How to understand the production mechanism of gas in transformer oil?
Oil and paper are the main insulating materials of oil-filled electrical equipment, and the production mechanism of gas in oil is related to the properties of materials and various factors.
First, transformer oil deterioration and gas production
Transformer oil is a kind of mineral oil obtained by distillation and refining of natural petroleum. It is a mixture of various hydrocarbons, of which carbon and hydrogen account for 95% to 99% of its total weight, and the other is sulfur, nitrogen, oxygen and a very small amount of metal elements. Petroleum-based hydrocarbons include cycloalkanes (CnH2n), alkanes (CnH2n+2), aromatics (CNH2N-M), and other components.
Cycloalkanes have good chemical and dielectric stability, and the viscosity changes little with temperature. The chemical stability and dielectric stability of aromatic hydrocarbons are also good, and they do not precipitate gas under the action of electric field, and they can absorb gas. If the content of aromatic hydrocarbons in transformer oil is high, the oil has strong inspiratory property, and vice versa, the inspiratory property is poor. However, there are more carbon particles of aromatic hydrocarbons under the action of electric arc, which will reduce the electrical properties of oil. Aromatic hydrocarbon is flammable, and with the increase of its content, the specific gravity and viscosity of oil increase, and the freezing point increases. Paraffin hydrocarbon in naphthene has good chemical stability and easy to make the oil solidify, easily ionize and precipitate gas under the action of electric field, and form dendritic X-wax, which affects the thermal conductivity of oil.
Transformer oil in operation due to temperature, electric field, oxygen and water and copper, iron and other materials catalytic action, oxidation, cracking and carbonization reaction, to produce some oxidation products and condensates (sludge), hydrogen and low molecular hydrocarbon gas and solid X wax.
In short, under the action of heat, electricity and oxygen, the deterioration process of transformer oil is carried out by free chain reaction, and the reaction rate increases with the rise of temperature. The presence and content of oxygen and water have a great influence on the reaction, and metals such as copper and iron also act as catalysts to accelerate the reaction, and the acids generated after aging and H2O and sludge endanger the insulation characteristics of the oil. The refined transformer oil does not contain low molecular weight hydrocarbon gas, but the transformer oil will decompose into carbon dioxide, low molecular weight hydrocarbon gas and hydrogen gas when it is subjected to high temperature in operation.
In summary, transformer oil is a mixture of many hydrocarbon molecules of different molecular weights, which contain the CH3*,CH2* and CH* chemical groups, and are held together by C-C bonds. Due to electrical or thermal failure, some C-H bonds and C-C bonds can be broken, accompanied by the formation of a small number of active hydrogen atoms and unstable hydrocarbon free radicals, which are rapidly recombined through complex chemical reactions to form hydrogen and low molecular hydrocarbon gases, such as methane, ethane, ethylene, acetylene, etc. Solid particles of carbon and hydrocarbon polymers (paraffin) may also be formed. In the initial stage of failure, the gas formed is dissolved in the oil; When the fault energy is large, it may also accumulate into free gas. The temperature of oil carbonization to generate carbon particles is 500-800 ° C, and solid particles of carbon and hydrocarbon polymers can be deposited in the interior of the equipment. Low-energy discharge faults, such as partial discharges, recombine hydrocarbon gases by an ionic reaction that causes the formation of the weakest C-H bond (338kJ/mol), which in turn requires higher and higher temperatures and more and more energy.
Ethylene is produced in small quantities at lower temperatures, but mainly at temperatures higher than methane and ethane, about 500 ° C. Acetylene is generally generated at a temperature of 800-1200 ° C, and when the temperature is reduced, the reaction is rapidly suppressed and accumulates as a stable product of the re-compound. Therefore, although a small amount of acetylene is produced at lower temperatures (below 800 ° C), a large amount of acetylene is produced in the arc path. In addition, when the oil is oxidized, a small amount of CO and CO2 is generated, and CO and CO2 can accumulate for a long time and become a significant number of characteristic gases.
2. Decomposition and gas of solid insulating materials
Oil paper insulation includes insulating paper, insulating cardboard, etc., their main component is cellulose. The main gas components of cellulose thermal decomposition are CO and CO2. The effective temperature of polymer cracking is higher than 105℃, and the complete cracking and carbonization are higher than 300℃. While water is generated, a large amount of CO and CO2 and a small amount of hydrocarbon gases and furan compounds are generated, while oil is oxidized. The production of CO and CO2 not only accelerated with the increase of temperature, but also increased with the increase of oxygen content in oil and humidity of paper.
