Transformer Oil Degassing

Transformer oil degassing is often one of the most effective methods for addressing specific issues that could eventually lead to transformer failure. All transformers release various gases during normal operation. Detecting and removing dissolved gases allows operators to identify potential problems at an early stage.

This proactive approach helps extend the service life of the equipment by enabling timely maintenance and preventing more serious damage.

How Gases Enter Transformer Oil

Gases can enter transformer oil not only from the surrounding environment, but also as a result of accelerated insulation aging. This process is often caused by excessive transformer temperatures. When the oil decomposes due to overheating, gases such as ethane, methane, and hydrogen are formed. These gases have high solubility in insulating fluids. Decomposition of oil-impregnated paper insulation, on the other hand, releases carbon dioxide, hydrogen, and carbon monoxide into the oil. Gases can also form in transformer oil as a result of partial discharges within the insulating paper.

A primary cause of transformer oil oxidation and decomposition is the presence of air in the oil. Routine degassing can help inhibit these processes.

During operation, transformers are subjected to thermal and electrical loads. As the oil degrades, various gases are formed. High electrical stress further triggers chemical changes and gas generation. Most electrical equipment operates immersed in transformer insulating oil. At high voltage levels, instantaneous or edge discharges may cause the oil to release gases.

In general, transformer oil does not readily absorb hydrogen. Certain transformer oils with specific viscosity characteristics can delay the release of hydrogen bubbles from the oil phase, leading to the formation of free hydrogen pockets — gas pockets in the insulating oil. These pockets can severely compromise the operational safety of transformers.

Impact of Gases on Transformer Oil

Dissolved gases in transformer oil reduce its dielectric strength by 20–30%. Gas bubbles further decrease this strength. Factors such as temperature changes, high-intensity electric fields, and excessive oil flow through pipelines contribute to this phenomenon.

Contact between gas-laden oil and transformer paper insulation accelerates paper degradation, often causing more damage than oxidation by-products in the oil. Although the harmful effects of dissolved gases on transformer oil are well documented, knowledge about the solubility of various gases in insulating liquids remains limited. For instance, the gas content of oil is known to be directly proportional to the gas pressure above the liquid. Higher temperatures also increase gas solubility. However, gas solubility varies among different oils and gases.

Standards for Gas Content of Transformer Oil

Sealed Transformers:

• Dissolved air: ≤ 0.5 % by volume.
• Water content: ≤ 0.001 %.

Unsealed Transformers:

• Dissolved air: ≤ 0.5 % by volume.
• Water content: ≤ 0.0025 %.

For transformers with film or nitrogen protection, hermetic bushings, and sealed instrument transformers, the air content of oil should not exceed 0.5 % (measured via gas chromatography). Water content should be not more than 0.001 % by mass. For transformers without film protection and unsealed bushings, oil with up to 0.0025 % water content by mass is acceptable. Mechanical impurities, measured as purity grade, shall not exceed Grade 11 for equipment operating at voltages up to 220 kV and Grade 9 for voltages above 220 kV.

Transformer Oil Degassing Technology: Fluidex FLD D

Transformer oil degassing occurs simultaneously with transformer oil dehydration. In order to prevent contact gases from reentering the insulating liquid after degassing, protective measures such as specialized membranes, nitrogen blankets, and other solutions can be employed.

Modern oil dehydration techniques are highly effective in removing almost all gases from the oil, leaving residual gas levels so low that they are difficult to measure accurately. However, during transformer operation, the oil can absorb gases again if a protective membrane is not provided. Alternative protection methods, such as the use of expansion tanks or inert gas cushions, are less effective in maintaining the oil purity.  Highly efficient transformer oil degassing can be achieved using specialized equipment such as the Fluidex FLD D units line. These systems are capable of removing gases and other contaminants from the oil, as well as performing transformer vacuuming. The units can process transformer and silicone oils, Midel 7131, FR3, and Fyrquel fluids and operate in the following modes:

• Equipment heating
• Vacuuming
• Oil degassing

Dissolved air near saturation levels can significantly impair transformer insulation, potentially causing failure under certain conditions. Furthermore, gases in the oil accelerate the aging of the insulation system. In order to counter these effects, it is essential that transformer oil degassing be performed prior to filling of sealed transformers.