Why does transformer oil need to be degassed?

Transformer oil degassing is a critical maintenance process that ensures the safe, reliable, and efficient operation of power transformers. By removing dissolved gases and moisture from insulating oil, degassing restores dielectric strength, slows insulation aging, and prevents electrical failure.

This guide explains what transformer oil degassing is, why it is necessary, how it works, and which standards it must meet, followed by practical FAQs commonly asked by engineers and maintenance professionals.

What Is Transformer Oil Degassing?

Transformer oil degassing is the process of removing dissolved gases and air bubbles from insulating oil used in oil-filled transformers. These gases commonly include oxygen, nitrogen, hydrogen, and carbon dioxide.

Degassing is usually performed as part of a vacuum oil purification process, where degassing, dehydration, and filtration work together to restore oil quality.

Why Is Transformer Oil Degassing Important?

During normal transformer operation, insulating oil is exposed to:

• High temperatures

• Electrical stress

• Oxidation

• Aging insulation paper

These factors cause gases and moisture to dissolve into the oil. If not removed, they can lead to:

• Reduced dielectric strength

• Partial discharge and arcing

• Accelerated insulation aging

• Increased risk of transformer failure

Moisture and dissolved gases significantly lower the breakdown voltage (BDV) of transformer oil. Degassing improves the oil’s ability to withstand electrical stress and protects internal insulation materials.

What Gases Are Removed During Transformer Oil Degassing?

Common gases removed during the degassing process include:

• Oxygen – accelerates oil oxidation

• Nitrogen – reduces dielectric strength under electrical stress

• Hydrogen and hydrocarbons – indicators of insulation deterioration

• Carbon dioxide – produced by insulation aging

Removing these gases improves insulation performance and slows chemical degradation inside the transformer.

Common Transformer Oil Degassing Methods

1. Vacuum Degassing (Industry Standard)

Vacuum degassing is the most effective and widely used method. Oil is exposed to low pressure in a vacuum chamber, allowing dissolved gases and moisture to vaporize and be removed continuously.

Advantages:

• High gas and moisture removal efficiency

• Suitable for high-voltage and extra-high-voltage transformers

• Often combined with controlled heating for better results

2. Thermal Degassing

Thermal degassing removes gases by heating transformer oil. While it can release some dissolved gases, it is less effective than vacuum degassing and is rarely used for critical transformer applications.

3. Centrifugal or Mechanical Degassing

This method uses centrifugal force to separate free gas bubbles from oil. It is mainly used in mobile oil treatment units for quick field operations but cannot remove deeply dissolved gases or moisture.

Among all methods, vacuum degassing is the preferred solution for modern transformer maintenance.

What Equipment Is Used for Transformer Oil Degassing?

Most degassing operations use a vacuum transformer oil filtration machine, which integrates multiple purification functions.

A standard system includes:

• Oil heating unit to enhance gas release

• Multi-stage vacuum chamber for deep degassing

• High-precision filters for solid particle removal

• Vacuum pumps and Roots pumps for stable low-pressure operation

Modern systems feature automated controls, real-time temperature and pressure monitoring, and digital displays. Both portable and stationary models are available, depending on transformer size and site conditions.

Transformer Oil Degassing Standards and Quality Requirements

After degassing, transformer oil must meet international standards such as IEC 60296 and ASTM D3487.

Key quality indicators include:

• Gas content: ≤ 0.3% by volume

• Breakdown Voltage (BDV): ≥ 70 kV

• Moisture content: ≤ 5 ppm (high-voltage transformers)

• Particle size: < 1 micron

Routine testing of BDV and moisture content ensures consistent oil quality and system effectiveness.

How Degassing Fits into Transformer Oil Purification

Transformer oil degassing is part of a complete oil purification cycle, which includes:

1. Filtration – removes solid contaminants and sludge

2. Dehydration – eliminates dissolved and free water

3. Degassing – removes air and dissolved gases

When combined in a vacuum oil filtration system, these processes restore the oil’s dielectric, physical, and chemical properties, ensuring long-term transformer reliability.

How often should transformer oil be degassed?

Degassing frequency depends on transformer load, voltage level, and operating environment. Most transformers benefit from periodic oil testing and degassing when BDV or moisture levels exceed recommended limits.

Can degassing extend transformer life?

Yes. By improving dielectric strength and slowing insulation aging, transformer oil degassing significantly extends transformer service life and reduces the risk of unexpected failures.

Is degassing necessary for new transformer oil?

New oil may still contain dissolved gases and moisture from handling and transportation. Degassing ensures the oil meets performance standards before commissioning.

What happens if transformer oil is not degassed?

Untreated oil can lead to reduced insulation strength, partial discharge, overheating, and ultimately transformer insulation failure.

Conclusion

Transformer oil degassing is a fundamental maintenance practice for ensuring transformer safety, efficiency, and longevity. By removing dissolved gases and moisture, degassing restores oil dielectric strength, protects insulation materials, and prevents costly equipment failures.

Integrated solutions such as SINO-NSH transformer oil filtration machines combine vacuum degassing, dehydration, and filtration in one system. Designed to meet international standards, these systems help maintain clean, dry, and gas-free transformer oil—ensuring reliable power transmission for years to come.