Last Updated on October 28, 2023 by Electricalvolt
The dissolved gas analysis test of transformer oil is a tool to detect gases dissolved 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 the fault in the electrical network, the large current flowing through the transformer winding causes thermal and electrical stresses, and certain gases are produced due to the decomposition of the transformer oil.
When there is a major fault in the transformer, the decomposed gas gets collected in the Buchholz relay, and the gas actuates its trip contact, which trips the transformer breaker. The gas can be released from the Buchholz relay, and the gas is thus taken out. However, if the fault in the transformer is minor, the evolved gas gets enough time, and it gets dissolved in the transformer oil. When the dissolved gases in the transformer oil exceed the specified limit, the transformer oil is prone to fail.
The analysis of the dissolved gases gives a clearer picture of the healthiness of the transformer oil and the paper insulation. The test conducted to analyze the transformer insulation condition by examining the transformer oil is called the DGA Test. DGA test must be conducted periodically to ensure the healthiness of the transformer for trouble-free operation. The DGA results indicate the healthiness of the insulation system of the transformer.
Advantages of Dissolved Gas Analysis Test of Transformer Oil
- The current state of insulating material inside the transformer
- 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.
Even if the transformer oil passes the breakdown dielectric voltage(BDV) test, it may fail in the DGA test. Therefore, the Dissolved Gas Analysis is a very important test to ensure the overall health of the transformer.
The dissolved gas analysis test of transformer oil comprises testing of the following gases.
1. Hydrogen (H2)
2. Methane (CH4)
3. Ethane (C2H6),
4. Ethylene (C2H4),
5. Acetylene (C2H3),
6. Carbon Monoxide (CO),
7. Carbon dioxide (CO2),
8. Nitrogen (N2)
10. Oxygen(O2).
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.
Methods of determining gas amount in transformer oil
The following methods are used to determine the content of the gases in the transformer oil.
- Vacuum gas extraction
- Gas Chronographs
The oil is stirred in the oil under vacuum to extract the gases from the transformer oil. Gas chronographs then analyze the extracted gas for measurement of each component.
If the temperature of the transformer rises from 150°C to 300 °C during thermal stress, the hydrogen(H2) and methane((CH4) is produced in large quantity. If the temperature of the transformer exceeds 300°C, ethylene (C2H4) is produced in a large quantity. Hydrogen(H2) and ethylene (C2H4) are produced if the temperature rises above 700°C. The gas generation at different temperatures is shown in the chart below.
If there is a hot spot inside the transformer, the content of ethylene (C2H4) will get increase. The large quantity of carbon monoxide(CO) and carbon dioxide(CO2) in the transformer oil indicates paper insulation quality deterioration.
If the amount of the gases in the transformer value exceeds the specified highest limit, the following types of fault, as shown below, can be detected.
| 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 is used to isolate 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 part of the paper was mixed up in the transformer oil. When the transformer is in service, it is not feasible to remove a small piece of paper to test the health condition of the cellulose paper. The method used to examine the paper condition without having a small portion of the paper insulation from the transformer. The test is called the Furfurldehyde test or the Furfufal Test.
In the Dissolved Gas Analysis test, it is possible to predict the condition of the cellulose paper insulation. According to the IEC-599 guideline, if the ratio of CO2 and CO is more than 11, it is inferred that the insulation quality of the paper inside the transformer is not good. If the ratio is between 4 to 11, the paper insulation quality may be good. This method of testing does not give accurate results because CO2 and CO are produced during the oil breakdown as well. The ratio may mislead the prediction.
When oil is soaked in the paper, the paper gets damaged by heat, and some oil soluble compounds are realized and get mingled in the transformer oil along with CO2 and CO. These compounds belong to the Furfurldehyde group.
Among all Furfural compounds, 2-Furfural is most predominant, and these 2- 2-Furfural family compounds are released when there is destructive heating of cellulose paper. Damage to a few grams of cellulose paper is noticeable through this test.
The rate of rise of the Furfural family compounds in oil with respect to time is a tool for assessing the condition and the remaining life of paper insulation in transformers.
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