Generally, the gasses found in the oil in service are hydrogen (H2), methane (CH4), Ethane (C2H6), ethylene (C2H4), acetylene (C2H3), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and oxygen(O2).
The generation of gas in transformer oil can generally come from the following three aspects:
1. Introduced from outside
(a) Air: Dissolved in oil, leaked from pipes and oil pumps, and discharged after being dissolved in oil;
(b) Hydrogen: nitrogen-filled transformers contain hydrogen due to impure nitrogen;
(c) Carbon dioxide: some imported transformers are protected by carbon dioxide during transportation, so when the exhaust gas is filled with oil, it will be partially dissolved in the oil;
(d) Methane, ethylene, etc.: When the Transformer Oil Regeneration or transformer oil polishing is under process, the conditions are not properly controlled, and the over-temperature of the tubular heater causes the decomposition of the transformer oil;
(e) Acetylene: gas welding near the transformer oil filtration site.
2. Internally generated (non-faulty)
(a) normal aging of transformer oil and slow cracking of solid insulation;
(b) the normal vapor pressure of low molecular weight hydrocarbons in the oil;
(c) Transformer enters water and decomposes under high electric field to produce hydrogen and oxygen. The reaction of water and iron can also produce hydrogen and dissolve in oil;
(d) Defects of auxiliary equipment: the forced oil circulation pump and the flow relay (reverse current circuit breaker) have poor electrical contact, and sparks produce acetylene dissolved in oil;
(e) The use of inappropriate insulating materials such as 1030 alkyd paint will decompose at 85°C. In tangled transformers, CO can be as high as 0.3% or more.
3. The gas generated inside the transformer in the presence of the fault
The fault gas mainly comes from insulating media, such as insulating paper, transformer oil, etc. Insulating paper is mainly composed of cellulose, which is a high molecular carbohydrate, and transformer oil is a mixture of various hydrocarbons. When there is a failure inside the equipment to release heat, the oil and paper will be cracked, and the generated gases are some gases containing carbon, hydrogen, and oxygen, such as hydrogen, carbon monoxide, carbon dioxide, and low-molecular-weight hydrocarbon gases.
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