Review of Dissolved Gas Analysis(DGA) 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 Chromatographs. By this apparatus first gasses are extracted from oil by stirring it under vacuum. These extracted gasses are then introduced in gas chromatographs for measurement of each composition.
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.
