How to do if the transformer oil contains excessive hydrogen?

February 08, 2024

How to do if the transformer oil contains excessive hydrogen?

Whether the transformer is a thermal fault or an electrical fault, it will lead to the cracking of the insulating medium and produce some characteristic gases. Due to the low chain energy between the C-H bond, hydrogen gas is generally generated during the decomposition process of the insulating medium, and hydrogen is also one of the main components of various fault gases.

1. The harm of increased hydrogen content in transformer oil

Hydrogen gas is mixed with dissolved air in the oil to exist in the transformer oil in a dissolved or suspended state. When operating conditions, such as oil temperature or oil pressure, change, hydrogen is removed from the oil in the form of tiny bubbles. Gradually accumulating in long narrow gaps and adhering to the insulating surface, this forms the conditions for bubble corona discharge. If this discharge occurs between the wire insulation and the pad or the gap between the wire insulation and the stay, the harm is greater.

2. The cause of hydrogen production in transformer oil

2.1 Decomposition of transformer oil under the action of electromagnetic field

Under normal circumstances, the transformer oil used in the transformer in the power grid of 110kV and below is No. 25 transformer oil, which belongs to the paraffin Base Oil. Paraffin oil has a large proportion of alkane, its chemical properties are relatively stable, its oxidation resistance is good, but its heat resistance is poor, especially under the action of electric field, it is easy to dehydrogenate.

2.2 Influence of moisture on transformer oil

Usually the moisture in the transformer oil is mainly caused by the transformer being damp and producing water. Due to the strong polarity of water molecules, water molecules are polarized in the electric field and form a dipole, and rotate in the direction of the electric field to form a water bridge with a large leakage current, which causes the water molecules to vaporize and form bubbles.

Under the action of electric field, the bubble forms a gas bridge, and the dielectric constant of the bubble is smaller than that of the oil. At this time, the electric field strength of the bubble is higher, causing corona discharge, and the gas water molecules are first ionized to produce hydrogen and oxygen.

The paper insulation drying is not complete or the air moisture intrusion and other reasons will also cause the production of hydrogen, because the initial field strength of the oil-impregnated paper insulation discharge increases with the degree of drying of the solid insulation.

2.3 Metal promotes transformer oil dehydrogenation reaction

Because a part of the stainless steel material is used in the transformer, during the gradual oxidation of the transformer oil, the nickel molecules in the stainless steel material will promote the dehydrogenation reaction of the transformer oil. For a solid to become a catalyst, the ability to adsorb reactants is a basic condition. During the catalytic process, the physical adsorption energy significantly reduces the activation energy of subsequent chemisorption. At the same time, transformer oil is a hydrocarbon compound.

Due to the pyrolysis of hydrocarbon molecules or the collision of oxygen molecules, the free radical R is generated, and R combines with the free valence of oxygen molecules to form a peroxide radical R+O2 - >ROO, and then ROO combines with new hydrocarbon molecules in oil to produce a new free radical. In this process, iron, copper and other metals can strengthen the oxidation reaction of oil. Because of their variable valence, they promote the decomposition of peroxides, act as catalysts for oxidation reactions, and produce a large amount of hydrogen.

2.4 Gas evolution of transformer oil

The gas evolution of transformer oil refers to the phenomenon that transformer oil will produce gas discharge or suction under the action of electric field and ionization. The phenomenon of oil gassing occurs because the bubbles dissolved in the oil, under the action of high electric field strength, dissociate and form high-energy electrons or ions. These high-energy particles produce violent collisions with oil molecules to break the C-H or part of the C-C chain of oil molecules, produce active hydrogen and active alkyl groups, and produce inspiratory or venting phenomena through the action of active hydrogen on hydrocarbon molecules.

2.5 Release of hydrogen adsorbed in insulation materials

Under the action of heat and electricity such as transformer drying, impregnation and high voltage test, the insulating materials decompose to produce hydrogen and hydrocarbon gases, which are adsorbed in the porous and thick solid insulating fiber materials and are difficult to release into the oil in the short term.

Because the transformer insulation materials are used more, it takes a long time for the absorbed gas inside the insulation layer to completely release into the oil, so the gas in the oil and paper has not reached the dissolution balance during the factory test, and the hydrogen content is low. After a period of time, when the transformer arrived at the site for acceptance, the hydrogen content was low. After a period of time, when the transformer arrives at the site for acceptance, the gas absorbed in the paper is gradually released, so the content of dissolved gas in the oil, especially hydrogen, is significantly increased.

At the same time, some metal materials such as carbon steel and stainless steel can also promote the transformer oil dehydrogenation reaction, so that hydrogen is released into the transformer oil, resulting in increased hydrogen content in the oil. This is why the transformer contains some characteristic gases before it is put into service.

3. Prevention of increased hydrogen content in transformer oil

3.1 The exposed metal inside the transformer, such as copper, iron and stainless steel materials, must be covered with insulating paint on its surface to prevent the reaction with the moisture in the transformer oil or as a catalyst to accelerate the hydrogenation of the transformer oil. Moreover, all surface insulating paints of metal materials must be thoroughly cured before tank filling.

3.2 Strictly implement transformer process regulations and minimize the moisture content of transformer insulation materials. When the transformer is finished, its exposure to the air should be shortened as far as possible to prevent the intrusion of water and avoid the ionization of water under the action of electric field.

3.3 In the case of confirming that there is no fault point inside the transformer, there are usually two ways to deal with the abnormal increase of hydrogen content in the oil: one method is to use on-site oil change, and the transformer oil treated by this method is more thorough in removing hydrogen, but the cost is higher; The second method is to use hot oil circulation or vacuum separation method.

5. Conclusion

In operation, power transformers often have excessive hydrogen content in oil, and the growth of hydrogen content is in many cases a precursor of electrical failure, which will also cause defects in transformer insulation, so hydrogen content is also one of the important indicators of whether the transformer can continue to operate safely, and should be paid attention to.

Whether it is before operation or after normal operation, the transformer oil should be tested in accordance with the requirements of relevant specifications, such as gas chromatography test, to ensure the safe operation of the transformer.

Author:

Mr. James

E-mail:

sales@cn-lubricantadditive.com

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+86 13783582233