Dissolved Gas Analysis Test of Transformer Oil

Dissolved gas analysis test of transformer oil is a tool to detects gases dissolved in the oil for diagnostics in order 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 sample. Online DGA test can also be done through portable online DGA test machine.

During the fault in the electrical network, the large current flowing through the transformer winding cause thermal and electrical stresses and certain gases are produced due to 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 the gas 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 is more than the specified limit the transformer oil is prone to fail.

The analysis of the dissolved gases gives the clearer picture of the healthiness of the transformer oil and the paper insulation. The test conducted for analysis of 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 of the transformer. The DGA results indicates about the healthiness of the insulation system of the transformer.

Advantages of Dissolved Gas Analysis Test of Transformer Oil

Current state of insulating material inside the transformer
Remaining life of the transformer

Identifies degradation before it lead to failure
It is a good tool for effective maintenance and replacement strategies.
It is low cost process

Even if the transformer oil pass the breakdown dielectric voltage(BDV) test, it may fails in the DGA test. Therefore, the Dissolved Gas Analysis is very important test to ensure the over all health of the transformer.

Dissolved gas analysis test of transformer oil comprise 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 is evolved during fault and gets 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 is extracted from oil and the amount of gas present 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. The extracted gas is then analyzed by gas chronographs for measurement of each component.

If the temperature of the transformer rises up to 150^°C to 300° C. during thermal stress, the hydrogen(H₂) and methane((CH₄) is produced in a large quantity. If the temperature of the transformer exceeds above 300°C ethelene(C₂H₄) is produced in a large quantity. Hydrogen(H₂) and ethelene (C₂H₄) are produced if temperature rises above 700°C. The gas generation at different temperature is as shown in below given chart.

Dissolved gas analysis of transformer oil

If there is a hot spot inside the transformer, the content of ethelene(C₂H₄) will get increased. 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 is more than the specified highest limit the following types of fault as shown in the below given table 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 a oil filled transformer paper insulation is used to isolate the winding and the core of the transformer. The paper insulation has base of cellulose of long chain of the molecules. The long chain starts breaking into number of shorter parts with time and paper becomes brittle. The broken part of the paper mixed up in the transformer oil. When the transformer is in service, it is not feasible to remove a small piece of paper for testing the health condition of the cellulose paper. The method used to examine the condition of paper without having a small portion of the paper insulation from transformer. The test is called Furfurldehyde test or Furfufal Test.

In Dissolved Gas Analysis test it is possible to predict the condition of the cellulose paper insulation. According to 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. With this method This method of testing does not give the 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 of oil 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- Furfural family compounds released when there is destructive heating of cellulose paper. Damage of a few gram 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 transformer.

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