What Gases Does DGA Monitor in Transformer Oil?
Dissolved Gas Analysis (DGA) commonly evaluates hydrogen (H₂), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), carbon monoxide (CO), and carbon dioxide (CO₂) in transformer oil. Some systems also measure oxygen (O₂), nitrogen (N₂), and moisture. These DGA gases are generated by different electrical, thermal, and insulation-related processes inside a transformer, which is why transformer DGA gases are used as diagnostic indicators rather than simple pass/fail readings.
The presence of a particular gas does not by itself prove a specific fault. Gas concentrations, combinations, ratios, and trends should be considered together before drawing any diagnostic conclusion.
This article mainly introduces What Do DGA Gases Mean. It explains the types, generation mechanisms and diagnostic logic of characteristic gases in DGA testing. Insulation aging, local overheating or discharge faults inside transformers decompose the oil and produce various characteristic gases. Operation and maintenance staff continuously track changes in gas data via DGA monitoring, conduct comprehensive analysis using ratio methods, warn of potential hazards in advance, prevent sudden transformer outages, and ensure stable operation of power systems.
DGA Gas Meaning at a Glance
| DGA Gas | Formula | Typical Diagnostic Association |
|---|---|---|
| Hydrogen | H₂ | Partial discharge / electrical activity |
| Methane | CH₄ | Low-temperature thermal activity |
| Ethane | C₂H₆ | Thermal fault |
| Ethylene | C₂H₄ | Higher-temperature thermal activity |
| Acetylene | C₂H₂ | Arcing / high-energy discharge |
| Carbon Monoxide | CO | Cellulose insulation degradation |
| Carbon Dioxide | CO₂ | Cellulose insulation degradation / aging |
Hydrogen in Transformer Oil: What Does H₂ Mean?
Hydrogen in transformer oil is one of the most commonly monitored DGA gases and is often the first indicator of an electrical or thermal abnormality. Its presence is measured as H₂ concentration, typically reported in ppm.
What does hydrogen indicate?
Elevated hydrogen in transformer oil may indicate:
- Partial discharge
- Low-energy electrical discharge
- Certain thermal or electrical processes
Because hydrogen is a relatively light molecule and is generated by several different mechanisms, it is not specific to a single fault type. It is best understood as an early indicator that some form of abnormal activity may be occurring.
How is hydrogen generated?
Electrical stress can break down hydrocarbon molecules in the oil, contributing to hydrogen formation. Thermal stress can also generate hydrogen, along with other hydrocarbon gases. This is why hydrogen is often detected before other fault gases become noticeable.
What should be considered?
Hydrogen should always be interpreted together with other gases and historical trends. For example:
- Hydrogen rising alongside CH₄ may suggest partial discharge or low-energy discharge.
- Hydrogen rising alongside C₂H₂ may indicate arcing or high-energy discharge.
- Hydrogen rising alongside C₂H₄ may point to a thermal fault.
A single hydrogen reading does not confirm a specific fault. Its concentration, rate of increase, and relationship with other gases all contribute to a meaningful interpretation.
Methane in Transformer Oil: What Does CH₄ Mean?
Methane in transformer oil is one of the key DGA gases monitored for transformer condition assessment. It is typically reported as CH₄ concentration in ppm and is often evaluated alongside hydrogen, ethane, ethylene, and acetylene to build a more complete diagnostic picture.
Methane Generation
Methane is generated primarily by thermal decomposition of transformer oil. It forms at relatively low temperatures compared with ethylene and acetylene, making it an early indicator of thermal activity inside the transformer.
Low-Temperature Thermal Faults
Elevated methane may be associated with thermal activity, particularly when interpreted together with ethane, ethylene, hydrogen, and other DGA gases. A gas pattern dominated by CH₄ and C₂H₆, with relatively low C₂H₄ and C₂H₂, is often associated with low-temperature thermal faults.
Electrical Discharge
Methane can also be generated during electrical discharge, though usually in combination with hydrogen and other gases. When CH₄ rises alongside H₂, it may indicate partial discharge or low-energy discharge rather than a purely thermal condition.
