Dissolved Gas Analysis (DGA) in Power Transformers: Content, Types, and Diagnostic Properties
Dissolved Gas Analysis (DGA) is a critical diagnostic technique used to assess the internal condition of oil-filled power transformers. It involves the study of gases dissolved in the insulating oil. Abnormal thermal and electrical stresses within the transformer cause the decomposition of the insulating fluid and solid insulation (cellulose), producing characteristic fault gases. The concentration and composition of these gases allow engineers and technicians to accurately predict the nature and severity of internal faults.
Key Fault Gases and Their Diagnostic Significance
The primary combustible gases generated by the decomposition of transformer oil and solid insulation are known as key fault gases. Their presence and relative concentrations are directly correlated with specific internal fault types, such as partial discharge, overheating, and arcing.
| Gas | Chemical Formula | Primary Fault Indication | Temperature Range / Condition | Key Diagnostic Properties |
| Hydrogen | H₂ | Partial Discharge (PD), Low-Energy Thermal Faults | Starting around 200°C | The universal choice for single-gas fault monitoring. Its presence indicates partial discharge activity, oil overheating, or electrolysis of free water. Concentration increases with temperature. |
| Methane | CH₄ | Low-to-Medium Temperature Thermal Faults, Partial Discharge | Starting around 200°C | Produced across the fault temperature spectrum. Concentration decreases as fault temperatures rise above 250°C. |
| Ethane | C₂H₆ | Medium Temperature Thermal Faults | 300°C – 700°C (Medium) | Indicates overheating of oil. Typically found starting around 250°C, with concentration decreasing as temperatures approach 300°C. |
| Ethylene | C₂H₄ | High Temperature Thermal Faults (Hot Spots) | Starting around 350°C | A strong indicator of very high-temperature hot spots. Its concentration decreases under arcing conditions (above 700°C). |
| Acetylene | C₂H₂ | Arcing (Electrical Discharge) | Small quantities around 500°C; rapid increase starting at 700°C | The definitive indicator of arcing or high-energy electrical discharge. Any detectable concentration, particularly in a new transformer, is considered a serious indicator of a critical fault. |
| Carbon Monoxide | CO | Cellulose Insulation Breakdown (Paper Overheating) | N/A | Formed primarily from the breakdown of solid cellulose insulation (paper). The CO₂/CO ratio provides additional information on the fault’s nature. |
| Carbon Dioxide | CO₂ | Cellulose Insulation Breakdown (Paper Overheating) | N/A | Formed from cellulose breakdown. Its rate of formation is influenced by the amount of paper in the fault region. It can also be generated from insulating oil, especially with high oxygen content. |
Atmospheric and Other Gases
Two other gases, Oxygen and Nitrogen, are typically present in transformer oil but are not products of oil or paper decomposition due to electrical or thermal faults. Their presence is primarily related to the transformer’s breathing or blanketing system.
Oxygen (O₂)
Oxygen is always present in varying amounts, typically originating from residual air or air ingress.
- Indication: May indicate leaky gaskets on the transformer tank or tap changers.
- Significance: High concentrations can increase the rate of formation of other measured fault gases and are capable of carrying moisture into the transformer fluid.
Nitrogen (N₂)
Nitrogen is present from residual air, air ingress, or, most commonly, from an inert nitrogen pressure (gas blanketing) system used to protect the oil from atmospheric contamination.
- Indication: High nitrogen levels can indicate incomplete transformer degassing or air ingress.
- Significance: May also indicate extreme overpressure in a gas blanketing system.
The analysis of these gases, particularly the ratio of the key fault gases, forms the basis for established diagnostic methods like the Duval Triangle and Rogers Ratios, allowing for precise fault identification.
