What are the dissolved gases in transformer oil? Seven kinds of fault characteristics of the gas in detail
Date: May 15, 2026 10:09:25
- faulty gas: oil-immersed transformer internal overheating or discharge faults occur, the insulating oil and solid insulating materials will decompose and produce characteristic gas dissolved in the oil, the gas components and content directly reflects the type and severity of the fault
- Seven core gasesHydrogen (H₂), Carbon Monoxide (CO), Carbon Dioxide (CO₂), Methane (CH₄), Ethane (C₂H₆), Ethylene (C₂H₄), Acetylene (C₂H₂), each of which corresponds to a different failure mode
- Diagnostic Principles: The combination of gases generated by different fault types has a fingerprint level of regularity, and the fault types can be accurately identified by analyzing the relationship between the concentration and ratio of each gas.
- Means of detectionOnline monitoring system automatically completes the whole process of oil extraction, degassing, separation and detection, and can simultaneously output the precise concentration of seven gases in a single analysis.
1. Mechanisms of faulty gas generation
During normal operation of the transformer, the insulating oil and solid insulating materials will slowly age due to electric, thermal and mechanical stresses, producing trace amounts of gas dissolved in the oil. When an internal abnormality occurs - either localized overheating, overall overheating or discharge - the corresponding organic molecular chain breakage speeds up dramatically and the gas production rate increases exponentially.
Superheat at different temperatures acts on different molecular structures to produce different cracking products; different types of discharges (partial discharges, spark discharges, arc discharges) release vastly different energy levels, resulting in very different types and proportions of gases. This is the scientific basis for diagnosing faults by analyzing dissolved gases in oil.
2. Cross-reference table of the seven characteristic gases
| Gas name | chemical formula (e.g. water H2O) | Main Fault Types | production mechanism | Key features |
|---|---|---|---|---|
| hydrogen (gas) | H₂ | Partial Discharge, Low Energy Discharge, Corona | Oil molecules break the C-H bond in the presence of an electric field to produce hydrogen radical binding | Lightest gas, most likely to escape from oil; earliest sign of trouble |
| methane CH4 | CH₄ | Low temperature overheating in oil (300~500°C) | Alkane chains in oil break at moderate temperatures to form methyl radicals | Marker of low temperature overheating, often concomitant with ethane |
| ethane (C2H6) | C₂H₆ | Low temperature overheating in oil (300~500°C) | Binding of two methyl radicals or C₂ chain breaks | Confirmation of low and medium temperature thermal faults when accompanied by methane |
| vinyl | C₂H₄ | High temperature overheating in oil (>500°C) | Massive C-C bond breaking at high temperatures to recombine into unsaturated hydrocarbons | A sign of high temperature overheating, occurring in large numbers at temperatures exceeding 500°C |
| ethyne C2H2 | C₂H₂ | Arc discharge, high energy discharge | C≡C triple bond formation at very high arc temperatures | Discharge failures are decisive indicators, trace occurrences require outage investigation |
| carbon monoxide CO | CO | Overheating or deterioration of solid insulation | Cellulose in insulating paper/board decomposes under the action of heat | CO/CO₂ ratio is a central parameter for determining the degree of insulation aging |
| carbon dioxide CO2 | CO₂ |
3. Detailed diagnostic value of individual gases
3.1 Hydrogen - the most sensitive precursor to failure
Hydrogen is the smallest molecular weight and fastest diffusing fault gas. Almost all types of faults initially produce hydrogen, making it the most sensitive but least specific indicator. Elevated hydrogen alone usually points to a partial discharge or corona; if it is accompanied by elevated levels of other hydrocarbon gases, further judgment is required by the law of combination.
3.2 Hydrocarbon Gases - Classification Indicators for Thermal Failures
Methane and ethane represent low-temperature overheating, ethylene represents high-temperature overheating, and acetylene represents arc discharge. The ratio of these four hydrocarbon gases is the core basis for determining the temperature range and energy level of the fault. For example, the higher the ethylene/ethane ratio, the higher the superheat temperature; once acetylene is present, no matter how low the concentration means there is a serious discharge.