Combination With H₂
The CH₄/H₂ ratio is a commonly used diagnostic indicator. A relatively high CH₄/H₂ ratio tends to suggest thermal activity, while a lower ratio may point toward discharge-related conditions.
Combination With C₂H₆
When methane rises together with ethane (C₂H₆), the pattern often indicates thermal decomposition of oil at low to moderate temperatures. The CH₄/C₂H₆ relationship helps distinguish thermal fault severity and is used in several ratio-based diagnostic methods.
What Should Be Considered?
Methane concentration should not be interpreted in isolation. Its significance depends on:
- Absolute concentration level
- Rate of increase over time
- Relationship with other gases such as H₂, C₂H₆, C₂H₄, and C₂H₂
- Historical trends and transformer operating conditions
A single elevated methane reading does not confirm a specific fault, but it may indicate the need for closer monitoring and further diagnostic evaluation.
Ethane in Transformer Oil: What Does C₂H₆ Mean?
Ethane (C₂H₆) is one of the key hydrocarbon gases monitored in transformer DGA. While it is often less prominent than hydrogen, methane, ethylene, or acetylene, it plays an important role in completing the gas picture — particularly for identifying thermal faults.
Thermal Decomposition
Ethane is primarily generated by thermal decomposition of transformer oil. It forms at relatively low temperatures, similar to methane, and is therefore associated with lower-temperature thermal activity rather than high-energy arcing.
Lower-Temperature Thermal Activity
When ethane is elevated alongside methane, the gas pattern typically points to thermal decomposition of oil at low to moderate temperatures. In this context:
- CH₄ + C₂H₆ dominant → low-temperature thermal fault
- C₂H₄ increasing → higher-temperature thermal activity
- C₂H₂ present → arcing or high-energy discharge
This progression makes ethane a useful reference point for assessing thermal fault severity.
Relationship With CH₄ and C₂H₄
The relationships between ethane and other hydrocarbon gases provide diagnostic value:
- CH₄/C₂H₆ ratio helps characterize low-temperature thermal conditions
- C₂H₄/C₂H₆ ratio is used in Rogers Ratios and IEC 60599 to evaluate thermal fault characteristics and distinguish temperature ranges
Ethane itself is rarely a standalone indicator, but its presence and proportion relative to methane and ethylene help refine the thermal fault classification.
What Should Be Considered?
C₂H₆ should be interpreted together with CH₄, C₂H₄, C₂H₂, H₂, and historical trend data. A stable ethane level may reflect normal aging, while a rising trend alongside increasing ethylene may indicate a developing thermal fault requiring closer monitoring.
Ethylene in Transformer Oil: What Does C₂H₄ Mean?
Ethylene (C₂H₄) is one of the most diagnostic hydrocarbon gases in transformer DGA. It is primarily generated by thermal decomposition of transformer oil at higher temperatures and serves as a key indicator of thermal fault severity.
Thermal Decomposition
Ethylene forms when transformer oil is subjected to elevated thermal stress. Compared with methane and ethane, which are associated with lower-temperature thermal activity, ethylene indicates more intense heating. Increasing ethylene can indicate increasingly severe thermal decomposition, but its interpretation depends on the overall gas pattern.
Higher-Temperature Thermal Activity
A gas pattern dominated by C₂H₄, with relatively low C₂H₂, typically points to a high-temperature thermal fault involving the oil. If C₂H₂ also appears, the condition may involve arcing or a combination of thermal and electrical faults.
Relationship With Ethane (C₂H₆)
The C₂H₄/C₂H₆ ratio is a widely used diagnostic indicator:
- Low C₂H₄/C₂H₆ ratio → lower-temperature thermal activity
- High C₂H₄/C₂H₆ ratio → higher-temperature thermal fault
This ratio forms part of the Rogers Ratios and IEC 60599 diagnostic methods.
