Several methods of degassing transformer oil

Gases in transformer oil (such as oxygen, nitrogen, methane, acetylene, etc.) will affect the insulation performance and oxidation stability of the oil, so degassing is an important part of oil quality maintenance. The following are common transformer oil degassing methods and their principles, advantages and disadvantages, and applicable scenarios:

Vacuum degassing method (mainstream method)

(1) High vacuum spray degassing

- Principle:

Heat the oil to 50~60℃, spray it into the vacuum tank (vacuum degree <1kPa) through a nozzle atomization, increase the contact area between the oil and the vacuum, and allow dissolved gases (such as O₂, N₂, CO, etc.) to escape and be pumped out by the vacuum pump.

- Equipment:

Vacuum oil filter (with spray or thin film evaporation system).

- Advantages:

- Efficient removal of dissolved gases (can reduce the gas content to below 0.1%).

- Simultaneous removal of moisture (suitable for comprehensive purification).

- Disadvantages:

- High equipment cost, and regular maintenance of the vacuum system is required.

(2) Thin film degassing

- Principle:

The oil forms a thin layer and flows in the vacuum tank (such as a packing tower or a rotating thin film device), which prolongs the gas escape time.

- Applicable scenarios:

Occasions with higher requirements for degassing efficiency (such as oil for ultra-high voltage transformers).

Gas replacement method

(1) Nitrogen bubbling degassing

- Principle:

Dry nitrogen is introduced into the oil, and the partial pressure difference between nitrogen and dissolved gas (such as O₂) is used to ｒｅｐｌａｃｅ oxygen and other gases, and then the residual gas is removed by vacuum.

- Steps:

1. Heat the oil to 40~50℃.

2. Nitrogen is introduced into the bottom (flow control to avoid oil foam carryover).

3. Combined with vacuum exhaust.

- Advantages:

- Prevent oil oxidation (nitrogen inert protection).

- Disadvantages:

- Requires continuous nitrogen supply and is more expensive.

(2) Vacuum-inert gas cycle degassing

- Method:

After vacuum degassing, inject nitrogen or dry air, then evacuate, and cycle multiple times to completely remove the gas.

Adsorption degassing method

(1) Molecular sieve adsorption

- Principle:

Use 3Å or 4Å molecular sieves to selectively adsorb gases in oil (such as O₂, CO₂), and need to be used with an oil filter cycle.

- Advantages:

- No heating is required, suitable for small systems.

- Disadvantages:

- The adsorbent needs to be regenerated or replaced regularly.

(2) Activated carbon adsorption

- Applicable gases:

Mainly adsorbs hydrocarbon gases (such as CH₄, C₂H₂), but the effect on oxygen is poor.

Thermodynamic degassing (auxiliary method)

(1) Heating and standing degassing

- Principle:

Heat the oil to 60~70℃ and then stand it, the gas solubility decreases and escapes naturally (need to be equipped with an exhaust valve).

- Disadvantages:

- Low efficiency, only suitable for slight gas content.

(2) Ultrasonic degassing

- Principle:

Ultrasonic cavitation promotes the aggregation and precipitation of gas microbubbles.

- Current status:

Laboratory stage, less industrial application.

Key points

1. Degassing target:

- Oxygen content ≤0.5%, total gas content ≤1% (GB/T 7595 standard).

2. Safe operation:

- Prevent oil splashing or oil return during vacuum degassing.

- Avoid violent fluctuations in the oil surface during nitrogen replacement.

3. Comprehensive treatment:

- If the oil contains both water and gas, vacuum dehydration is preferred before degassing (water will hinder the escape of gas).

Method selection recommendations

| Scenario | Recommended method |

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

| Degassing before new oil injection | High vacuum spray degassing |

| Slightly gas-containing operating oil (O₂≤2%) | Nitrogen bubbling + short-term vacuum |

| Removal of hydrocarbon gas after failure | Vacuum degassing + activated carbon adsorption |

| Online maintenance | Installation of vacuum degassing device or molecular sieve oil filter |

Detection standard

- Gas content determination: gas chromatography (GB/T 17623) or vacuum degassing (IEC 60567).

- Oxygen detection: electrochemical sensor or coulometric method.

Timely degassing can delay oil aging and improve transformer operation reliability!