Dissolved gas analysis (DGA) is the study of dissolved gases in transformer oil. It is also referred to as a DGA test. Whenever a transformer undergoes abnormal thermal and electrical stresses, certain gases are produced due to the decomposition of the transformer oil. When the fault is major, the production of decomposed gases are significant and they get collected in a Buchholz relay. But when abnormal thermal and electrical stresses are not significantly high the gasses due to decomposition of transformer insulating oil will get enough time to dissolve in the oil.
Hence by only monitoring the Buchholz relay it is not possible to predict the condition of the total internal healthiness of electrical power transformer. That is why it becomes necessary to analyze the number of different gasses dissolved in transformer oil in service. Using DGA of transformer oil, one can predict the actual condition of the internal health of a transformer.
It is preferable to conduct the DGA test of transformer oil in a routine manner to get historical information about the internal health of a transformer over its lifetime. In a DGA test, the gases in oil are extracted and analyzed to determine the quantity of gasses in a specific amount of oil. By observing the percentages of different gasses present in the oil, you can predict the internal condition of the transformer.
Generally, the gasses found in the oil in service are hydrogen (H2), methane (CH4), Ethane (C2H6), ethylene (C2H4), acetylene (C2H3), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2) and oxygen(O2).
Most commonly used method of determining the content of these gases in oil, is using a Vacuum Gas Extraction Apparatus and Gas Chronographs. Using this apparatus, gasses are extracted from oil by stirring it under vacuum. These extracted gasses are then introduced in gas Chronographs for measurement of each component.
Generally it is found that hydrogen and methane are produced in large quantity if the internal temperature of power transformer rises up to 150oC to 300oC due to abnormal thermal stresses. If the temperature goes above 300oC, ethylene (C2H4) is produced in large quantity. At the temperature is higher than 700oC a large amount of hydrogen(H2) and ethylene(C2H4) are produced.
Ethylene (C2H4) is an indication of a very high-temperature hot spot inside an electrical transformer. If during DGA test of transformer oil, CO and CO2 are found in large quantity it is predicted that there is decomposition of proper insulation.
Furan Analysis of Transformer Oil
Transformer core and winding have mainly paper insulation. The base of the paper is cellulose. The cellulose structure is a long chain of molecules. As the paper becomes aged, these long chains are broken into a number of shorter parts. This phenomenon we often observe in our home. The pages of very old books become brittle over time.
In a transformer, the aging effect of paper insulation is accelerated due to the oxidation that occurs in oil. When insulating paper becomes mechanically weak, it can not withstand the mechanical stresses applied during an electrical short circuit – leading to electrical breakdown. It is therefore necessary to monitor the condition of paper insulation inside a power transformer.
It is not possible to bring out a piece of paper insulation from a transformer in service for testing purpose. But we are lucky enough, that there is a testing technique developed, where we can examine the condition of paper insulation without touching it. The method is called Furan analysis.
Although by dissolved gas analysis one can predict the condition of the paper insulation primarily, it is not a very sensitive method. There is a guideline in IEC-599, where it is stated that if the ratio of CO2 and CO in DGA results is more than 11, it is predicted that the condition of paper insulation inside the transformer is poor.
Healthy cellulose insulation gives that ratio in a range of 4 to 11. But still it is not a very sensitive way of monitoring the condition of paper insulation. Because CO2 and CO gases also produced during oil breakdown and sometimes the ratio may misleads the prediction.
When oil is soaked into paper, it is damaged by heat and some unique oil soluble compounds are realized and dissolved in the oil along with CO2 and CO. These compounds belong to the Furfuraldehyde group. These are sometimes called Furfural in short. Among all Furfurals compounds 2- Furfural is the most predominant. These Furfural family compound can only be released from destructive heating of cellulose or paper.
Furan analysis is very sensitive, as damage to a few grams of paper is noticeable in the transformer oil – even in a large transformer. It is a very significant diagnostic test, and bis generally considered the best test for assessing the life of a transformer.
The percentage rate rise of Furfurals products in transformer oil over time is used to assess the condition and remaining life of the paper insulation in a transformer.