Dissolved Gas Analysis in Mineral Oil-Immersed Transformers: An Overview
- Augusto Moser
- Jun 10
- 4 min read
Updated: Jul 22
Dissolved Gas Analysis (DGA) is a vital diagnostic technique employed to evaluate the health of mineral oil-immersed transformers. By examining the gases dissolved in the insulating oil, engineers can detect potential faults and prevent catastrophic failures. This article explores the nature, purpose, and application of DGA, emphasizing the key gases used in fault identification and other related gases, based on Section 4 of the IEEE Std C57.104-2019 standard.
The Nature of Dissolved Gas Analysis in Transformers
DGA involves identifying, measuring, and interpreting gases dissolved in the transformer's insulating oil. The primary gases, referred to as "fault gases," are critical for diagnosing transformer conditions. These include: Hydrogen (H₂), Methane (CH₄), Ethane (C₂H₆), Ethylene (C₂H₄), Acetylene (C₂H₂), Carbon Monoxide (CO) and Carbon Dioxide (CO₂). Additionally, Oxygen (O₂) and Nitrogen (N₂) are measured and used in the interpretation process, though they are not direct fault byproducts.
The Purpose of DGA
DGA aims to detect and diagnose abnormal conditions that could lead to transformer failure. By analyzing gas types and concentrations, engineers can:
Confirm the presence of a fault.
Identify the fault type (thermal, electrical, or combined).
Evaluate fault severity.
Track fault progression over time.
This non-invasive method enables early intervention, enhancing transformer reliability and reducing maintenance costs.
The Application of DGA
DGA is applied in multiple scenarios to ensure transformer performance:
Routine Monitoring:
Regular testing tracks gas trends, identifying gradual changes indicative of emerging faults.
Post-Fault Analysis:
Following an incident, DGA reveals the fault’s nature and severity, aiding repair or replacement decisions.
Commissioning and Acceptance Testing:
New or refurbished transformers are tested to verify fault-free operation before service.
Research and Development:
DGA data informs the improvement of diagnostic techniques and transformer design.
Gas Formation Mechanisms
Gases are generated through several processes within the transformer:
Thermal Decomposition:
Oil Decomposition: Mineral oils, composed of hydrocarbon molecules, break down under thermal stress, fracturing carbon-hydrogen and carbon-carbon bonds. This produces hydrogen, methane, ethane, ethylene, and acetylene, depending on the temperature and energy involved.
Cellulosic Decomposition: Cellulose insulation degrades thermally, yielding carbon monoxide (CO) and carbon dioxide (CO₂), along with minor amounts of hydrogen and methane.
Electrical Discharges:
Partial Discharges: These occur in gas bubbles or insulation voids, primarily generating hydrogen, with small quantities of methane and ethane.
Arcing: High-energy discharges produce a wider range of gases, notably acetylene, signaling severe electrical faults.
The production of these gases varies with temperature, energy distribution, and stress duration, reflecting the complexity of transformer oil chemistry.
Table 1. Typical faults types
Abbreviation | Fault Description |
Basic Fault Types (Table C.1) | |
PD | Partial discharges |
D1 | Discharges of low energy |
D2 | Discharges of high energy |
T1 | Thermal fault, t < 300 °C |
T2 | Thermal fault, 300 °C ≤ t < 700 °C |
T3 | Thermal fault, t ≥ 700 °C |
Additional Sub-Types of Faults (Table C.2) | |
S | Stray gassing |
O | Overheating of paper or mineral oil |
C | Possible carbonization of paper |
R | Catalytic reactions |
The Gases and The Fault Identification
The fault gases are pivotal in pinpointing specific transformer issues, while not fault indicators, Oxygen and Nitrogen play supporting roles:
Table 2. Gases and transformer issues
Gas | Description |
Hydrogen (H₂) | Generated by partial discharges and low-energy faults; may also indicate stray gassing or catalytic reactions. |
Methane (CH₄) | Produced by low-temperature oil decomposition or partial discharges, signaling early thermal stress. |
Ethane (C₂H₆) | Formed during moderate-temperature oil decomposition, suggesting sustained thermal exposure. |
Ethylene (C₂H₄) | Indicates higher-temperature thermal faults, often linked to severe overheating. |
Acetylene (C₂H₂) | A hallmark of high-energy arcing, its presence demands urgent attention due to the risk of serious faults. |
Carbon Monoxide (CO) | Arises from cellulose insulation overheating, reflecting solid insulation degradation. |
Carbon Dioxide (CO₂) | Produced from cellulose breakdown; its ratio with CO helps assess insulation condition severity. |
Oxygen (O₂) | Accelerates oil and insulation aging; its levels assess sealing system integrity. Not a fault indicator. |
Nitrogen (N₂) | Used as a blanket gas in sealed transformers; its concentration monitors seal effectiveness. Not a fault indicator. |
Conclusion
Dissolved Gas Analysis is an essential practice for maintaining mineral oil-immersed transformers. By leveraging fault gases—hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, and carbon dioxide—alongside oxygen and nitrogen, engineers gain critical insights into transformer health. This enables proactive maintenance, ensuring operational reliability and longevity. For deeper exploration, consult the IEEE Std C57.104-2019 standard and related technical resources.
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References
[1] "IEEE Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers," in IEEE Std C57.104-2019 (Revision of IEEE Std C57.104-2008) , vol., no., pp.1-98, 1 Nov. 2019, doi: 10.1109/IEEESTD.2019.8890040.
