Dissolved Gas Analysis Test of Transformer Oil

Last Updated on February 2, 2024 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 to dissolve 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 tests must be conducted periodically to ensure the health 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 the DGA test. Therefore, the Dissolved Gas Analysis is a very important test to ensure the transformer’s overall health.

The dissolved gas analysis test of transformer oil comprises testing 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) are produced in large quantities. 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. 

Dissolved gas analysis of transformer oil

If there is a hot spot inside the transformer, the content of ethylene (C2H4) will 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.NParametersIndicative faultUnit of MeasurementViolation Limits
1.Methane    
(CH4)
Local Overheatingppm120 ppm
2.Ethylene       
(C2H4)
Thermal Degradation of oilppm30 ppm
3.Acetylene        (C2H2)Overheatingppm65 ppm
4.Ethane   
 (C2H) 
Arcing & sparkingppm35 ppm
5.Hydrogen         (H2)
 
Electrical / Thermal Fault
 
ppm100 ppm
6.Carbon Mono Oxide (CO)Thermal Aging of Paperppm350 ppm
7.Carbon Dioxide    (CO2)Cellulose DecompositionppmN/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. This method was 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  CO 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 CO 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 CO 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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About Satyadeo Vyas

Satyadeo Vyas, M.Tech,M.B.A. is an electrical engineer and has more than 36 years of industrial experience in the operation, maintenance, and commissioning of electrical and instrumentation projects. He has good knowledge of electrical, electronics, and instrumentation engineering.

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