Relationship With Acetylene (C₂H₂)
The relationship between ethylene and acetylene helps distinguish thermal faults from electrical discharge:
- C₂H₄ dominant, low C₂H₂ → thermal fault
- C₂H₂ dominant, with C₂H₄ present → arcing or high-energy discharge
The C₂H₂/C₂H₄ ratio is particularly important for identifying discharge-related conditions.
What Should Be Considered?
C₂H₄ should be interpreted together with H₂, CH₄, C₂H₆, C₂H₂, and historical trend data. Rising ethylene alongside stable acetylene suggests a developing thermal fault, while rising ethylene together with acetylene may indicate a more complex or severe condition. As with all DGA gases, a single value does not confirm a fault — concentration, ratio, and trend should be assessed together.
Acetylene in Transformer Oil: What Does C₂H₂ Mean?
Acetylene (C₂H₂) is one of the most critical gases in transformer DGA. It is particularly important because its presence can be associated with high-energy electrical discharge or arcing — conditions that may develop rapidly and require prompt attention.
Arcing and High-Energy Electrical Discharge
Acetylene is primarily generated by high-energy electrical discharge, including arcing. When C₂H₂ appears in transformer oil, it typically indicates that the oil has been exposed to extreme localized energy — far more intense than the conditions that produce methane or ethylene. Even relatively low concentrations of acetylene are often treated as a warning sign.
Thermal and Electrical Fault Conditions
While acetylene is most strongly associated with arcing, it can also appear in certain thermal fault conditions where local temperatures are extremely high. In some cases, combined thermal and electrical fault conditions may produce both ethylene and acetylene. This is why the concentration and combination with other gases matter.
Relationship With Other Gases
The diagnostic value of C₂H₂ depends on how it relates to other gases:
- C₂H₂ + H₂ → may indicate arcing or high-energy discharge
- C₂H₂ + C₂H₄ → the C₂H₂/C₂H₄ ratio helps distinguish arcing from thermal faults
- C₂H₂ alone at low level → may reflect a transient event, but still requires monitoring
What Should Be Considered?
Acetylene should never be interpreted from a single reading. Key factors include:
- Absolute concentration
- Rate of increase — a sudden rise is more concerning than a stable level
- Relationship with H₂, C₂H₄, and other gases
- Historical trend and transformer operating conditions
Because acetylene is associated with high-energy discharge, its appearance — especially a rising trend — typically warrants closer monitoring, more frequent sampling, and further diagnostic investigation.
Carbon Monoxide in Transformer Oil: What Does CO Mean?
Carbon monoxide (CO) in transformer oil is primarily associated with the thermal degradation of cellulose-based insulation rather than directly indicating an electrical discharge. It is therefore interpreted differently from hydrocarbon gases such as hydrogen, methane, or acetylene.
Cellulose Insulation and Paper Insulation Degradation
Transformer solid insulation consists mainly of cellulose-based paper and pressboard. When this insulation is exposed to excessive heat, it decomposes and releases CO and CO₂ into the oil. CO is generally the more sensitive indicator of active cellulose degradation, while CO₂ reflects the broader condition of the insulation system.
Overheating of Solid Insulation
Elevated CO may indicate overheating of solid insulation, particularly when accompanied by rising CO₂. This type of condition can occur due to:
- Localized hot spots in the winding or core
- Restricted oil flow affecting heat dissipation
- Long-term thermal aging of paper insulation
Unlike hydrocarbon gases, which primarily reflect oil decomposition, CO directly relates to the solid insulation — a critical factor in transformer life assessment.
Relationship With CO₂
The CO/CO₂ relationship provides additional diagnostic value:
- Rising CO with relatively stable CO₂ → may indicate active cellulose degradation
- Rising CO and CO₂ together → may indicate more extensive insulation involvement
- High CO/CO₂ ratio → may suggest severe localized overheating of paper insulation
What Should Be Considered?
CO should be interpreted together with CO₂, hydrocarbon gases, and historical trends. A single elevated CO reading does not confirm a fault, but it may indicate the need for closer monitoring — especially since cellulose insulation degradation is largely irreversible and directly affects transformer service life.
