The cause for the gas generation in the mineral oil is the breaking of the chemical bonds in the hydrocarbon molecules of the mineral oil. Energy is needed for the breaking of the chemical bonds and this comes from the energy contained in the fault of the transformer. The gases generated include hydrogen(H2), methane(CH4), ethane(C2H6), ethylene(C2H4), acetylene(C2H2), carbon dioxide(CO2) and carbon monoxide(CO).
These generated gases will initially dissolve in the oil and cannot be seen by the naked eye. As the volume of generated gases increases, more of these gases will dissolve into the oil. There will come a point when the oil will be totally saturated with the dissolved gas, and any further increase in gases cannot be contained as dissolved gas in the oil. The excess gases will come out as free gas.