3.3 Carbon Oxide - Life Indicator for Solid Insulation
CO and CO₂ come from the thermal decomposition of insulating paper and board and are a completely different source of hydrocarbon gases from the decomposition of oil Elevated CO/CO₂ ratios usually mean that the solid insulation is deteriorating at an accelerated rate. This part of the data is essential for assessing the overall remaining life of the transformer.
4. Correspondence between gas combinations and fault types
4.1 Thermal failure modes
Pure oil superheating (e.g. localized superheating due to poor contact with the on-load tap-changer): Mainly methane and ethylene, small amounts of ethane, almost no acetylene. Solid insulation overheating: Significantly higher CO and CO₂ on the basis of oil overheating gases.
4.2 Discharge Failure Modes
Localized discharge: Hydrogen predominates, accompanied by a small amount of methane. Spark discharge: hydrogen + acetylene at the same time, acetylene content is not high. Arc discharge: Acetylene rises sharply, while ethylene and hydrogen both increase dramatically, is the most serious fault signal inside the transformer.
5. Frequently Asked Questions FAQ
5.1 Q. What is the normal value of dissolved gases in oil?
A: The gas values vary for transformers of different voltage levels and capacities. Generally speaking, the hydrogen note value of transformer in operation is about 150 μL/L, the acetylene note value is about 5 μL/L (220kV and above), and the total hydrocarbon note value is about 150 μL/L. The specific values should be determined by referring to the equipment factory test report and operation regulations.
5.2 Q. Why is hydrogen elevated but other gases normal?
A: This situation usually points to partial or corona discharges. Because low energy discharge is mainly broken C-H bond to produce hydrogen, not enough to break the C-C bond to produce hydrocarbon gases. But also need to exclude water in the oil electrolysis hydrogen, stainless steel material catalytic hydrogen and other non-fault factors.
5.3 Q. Does the presence of acetylene necessarily imply a fault?
A: The presence of acetylene in an operating transformer requires high attention. Even if the concentration is very low (1~2 μL/L), it is necessary to shorten the testing period and monitor it intensively. If acetylene shows a continuous rising trend, basically can confirm the existence of internal discharge faults, should be arranged as soon as possible out of service inspection.
5.4 Q. How is the CO/CO₂ ratio interpreted?
A: A CO/CO₂ ratio greater than 0.1 or a steady increase indicates that the solid insulation is undergoing abnormal thermal aging. The higher the ratio, the steeper the trend, the faster the insulation aging rate. However, it should be noted that the CO and CO₂ will also increase slowly at the beginning of the operation of a new transformer, which is a normal aging process.
5.5 Q: What about discrepancies in gas data between on-line and off-line oil chromatography monitoring?
A: It is normal for there to be some error between on-line monitoring and off-line detection, due to different sampling methods, degassing methods, detector differences, etc. The key is to look at the trend. The key is to see if the trend is the same - if both show the same gas is continuously rising, even if there is a difference in the absolute value, the trend should be taken to determine the fault.
6. How can gas data be used to guide O&M decisions?
6.1 Establishment of gas baselines - A "fingerprint baseline" for each gas should be established as soon as possible after commissioning or overhaul of the equipment, and all subsequent analyses should be based on the baseline as a reference.
6.2 Focus on trends rather than single points - a single data anomaly may be a sampling or detection error; a continuing upward trend is a true warning sign. The advantage of online monitoring is that it provides dense trend data.
6.3 Multi-gas joint judgment - not only look at a single gas, but also look at the combination of gases and the ratio relationship, combined with the three-ratio method or David's Triangle method of integrated diagnosis.
Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements. The technical data and cases involved in this article are from public information and engineering practice, if updated without notice.
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