Top Methods for Transformer Oil Degassing and Dehydration

Transformer Oil Degassing Methods: How to Remove Moisture and Gases Effectively

Transformer oil degassing is essential during transformer installation and maintenance. Its purpose is to remove dissolved moisture and gases (oxygen, nitrogen, CO₂) from oil, improving insulation strength, preventing aging, and avoiding partial discharge from air bubbles.

Below are the four main transformer oil degassing methods, from traditional to modern.

1. Hot Oil Circulation Method (Heating + Vacuum Filtration)

Heated transformer oil (50–65 °C) becomes less viscous, allowing moisture and gases to escape more easily. In a vacuum tank, the oil is atomized or forms a thin film; water and gases evaporate and are removed by the vacuum pump.

Equipment:

Vacuum oil purifier with heating, filtration, and vacuum degassing.

Advantages:

Excellent dehydration and degassing.

Can operate online during vacuuming.

Mature and reliable.

Limitations:

Large equipment and higher power use.

Speed is limited by vacuum level and flow rate.

2. Thin-Film Evaporation Degassing Method

Principle:

Oil flows through plates or packing to form an ultra-thin film under vacuum, shortening gas diffusion paths and completing degassing almost instantly.

Advantages:

Faster, more efficient than spray-type systems.

Compact design and lower energy use.

Removes dissolved oxygen effectively.

Limitations:

Higher cost and strict cleanliness requirements.

3. Coalescing Separation Method

Principle:

Tiny water droplets and gas bubbles merge as oil passes through coalescing filter elements, separating naturally by gravity.

Advantages:

Fast dehydration, ideal for emulsified oil.

Removes solids and free air.

Limitations:

Ineffective on dissolved gases.

Usually used as pretreatment before vacuum degassing.

4. Vacuum Oil Injection Method

Principle:

While the transformer tank is under high vacuum (<133 Pa), pre-dehydrated and degassed oil is injected, preventing re-absorption of air and moisture.

Advantages:

Ensures dryness and gas-free insulation.

Works with hot oil circulation systems.

Limitations:

Transformer tank must withstand vacuum.

Maintains oil quality but doesn’t clean oil.

Standard Practice & Summary

Recommended process:

Vacuum oil filtration (hot oil or thin-film method).

Transformer body vacuuming.

Vacuum oil injection.

Breather installation for long-term moisture control.

For high-voltage transformers, thin-film vacuum degassing combined with vacuum oil injection offers the best efficiency and reliability, keeping insulation systems dry, gas-free, and long-lasting.