DGA testing of Power transformer

DGA or Dissolved Gas Analysis of Transformer Oil

Whenever electrical power transformer goes under abnormal thermal and electrical stresses, certain gases are produced due to decomposition of transformer insulating oil, when the fault is major, the production of decomposed gases are more and they get collected in 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 analyse the quantity of different gasses dissolved in transformer oil in service. From dissolved gas analysis of transformer Oil or DGA of transformer oil, one can predict the actual condition of internal health of a transformer. It is preferable to conduct the DGA test of transformer oil in routine manner to get prior information about the trend of deterioration of transformer health and life.

Actually in dissolved gas analysis of transformer oil or DGA of transformer oil test, the gases in oil are extracted from oil and analyze the quantity of gasses in a specific amount of oil. By observing percentages of different gasses present in the oil, one can predict the internal condition of 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. By this apparatus first 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 internal temperature of power transformer rises up to 150°C to 300°C due to abnormal thermal stresses. If temperature goes above 300°C, ethylene(C₂H₄) are produced in large quantity. At the temperature is higher than 700°C large amount of hydrogen(H₂) and ethylene(C₂H₄) are produced.

Ethylene(C₂H₄) is indication of very high temperature hot spot inside 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.

Furfural or Furfuraldehyde Analysis

Transformer core and winding have mainly paper insulation. Base of paper is cellulose. The Cellulose has a structure of long chain of molecules. As the paper becomes aged, these long chains are broken into number of shorter parts. This phenomenon we often observe in our home. The pages of very old books become very much brittle. In transformer, the aging affect of paper insulation is accelerated due to oxidation occurs in oil. When insulating paper becomes mechanically weak, it can not withstand the mechanical stresses applied during electrical short circuit and leads 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 Furfuradehyde analysis of in short Furfural test.

Although by dissolved gas analysis one can predict the condition of the paper insulation primarily, but it is not very sensitive method. There is a guide line in IEC-599, where it is told 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 not good. A 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 some times 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. Furfuraldehyde analysis is very sensitive as because damage of few grams of paper is noticeable in the oil even of a very large size transformer. It is a very significant diagnostic test. It is best test for assessing life of transformer. The rate of rise of percentage of Furfurals products in oil, with respect to time, is used for assessing the condition and remaining life of paper insulation in power transformer.