and cooling properties of transformer oil by removing moisture, dissolved gases, air, and solid contaminants through a combination of heating, high-vacuum dehydration, degassing, and precision filtration. Unlike conventional filtration systems that only remove particles, a high-vacuum transformer oil purifier simultaneously eliminates water and gases that significantly reduce dielectric strength and accelerate insulation aging.
Transformer Oil Purification Process
Step 1. Oil Intake
Contaminated transformer oil is pumped from the transformer or storage tank into the purifier. The oil may contain free water, dissolved moisture, air bubbles, dissolved gases, metal particles, carbon residues, sludge, and other contaminants accumulated during long-term operation.
Step 2. Controlled Heating
The oil passes through a high-efficiency heating system, where it is gradually heated to the optimal processing temperature—typically 45–65°C. Heating lowers the oil’s viscosity and improves the evaporation efficiency of moisture and dissolved gases while protecting the oil from thermal degradation.
Step 3. High-Vacuum Dehydration
The heated oil enters the vacuum chamber, where the pressure is reduced to a high vacuum level (typically ≤5 Pa). Under these conditions, free water and dissolved moisture rapidly vaporize at a much lower temperature than under atmospheric pressure and are extracted by the vacuum system.
Step 4. Vacuum Degassing
At the same time, dissolved gases such as hydrogen (H₂), methane (CH₄), acetylene (C₂H₂), carbon monoxide (CO), and air are separated from the oil under vacuum. Removing these gases improves insulation performance and reduces the risk of partial discharge, overheating, and internal electrical faults.
Step 5. Multi-Stage Precision Filtration
After dehydration and degassing, the oil flows through multiple filtration stages. Coarse filters remove larger debris, while fine filters—typically rated between 1 and 3 microns—capture metal particles, carbon deposits, insulation fibers, sludge, and other suspended contaminants.
Step 6. Clean Oil Return
The purified oil is returned directly to the transformer or collected in a clean storage tank. The treated oil exhibits significantly lower moisture and gas content, improved dielectric strength (BDV), and enhanced cooling performance, allowing the transformer to operate more safely and efficiently.
Transformer Oil Treatment Standards
Maintaining transformer oil according to recognized international standards is essential for ensuring reliable insulation performance, extending transformer service life, and reducing the risk of electrical failures. During oil purification, key parameters such as dielectric strength (BDV), moisture content, particle cleanliness, acidity, and dissolved gases should be evaluated against applicable IEC and ASTM standards. A high-quality transformer oil purifier helps restore insulating oil to meet or exceed these performance requirements.
| Category | Standard / Parameter | Scope & Description | Explicit Standard Values & Thresholds |
| IEC Standards | IEC 60296 | Specification for unused (new) mineral insulating oils for electrical equipment. | BDV: ≥ 30 to 70 kV (depending on testing gap/oil type) Water Content: ≤ 30–40 mg/kg (ppm) Acidity: ≤ 0.01 mg KOH/g |
| IEC Standards | IEC 60422 | Supervision and maintenance guide for mineral insulating oils in electrical equipment. | Intervention Limits (e.g., >170 kV systems): BDV: Must be > 50 kV Water Content: < 15–20 ppm Acidity: < 0.1 mg KOH/g |
| ASTM Standards | ASTM D877 | Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using Disk Electrodes. | Minimum Acceptance Threshold: Typically 25 kV to 30 kV depending on equipment voltage class (commonly applied to distribution transformers). |
| ASTM Standards | ASTM D1816 | Standard Test Method for Dielectric Breakdown Voltage of Insulating Oils of Petroleum Origin Using VDE Electrodes. | Minimum Acceptance Threshold: > 30 kV (1 mm gap) or > 47 kV (2 mm gap) for high-voltage power transformers. |
| Key Quality Parameters | Dielectric Breakdown Voltage (BDV) | Measures the electrical stress the oil can withstand without arc breakdown. | Purification Target: > 60 kV to 70 kV for high-voltage applications to ensure optimal dielectric strength. |
| Key Quality Parameters | Water Content | Measures free and dissolved moisture that degrades cellulose insulation and lowers BDV. | Purification Target: < 10 ppm (mg/kg) for high-voltage power transformers; < 20 ppm for standard equipment. |
| Key Quality Parameters | Particle Count / Cleanliness | Measures suspended solid contaminants, carbon particles, and sludge. | Purification Target: ISO 4406 cleanliness level better than 14/12/9 or NAS 1638 Class 6. |