Carbon Dioxide in Transformer Oil: What Does CO₂ Mean?
Carbon dioxide (CO₂) in transformer oil is mainly associated with the condition of cellulose-based insulation. Like CO, it is generated when paper and pressboard insulation undergo thermal degradation or long-term aging. CO₂ is generally produced in larger quantities than CO and reflects the broader state of the solid insulation system.
Cellulose Insulation and Paper Aging
Transformer paper insulation decomposes gradually over its service life, releasing CO and CO₂ into the oil. Normal aging produces a stable background level of both gases. When insulation is exposed to excessive heat or localized hot spots, CO₂ generation may increase alongside CO.
Thermal Degradation
Elevated CO₂ can indicate thermal degradation of cellulose insulation, particularly when accompanied by rising CO. The combination of both gases rising together often points to active insulation involvement rather than normal aging.
CO/CO₂ Relationship
The relationship between CO and CO₂ is more informative than either gas alone:
- Rising CO with relatively stable CO₂ → may indicate active cellulose degradation
- Rising CO and CO₂ together → may indicate more extensive insulation involvement
- High CO/CO₂ ratio → may suggest severe localized overheating of paper insulation
- Low CO/CO₂ ratio with both elevated → may reflect long-term aging rather than an acute fault
CO and CO₂ should often be evaluated together rather than interpreted independently.
What Should Be Considered?
CO₂ should be interpreted alongside CO, hydrocarbon gases, and historical trend data. A stable CO₂ level may simply reflect normal insulation aging, while a rising trend — especially with increasing CO — may indicate a developing thermal condition affecting solid insulation. Because cellulose degradation is largely irreversible, early detection matters for transformer life assessment.
CO and CO₂ in Transformer Oil: What Do They Indicate?
CO and CO₂ in transformer oil are primarily associated with cellulose insulation — the paper and pressboard materials used in transformer windings. Unlike hydrocarbon gases, which mainly reflect oil decomposition, CO and CO₂ indicate the condition of solid insulation.
| Gas / Indicator | What It Indicates |
|---|---|
| CO | Thermal degradation of cellulose insulation; more sensitive to active paper decomposition |
| CO₂ | Broader condition of cellulose insulation; produced in larger quantities than CO |
| CO/CO₂ relationship | Helps distinguish active degradation from long-term aging; a rising CO/CO₂ ratio may suggest localized overheating |
| Historical changes | Stable levels may reflect normal aging; rising trends may indicate developing thermal stress on insulation |
CO and CO₂ should be evaluated together rather than independently. Their relationship, combined with historical changes, provides insight into whether cellulose insulation is aging normally or undergoing active thermal degradation.
What Do Different DGA Gas Patterns Mean?
Individual gases provide useful clues, but DGA interpretation becomes more meaningful when gases are considered in combination. Different gas patterns may indicate different fault directions. The table below summarizes common patterns and their possible associations.
| Gas Pattern | Possible Association |
|---|---|
| H₂-dominant pattern | Possible partial discharge / electrical activity |
| CH₄ + C₂H₆ | Possible thermal activity at relatively low temperatures |
| C₂H₄-dominant pattern | More pronounced thermal decomposition at higher temperatures |
| C₂H₂ + H₂ | Possible high-energy electrical discharge or arcing |
| CO + CO₂ | Cellulose insulation degradation or aging |
These associations are indicative only. A gas pattern does not confirm a specific fault on its own. Actual DGA fault diagnosis requires combining gas concentrations, gas ratios, diagnostic methods, historical trends, and transformer operating conditions before drawing conclusions.
For example, an H₂-dominant pattern may suggest partial discharge, but it could also reflect other electrical or thermal processes. Similarly, the presence of C₂H₂ alongside H₂ is often associated with high-energy discharge, yet its significance depends on concentration level, rate of increase, and how other gases are changing over time.
In practice, gas patterns serve as a starting point for further analysis — not as a final diagnosis.
