The dissolved gas analysis (DGA) test of transformer oil is a tool to detect dissolved gases in the oil for diagnostics to detect incipient faults. In other words, Dissolved Gas Analysis(DGA) is the study of dissolved gases in the transformer oil to examine the insulation condition of the transformer. This test is a kind of laboratory test of transformer oil samples. Online DGA tests can also be done through portable online DGA test machines.
During faults in the electrical network, the current passing through transformer windings causes thermal and electrical stress, which can produce gases due to the decomposition of transformer oil.
When a major fault occurs in a transformer, the decomposed gas is collected in the Buchholz relay, which activates its trip contact and trips the transformer breaker. The gas can be released from the Buchholz relay and is thus taken out. If the fault in the transformer is not severe, the gas produced can dissolve in the transformer oil. However, if the concentration of dissolved gases in the transformer oil exceeds the limit specified, the oil may fail.
Monitoring the Buchholz relay alone is insufficient to predict the overall internal condition of an electrical power transformer. Therefore, analyzing the various gases dissolved in the transformer oil during service is necessary.
Analyzing the dissolved gases present in transformer oil gives a better understanding of the condition of the oil and paper insulation. This transformer’s oil analysis, known as the DGA test, is essential to regularly perform to ensure the transformer is in good health and operates smoothly. The DGA test results provide valuable insight into the transformer’s insulation system, indicating its overall health.
A Dissolved Gas Analysis (DGA) test on transformer oil ensures the transformer’s overall health, even if the oil passes the breakdown dielectric voltage (BDV) test.
During DGA test, various gases such as hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H3), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), and oxygen (O2) are typically found in the oil.
Methods of Determining Dissolved Gas in Transformer Oil
The gases evolve during fault and get dissolved in the transformer oil. In Dissolved Gas Analysis of transformer oil or DGA analysis of transformer oil, the gases dissolved in the transformer oil are extracted from oil, and the amount of gas in the oil is analyzed.
The Vacuum Gas Extraction Apparatus and Gas Chronograph methods are used to determine the content of the gases in the transformer oil.
The Vacuum Gas Extraction Apparatus extracts gas from oil by creating a vacuum. Then, gas components are analyzed using gas chronographs.
It has been observed that when the internal temperature of a power transformer rises between 150°C and 300°C due to abnormal thermal stresses, a large quantity of hydrogen and methane gases are produced. If the temperature exceeds 300°C, a substantial amount of ethylene (C2H4) is produced. At temperatures exceeding 700°C, a considerable amount of hydrogen (H2) and ethylene (C2H4) are produced.
The chart below displays the gas production at various oil temperatures.
Ethylene (C2H4) is a sign of a potentially dangerous hot spot inside the electrical transformer that has reached very high temperatures. During a DGA (Dissolved Gas Analysis) test of the transformer oil, high levels of CO and CO2 could indicate the decomposition of proper insulation.
Permissible Limits of Dissolved Gases in Transformer Oil
If the dissolved gases in the transformer oil exceed the specified limit, in that case, the following faults can be detected: The maximum amount of dissolved gases that a transformer oil can have, as per IS 10593, is provided below.
| S.N | Parameters | Indicative fault | Unit of Measurement | Violation Limits |
| 1. | Methane (CH4) | Local Overheating | ppm | 120 ppm |
| 2. | Ethylene (C2H4) | Thermal Degradation of oil | ppm | 30 ppm |
| 3. | Acetylene (C2H2) | Overheating | ppm | 65 ppm |
| 4. | Ethane (C2H6 ) | Arcing & sparking | ppm | 35 ppm |
| 5. | Hydrogen (H2) | Electrical / Thermal Fault | ppm | 100 ppm |
| 6. | Carbon Mono Oxide (CO) | Thermal Aging of Paper | ppm | 350 ppm |
| 7. | Carbon Dioxide (CO2) | Cellulose Decomposition | ppm | N/A |
Furan Analysis of Transformer Oil
In an oil-filled transformer, paper insulation separates the winding and the core of the transformer. The paper insulation has a cellulose base with a long chain of molecules. The long chain breaks into many shorter parts with time, making the paper brittle—the broken parts of the paper mix with the transformer oil. During the transformer’s operation, removing a small piece of paper to inspect the cellulose paper’s health condition is impractical. The Furfuryldehyde test, or the Furfural Test, is a method used to examine the condition of paper without having to extract a small portion of the paper insulation from the transformer.
In the Dissolved Gas Analysis test, the condition of the cellulose paper insulation inside the transformer can be predicted. According to the guideline provided by IEC-599, if the ratio of CO2 and CO is more than 11, the quality of paper insulation inside the transformer is not good. The paper insulation quality may still be good if the ratio is between 4 to 11. However, this testing method may not always provide accurate results as CO2 and CO are also produced during the oil breakdown. Therefore, the ratio may sometimes mislead the prediction.
When paper is soaked with oil and exposed to heat, it gets damaged, and some unique oil-soluble compounds are released. These compounds belong to the Furfuraldehyde family, also known as Furfural. Out of all the Furfural compounds, 2-furfural is the most common. These compounds can only be released through destructive heating of cellulose or paper, and along with CO2 and CO, they dissolve in the oil.
Furan analysis is an extremely sensitive diagnostic test that can detect even minor damage to a few grams of paper in transformer oil, even in large transformers. This makes it one of the most reliable tests for assessing the lifespan of a transformer.
The percentage rate increase of Furfural’s products in transformer oil over time is utilized to evaluate the condition and remaining lifespan of the paper insulation in a transformer.
When to Conduct the DGA Test?
It is advisable to conduct a Dissolved Gas Analysis (DGA) of transformer oil at least once a year and compare the results with those from previous years. The relevant standards for sampling, testing, and analyzing the results are ASTM D3613, ASTM D3612, and ANSI/IEEE C57.104.
After high voltage and temperature rise tests in the factory, the DGA test must be conducted.
If the transformer trips due to the transformer main protection trip, Buchholz relay trip, pressure relay trip, Differential relay trip, or overloading, a DGA test must be conducted.
Advantages of the DGA Test
- It indicates the health of the insulating materials inside the transformer.
- Identifies the remaining life of the transformer.
- Identifies degradation before it leads to failure.
- It is a good tool for effective maintenance and replacement strategies.
- It is a low-cost process.
Conclusion:
Dissolved Gas Analysis (DGA) is a test that can detect abnormal conditions inside a transformer at an early stage. This test involves analyzing the concentrations of gases dissolved in the transformer oil. To perform this test, small oil samples are taken using a clean and moisture-free container or syringe.
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