Why Can’t a Transformer Fault Be Identified From One DGA Gas?
A single DGA gas value cannot confirm a specific transformer fault. Different gases are generated by overlapping processes, and the same gas can appear in more than one fault type.
For example:
- High hydrogen does not automatically mean partial discharge.
- Acetylene does not automatically prove an arc fault.
- Elevated CO does not by itself determine the condition of the paper insulation.
Interpreting DGA correctly requires moving through several layers of analysis:
Individual Gas → Gas Combination → Gas Ratio → Trend → Diagnostic Method
Each layer adds context. Individual gas concentrations show what is present. Gas combinations reveal which fault directions are more likely. Gas ratios help distinguish thermal faults from electrical discharges. Historical trends indicate whether the condition is stable, developing, or accelerating. Diagnostic methods such as IEC 60599, the Duval Triangle, and Rogers Ratios then provide a structured framework for classification.
Only when these layers are considered together — along with transformer operating and maintenance data — can a reliable fault diagnosis be reached. For a complete step-by-step interpretation process, see the DGA Results Interpretation guide.
DGA Gases and Their Typical Fault Associations
| Gas | Main Diagnostic Association | Interpretation Context |
|---|---|---|
| H₂ | Electrical activity / PD | Check other gases and trend |
| CH₄ | Thermal / electrical activity | Analyze with H₂ and C₂ gases |
| C₂H₆ | Thermal activity | Compare with CH₄ and C₂H₄ |
| C₂H₄ | Higher-temperature thermal activity | Evaluate thermal gas pattern |
| C₂H₂ | Arcing / high-energy discharge | Check concentration and gas ratios |
| CO | Cellulose degradation | Evaluate with CO₂ |
| CO₂ | Cellulose degradation | Evaluate CO/CO₂ trend |
Frequently Asked Questions About Partial Discharge in Transformers
What gases does DGA monitor in transformer oil?
DGA monitors the key fault gases dissolved in transformer oil, including hydrogen (H₂), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), carbon monoxide (CO), and carbon dioxide (CO₂). Some systems also measure O₂, N₂, and moisture. These gases are generated by different electrical, thermal, and insulation-related processes inside a transformer.
What does hydrogen in transformer oil mean?
Hydrogen in transformer oil is often the first indicator of an electrical or thermal abnormality. It may be associated with partial discharge, low-energy electrical discharge, or certain thermal processes. Hydrogen should be interpreted together with other gases and historical trends, since a single reading does not confirm a specific fault.
What does methane in transformer oil indicate?
Methane in transformer oil is primarily associated with thermal decomposition of oil at relatively low temperatures. Elevated methane, particularly when accompanied by ethane, may indicate low-temperature thermal activity. When methane rises alongside hydrogen, it may also suggest partial discharge or low-energy discharge.
What does acetylene in transformer oil indicate?
Acetylene in transformer oil is particularly important because its presence can be associated with high-energy electrical discharge or arcing. Even relatively low concentrations are often treated as a warning sign. However, concentration level, rate of increase, and combination with other gases such as H₂ and C₂H₄ all matter for interpretation.
What does ethylene in transformer oil indicate?
Ethylene in transformer oil is primarily generated by thermal decomposition at higher temperatures. Increasing ethylene can indicate increasingly severe thermal decomposition. Its interpretation depends on the overall gas pattern — particularly the C₂H₄/C₂H₆ ratio and the presence or absence of acetylene.
What do CO and CO₂ indicate in transformer oil?
CO and CO₂ in transformer oil are associated with cellulose insulation condition. CO is more sensitive to active paper degradation, while CO₂ reflects the broader state of the insulation system. The CO/CO₂ relationship and historical changes help distinguish normal aging from active thermal degradation.
Which DGA gas is associated with arcing?
C₂H₂ (acetylene) is the DGA gas most closely associated with arcing and high-energy electrical discharge. However, acetylene should not be interpreted in isolation — concentration, rate of increase, and combination with other gases such as H₂ and C₂H₄ are needed before drawing any diagnostic conclusion.